Chapter 2: Population and Conservation Ecology
Every ten years, a small army of schoolteachers and clerks walks out across India with clipboards. They climb to hill hamlets, wait outside locked doors, count people sleeping on railway platforms, and knock on the doors of flats where nobody is home before nine at night. India has been counting itself this way, at fixed intervals, since the 1870s. It is one of the largest counting exercises any country has ever attempted.
When the results of the 2011 count were released, they contained a result that surprised many people who had only ever heard that India’s population was exploding. The country had indeed added a very large number of people over the decade — more than most nations contain in total. Yet the rate at which it was growing had fallen, and fallen more sharply than in any decade since Independence. Families were, on average, having fewer children than their parents had. The population was still rising, but the engine was slowing.
Both halves of that sentence are true at the same time, and understanding why is the beginning of population ecology. It is also the beginning of a harder question. A population is not only a number of people. It is a number of people consuming water, grain, fuel, forest and land — and the forest, unlike the census, does not fill in a form.
Diagram
Fig. 2.0-1 – a census enumerator with a clipboard at the door of a house in a crowded street, with a distant hillside forest edge visible behind the rooftops; no identifiable individual faces
This chapter will answer each of these questions.
2.1 Population Dynamics
A census counts people. Ecology asks a different question: not how many there are today, but why the number is what it is, and where it is heading. That is the business of population dynamics.
Population dynamics is the study of changes in the size and composition of a population over time, and of the causes of those changes. It applies equally to the people of a town, the rohu in a village pond, the sal trees in a forest block and the mosquitoes breeding in a water tank on a roof. The vocabulary is the same in every case, which is one of the reasons this part of ecology is so useful.
2.1.1 Population, Size and Density
In ordinary speech, “population” means people. In ecology it means something narrower and more precise.
A population is a group of individuals of the same species living in a defined area at a given time, capable of interbreeding with one another. Three conditions are doing real work in that sentence. The individuals must belong to one species; they must share a defined area, which the ecologist chooses and states; and they must be counted at a stated time, because the number will not be the same next month.
Size and density are not the same thing
Population size, written \(N\), is simply the total number of individuals in that area at that time. It is a plain count, and by itself it can mislead. Two forests may each hold 500 deer, but if one is ten times the size of the other, the animals in the smaller forest are living under very different pressure.
What captures that pressure is population density — the number of individuals per unit area or per unit volume of habitat. Density is used for organisms in water or soil as well, where volume is the more natural measure.
\[D = \frac{N}{A}\]
Here \(D\) is the population density, \(N\) is the number of individuals in the area, and \(A\) is the area, usually in square kilometres or hectares for land organisms. For plankton in a lake or bacteria in soil water, \(A\) is replaced by a volume in litres or cubic metres, and the unit of \(D\) changes accordingly. A density figure without its unit means nothing, so the unit is always stated.
Suppose a town of 24 150 residents occupies 30 km². Its density is
\[D = \frac{24\,150}{30} = 805 \text{ persons per km}^2\]
That single figure tells a planner more about the load on the town’s water supply and drainage than the head-count does. If the same 24 150 people were spread over 300 km² of farmland, the density would be 80.5 persons per km², and the water problem would be a completely different one.
How density is actually measured
You cannot count every grass plant in a meadow or every fish in a lake. For humans, a census — a complete enumeration of everyone in the defined area — is possible, and India conducts one at fixed intervals. For most other organisms, ecologists count individuals in several small sample plots of known area, average the counts, and scale up. The commonest version of this is the quadrat method, in which a square frame of fixed size is laid down at random points and everything of the target species inside it is counted.
Diagram
Fig. 2.1-1 – two rectangles of unequal area, each containing the same number of dots representing individuals; the smaller rectangle labelled “high density”, the larger labelled “low density”; both labelled with the same N and different A, and the computed D marked under each in individuals per hectare
2.1.2 Factors Causing Population Change
A population is not a fixed store. It is more like a tank with two taps running in and two drains running out. Only four processes can change the number of individuals in a defined area: individuals are born, individuals die, individuals arrive from elsewhere, and individuals leave. Nothing else adds or removes anyone.
Diagram
Fig. 2.1-2 – flow: births → population (drawn as a box labelled N) ← immigration, with two arrows leaving the box: → deaths and → emigration; the two inward arrows labelled “increase” and the two outward arrows labelled “decrease”
2.1.2.1 Natality
Natality, or the birth rate, is the number of new individuals added to a population by reproduction per unit time, usually per year. In human demography it is normally expressed as the crude birth rate, the number of live births per 1000 people in the population per year.
\[\text{Crude birth rate} = \frac{B}{N} \times 1000\]
where \(B\) is the number of live births in the year and \(N\) is the mid-year population. The word “crude” is a technical warning, not a criticism: the figure takes no account of how many women of childbearing age the population actually contains.
Ecologists distinguish two versions of this rate. Potential natality is the maximum number of offspring a population could produce under ideal conditions, with unlimited food and space and no disease or predation. Realised natality is the number actually produced under the conditions that prevail. Realised natality is always lower, and the gap between the two measures how far the environment is holding reproduction back.
Natality varies enormously between species, and this variation is not accidental — it is the core of the life strategies you will meet in section 2.3.
2.1.2.2 Mortality
Mortality, or the death rate, is the number of individuals dying per unit time, usually expressed per 1000 individuals per year. Like natality, it is a property of the population, not of an individual: a person dies, but only a population has a death rate.
Two refinements matter in human populations. The infant mortality rate is the number of deaths of children under one year of age per 1000 live births in a year, and it is among the most sensitive indicators of nutrition, sanitation and health care available anywhere. Life expectancy at birth is the average number of years a newborn would live if current age-specific death rates continued unchanged.
Mortality has a minimum it cannot go below. Even under perfect conditions, individuals eventually die of the physiological changes of old age, and this floor is called physiological mortality. What varies between one place and another is everything above that floor — deaths from infection, malnutrition, injury, predation and lack of care. Almost all of the historical fall in human death rates came from removing causes above the floor, not from extending the floor itself.
2.1.2.3 Immigration
Immigration is the permanent movement of individuals into a population from outside, adding to its numbers. A single defined area gains individuals this way without a single birth taking place. For a city, immigration from rural districts can dominate all other factors; you will meet the push and pull factors behind that movement again when urbanisation is discussed later in the course.
Immigration matters in non-human populations too. Seeds carried by a river into a floodplain, or fish moving upstream into a stretch of river, are immigration in exactly the ecological sense.
2.1.2.4 Emigration
Emigration is the permanent movement of individuals out of a population to another area, reducing its numbers. Emigration and immigration are the same event seen from two ends: the person who emigrates from a village immigrates into a city, and the national total is unchanged while both local totals shift.
This is worth holding on to. Births and deaths change the number of individuals in the world. Migration only moves them about. Which of the four factors dominates therefore depends entirely on how large an area you have defined.
2.1.3 The Relation Between the Four Factors
The four factors are not independent influences pulling in unrelated directions. They combine in a single arithmetic relation, which is the central equation of population dynamics.
\[N_t = N_0 + (B + I) - (D + E)\]
where \(N_t\) is the population size at the end of the period, \(N_0\) is the population size at the start of the period, \(B\) is the number of births during the period, \(I\) the number of immigrants, \(D\) the number of deaths and \(E\) the number of emigrants. All five counts refer to the same defined area and the same stated period, usually one year.
Read the equation as a balance sheet. Births and immigration are credits; deaths and emigration are debits. If the credits exceed the debits the population grows; if the debits exceed the credits it declines; if they are equal the population is stationary even though all four processes are still running at full speed.
That last case is worth pausing on. A stationary population is not a still one. A pond can hold the same number of fish for years while thousands are hatching and thousands are dying, and a country’s population can hold steady while millions are born, die, arrive and leave. Stability at the level of the total conceals enormous turnover underneath.
Working the equation
A small town has 24 000 residents on 1 January. During the year there are 520 births and 310 deaths, 180 people move in permanently and 240 move away.
\[N_t = 24\,000 + (520 + 180) - (310 + 240)\] \[N_t = 24\,000 + 700 - 550 = 24\,150\]
The town has grown by 150 people. Notice what the arithmetic reveals that a single headline figure would not: natural increase — births minus deaths — added 210 people, while net migration took 60 away. The town is growing despite losing people to migration, not because of it. A planner who knew only the final figure of 24 150 would miss that entirely, and might build for the wrong future.
2.1.4 Rate of Change of a Population
An increase of 150 people means one thing in a town of 24 000 and something completely different in a village of 300. To compare populations of different sizes, the change is expressed as a rate — relative to the size of the population producing it.
The growth rate of a population over a period is the change in numbers divided by the starting size, usually written as a percentage per year:
\[\text{Growth rate (\% per year)} = \frac{N_t - N_0}{N_0} \times 100\]
For the town above:
\[\frac{24\,150 - 24\,000}{24\,000} \times 100 = \frac{150}{24\,000} \times 100 = 0.625\% \text{ per year}\]
Under a quarter of one per cent per year sounds negligible. It is not. A quantity growing at a constant percentage adds a larger absolute number every year, because each year’s increase is calculated on a base that already includes last year’s. This is the property that makes population growth so easily misjudged, and it is the reason the exponential curve in section 2.5 climbs the way it does.
Natural increase and net migration
Two components of the growth rate are reported separately because they behave differently and are influenced by different policies.
The rate of natural increase is the birth rate minus the death rate, and it reflects the biology and health conditions of the population itself. Net migration is immigration minus emigration, and it reflects economic opportunity, conflict and policy elsewhere. A district can have a high rate of natural increase and a falling total population, if enough young adults are leaving for work.
This distinction also explains a puzzle in the news. When a country’s growth rate falls while its total population still rises by record numbers, both statements are true: the percentage has come down, but it is being applied to a much larger base than before. The rate is slowing; the absolute additions have not yet caught up.
2.2 Age Structure and Population Pyramids
Two districts have exactly the same population and the same density. In one, nearly half the people are under fifteen. In the other, nearly a third are over sixty. Their futures are not remotely alike — in schools needed, in workers available, in hospital beds, in the number of births that will occur over the next twenty years even if nobody’s family size changes at all.
The total tells you almost nothing about that. The composition tells you almost everything.
2.2.1 Age Structure and Its Significance
Age structure is the proportion of individuals in each age group in a population, usually expressed as a percentage of the total. It is a snapshot of composition rather than of size, and it is read alongside the size, never instead of it.
For studying growth, ecologists compress the age groups into three functional classes defined by reproduction rather than by convention:
- the pre-reproductive age group — individuals too young to reproduce;
- the reproductive age group — individuals currently capable of reproducing;
- the post-reproductive age group — individuals past reproductive age.
The ratio between these three classes determines the current reproductive status of the population, and this is what makes age structure predictive rather than merely descriptive.
Why the ratio predicts growth
Consider a population in which the pre-reproductive class is very large. Every one of those individuals is a future member of the reproductive class. As they mature, the number of potential parents rises, and the number of births rises with it even if each parent has fewer children than their own parents did. Growth is, in effect, already loaded into the age structure.
Now reverse it. If the post-reproductive class is large and the pre-reproductive class is small, the number of potential parents will shrink as time passes, and deaths will begin to outnumber births. The population is set to decline, and no immediate change in behaviour can prevent it for many years.
This built-in tendency has a name. Population momentum is the continued growth of a population for decades after fertility has fallen, caused by a large existing pre-reproductive group moving into its childbearing years. It is the single most common source of confusion in public discussion of population.
2.2.2 Reading a Population Pyramid
Age structure is almost always presented as a diagram, because the shape carries the information faster than a table of percentages can.
A population pyramid, also called an age pyramid or age–sex pyramid, is a pair of horizontal bar graphs drawn back to back. Age groups run up the vertical axis, from the youngest at the bottom to the oldest at the top, usually in five-year classes. The horizontal axis shows the number or percentage of people in each age group, with males plotted to the left of the centre line and females to the right.
Diagram
Fig. 2.2-1 – anatomy of a population pyramid; vertical axis age in five-year classes from 0–4 at the base to 80+ at the top; horizontal axis percentage of total population, males extending left of the centre line and females right; the three functional bands marked and labelled at the side — pre-reproductive, reproductive, post-reproductive
Three things are read off the diagram at once. The width of the base shows how large the youngest group is relative to the rest. The slope of the sides shows how quickly numbers fall away with age, which reflects mortality. And the left–right asymmetry at any level shows the sex ratio — the number of females per 1000 males — in that age group.
Irregularities in the outline are informative too. A sudden narrow band part way up marks a cohort thinned by famine, epidemic or a period of very low birth rates; a bulge marks a period when births were unusually high.
2.2.3 The Three Shapes of Pyramid
Real pyramids take countless outlines, but almost all of them approximate one of three general shapes. Each shape corresponds to a different relationship between the three functional age classes, and therefore to a different future.
Diagram
Fig. 2.2-2 – three population pyramids drawn side by side to the same scale; each with age in five-year classes on the vertical axis and percentage of population on the horizontal axis, males left and females right; (a) expanding — broad base tapering steeply to a narrow apex, triangular; (b) constant — nearly vertical sides up to about age 55 then tapering, bell-shaped; (c) declining — narrow base, widest at the middle age classes, urn-shaped; each labelled with its name and the predicted direction of change
The expanding pyramid
The expanding pyramid is broad at the base and tapers steeply, giving it a triangular outline. The pre-reproductive group is far larger than the reproductive group, which is in turn larger than the post-reproductive group. Both birth rates and death rates tend to be high, so few individuals survive into the upper bands.
Such a population is set to grow, and to grow for a long time, because of the momentum described above. Expanding pyramids are characteristic of populations in the earlier stages of the demographic transition you will meet in section 2.4.
The constant pyramid
The constant, or stationary, pyramid has nearly vertical sides through the younger and middle age classes, tapering only near the top, giving it a bell-shaped outline. The pre-reproductive and reproductive groups are roughly equal in size, indicating that each generation is being replaced by one of about the same size.
Birth and death rates are both low and roughly in balance. Such a population is close to stable in total size, and its age structure changes little from one decade to the next.
The declining pyramid
The declining, or constrictive, pyramid has a narrow base and is widest somewhere in the middle age classes, giving it an urn-shaped outline. The pre-reproductive group is smaller than the reproductive group above it — each generation is smaller than the one before.
Unless immigration compensates, the total population will fall as the large middle bands age and are replaced by smaller ones. The population also ages rapidly, since the proportion in the upper bands rises year after year.
2.2.4 Interpretation and Implications
A pyramid is read for what it implies, not admired for its shape. Four implications follow directly from the outline.
Dependency
Some people in a population produce goods and services, and some depend on those who do. The dependency ratio compares the two groups. In human populations it is conventionally calculated using age bands as a rough proxy:
\[\text{Dependency ratio} = \frac{P_{0-14} + P_{65+}}{P_{15-64}} \times 100\]
where \(P_{0-14}\) is the number of people below 15 years, \(P_{65+}\) the number aged 65 and above, and \(P_{15-64}\) the number in between. The result is the number of dependants per 100 people of working age.
Take a district of 80 000 people: 26 000 are under 15, 6 000 are 65 or over, and 48 000 are in between.
\[\frac{26\,000 + 6\,000}{48\,000} \times 100 = \frac{32\,000}{48\,000} \times 100 = 66.7\]
There are roughly 67 dependants for every 100 people of working age. The measure is deliberately crude — it counts a 16-year-old in full-time school as a producer and a healthy 66-year-old farmer as a dependant — so it indicates the shape of the burden rather than measuring it exactly.
The composition matters as much as the number. An expanding pyramid produces high dependency made mostly of children, which calls for schools and maternal health services; a declining pyramid produces high dependency made mostly of older people, which calls for pensions and long-term care. The ratio can be identical in the two cases and the policy implications opposite.
Predicting the next thirty years
Because people age at a fixed rate, a pyramid can be read forward. Every band simply moves up one level every five years, thinned a little by mortality. A large band at ages 10–14 today is a large band of new workers in ten years and a large band of new parents in fifteen — which is why age structure is used to project school enrolment, labour supply and future births long before any of them happen.
The working-age bulge
When a large cohort passes through the reproductive and working ages while the groups below and above it are smaller, dependency falls and the share of workers in the population reaches a peak. Economists point out that this period offers an opportunity for higher output and savings, but the gain is conditional rather than automatic — it depends on whether that cohort finds education, health care and employment. Where those are lacking, a large young population brings high unemployment instead. The pyramid describes the opportunity; it does not deliver it.
Sex composition
Reading the two sides against each other shows how the sex ratio varies with age. In most populations the upper bands hold more women than men, because female life expectancy tends to be higher. Marked asymmetry in the younger bands is treated as a signal worth investigating, since it cannot be explained by differences in longevity.
2.3 Survivorship Curves and Life Strategies
A female fish may shed lakhs of eggs into a river in a single season. An elephant produces one calf roughly every four or five years and guards it for a decade. Both species persist. Neither strategy is a mistake, and neither is an accident.
Section 2.1 treated natality and mortality as rates for the whole population. This section splits them by age, because the age at which individuals die is as informative as how many die — and because that pattern, once known, explains why reproduction differs so widely between species.
2.3.1 Cohorts and the Survivorship Curve
To study mortality by age you must follow a fixed group of individuals through time rather than take a snapshot of everyone alive today.
A cohort is a group of individuals of a population, all born at about the same time, that is followed together through their lives. Ecologists start with a cohort of known size — conventionally 1000 — and record how many are still alive at each successive age.
Plotting that record gives a survivorship curve: a graph of the number or percentage of individuals of a cohort surviving, against age. Age on the horizontal axis is usually expressed in relative terms, as a percentage of the maximum lifespan of the species, so that a mayfly and a tortoise can be plotted on the same axis and compared.
Read the three columns across, not down. Species A loses almost nobody until the second half of life and then collapses. Species B loses roughly the same proportion at every stage — from 1000 to 700, then 700 to about 420, and so on. Species C loses more than nine-tenths of the cohort before a tenth of the lifespan has passed, after which the few survivors are remarkably persistent. Those three patterns are not particular to these species; they are the three general shapes into which almost all survivorship curves fall.
2.3.2 The Three General Shapes
All three curves are drawn on one plot so that their shapes can be compared directly, as in Fig. 2.3-1.
Diagram
Fig. 2.3-1 – three survivorship curves on a single plot; x-axis age as percentage of maximum lifespan, 0 to 100; y-axis survivors per 1000 of the cohort on a logarithmic scale from 1000 down to 1; Type I drawn convex, staying high and dropping steeply near the right; Type II drawn as a straight descending line; Type III drawn concave, falling almost vertically at the left then flattening; each curve labelled with its type and one example group
Type I — the convex curve
The Type I curve stays high across early and middle life and then falls steeply in old age. Age-specific survival is high while individuals are young and mature, so most of the cohort reaches the later age classes and dies within a relatively narrow band of ages.
This pattern is typical of large-bodied mammals, including humans. The reasons are consistent: few offspring, prolonged parental care, large body size at birth, and comparatively few sources of juvenile mortality. Mortality is concentrated at the end because it is largely physiological rather than accidental.
Type II — the linear curve
The Type II curve is a straight descending line on a logarithmic axis, which means that the rate of mortality is roughly constant at every age. An individual’s chance of dying in the coming year is much the same whether it is young, middle-aged or old.
This intermediate pattern is reported for many songbirds, some reptiles and small mammals, and for organisms such as Hydra. Where mortality comes mainly from predation, accident and disease rather than from ageing, age confers little protection and little penalty.
Type III — the concave curve
The Type III curve drops almost vertically at the left and then flattens. The great majority of the cohort dies very young; the small proportion that survives the early phase goes on to live a long time with a low death rate.
This is the pattern of organisms that release enormous numbers of small offspring with no parental care — many fish and marine invertebrates, and plants that scatter very large numbers of seeds. Predation, starvation and simple failure to find a suitable place to settle remove most of the young. Those that get past that filter face far fewer threats.
2.3.3 r Strategies and K Strategies
The three survivorship shapes are not random. They correspond to two broadly different solutions to the same problem — how to convert limited resources into surviving descendants — and ecologists call these solutions life strategies.
The naming comes from two symbols already met in population growth. The letter \(r\) stands for the intrinsic rate of natural increase, the rate at which a population would grow per individual per unit time in the absence of any limitation:
\[r = b - d\]
where \(b\) is the birth rate and \(d\) the death rate, both expressed per individual per unit time. The letter \(K\) stands for the carrying capacity of the environment — the ceiling that its resources set on numbers, which section 2.4 takes up in detail.
The r strategy
An r strategy is a life strategy that maximises the rate of increase: produce many offspring quickly, cheaply and early, and accept that most will die. Species following it are typically small-bodied, mature early, have short generation times, breed once or a few times, invest almost nothing in each offspring after birth, and disperse their young widely.
Such species do well where conditions are unstable or unpredictable — a flooded plain, a freshly ploughed field, a gap opened in a forest by a fallen tree, a puddle that will dry in three weeks. In habitats that are frequently disturbed, the population is rarely anywhere near the resource ceiling, and the winner is whoever colonises and reproduces fastest. Their populations therefore fluctuate sharply, often crashing and recovering. Insects, many weeds and most microorganisms follow this route, and their survivorship is Type III.
The K strategy
A K strategy is a life strategy that maximises competitive ability near the carrying capacity: produce few offspring, invest heavily in each, and survive. Species following it are typically large-bodied, mature late, live long, breed repeatedly, and care for their young over extended periods.
Such species do well where conditions are stable and the habitat is already occupied close to its ceiling — a mature forest, a long-established grassland, a coral reef. There, extra offspring are of little use if they cannot compete for a place that is already taken, and the winner is whoever produces well-provisioned young that survive. Their populations tend to stay near the ceiling and to fluctuate comparatively little. Elephants, whales, primates and large trees are usually placed here, and their survivorship is Type I.
2.3.4 Differences Between the Two Strategies
The contrast is best set out trait by trait, because it is the whole cluster of traits, not any single one, that defines a strategy.
The strategies are ends of a spectrum
Most species sit somewhere between the two. Trees are the standard awkward case: they are long-lived, slow to mature and strongly competitive, which is K-like, yet they scatter thousands of seeds with no care whatever, which is r-like. Sea turtles are similar — large, long-lived and slow to mature, but laying very many unattended eggs.
This is not a flaw in the scheme so much as a warning about how to use it. The value of r and K selection is that it links a species’ reproduction to the stability of its habitat and predicts which traits travel together. It was never intended as a filing system into which every organism must fit.
The distinction also has a practical edge in conservation, which section 2.8 develops. A K-strategist that is heavily hunted recovers slowly, because it cannot replace losses quickly; an r-strategist pest, for the same reason, rebounds within a season of being controlled. Knowing which kind of species you are dealing with tells you how fast a population can be expected to come back — and how easily it can be lost.
2.4 Human Populations
Everything in sections 2.1 to 2.3 applies to human beings as much as to fish or grass. But one thing is different. Human populations can change the ceiling that their environment sets — by irrigating a dry field, by importing grain from another continent, by curing a disease that used to kill a fifth of all children. No other species negotiates with its own limits in this way.
That capacity is at the centre of the oldest argument in population studies, and of the model that eventually replaced it.
2.4.1 Carrying Capacity
Any environment can support only so many individuals of a species. Food, water, space, nesting or rooting sites and the ability of the environment to absorb waste all run out at some point, and beyond that point additional individuals cannot be sustained.
The carrying capacity, written \(K\), is the maximum population size of a species that a given environment can support indefinitely, under a stated set of conditions. Two parts of that definition do the work. It is a maximum that can be sustained without the resource base being run down, and it is tied to stated conditions — change the technology, the climate or the resource supply, and the value of \(K\) changes with it.
Three broad capacities of the environment together set the value of \(K\): its productive capacity, which supplies food and materials; its protective capacity, the air, water and climate systems that keep conditions liveable; and its assimilative capacity, its ability to absorb and break down wastes. A population can press against any one of these first. A city may have grain enough and still be limited by what its river can absorb.
2.4.2 The Malthusian View
In 1798 Thomas Robert Malthus, an English clergyman and economist, published an essay on population that has shaped the argument ever since. He was writing at a time when many of his contemporaries expected human society to improve without limit, and his purpose was to show why he thought it could not.
His argument rests on a contrast between two rates of growth. Population, if unchecked, grows geometrically — doubling repeatedly, in the sequence 1, 2, 4, 8, 16. The means of subsistence, chiefly food, can at best be made to grow arithmetically — 1, 2, 3, 4, 5 — because land is finite and yields improve only slowly. Two quantities growing at such different rates must eventually diverge, however comfortable the starting position.
Diagram
Fig. 2.4-1 – two curves on one plot; x-axis time in equal intervals, y-axis quantity as an index; one curve rising geometrically and labelled “population, unchecked”; one straight line rising arithmetically and labelled “food supply”; the crossing point marked and labelled “subsistence limit”
When the two lines cross, the population has outrun its means of subsistence. Malthus called this condition over-population — a population larger than the available resources can feed — and argued that it is not a permanent state, because forces come into play that bring numbers back down.
He grouped these forces into two kinds. Preventive checks reduce the birth rate: postponement of marriage and restraint in family size, which he considered the humane route. Positive checks raise the death rate: famine, epidemic disease and war. Malthus regarded famine as the last and most terrible of these, acting when all else has failed. The population crash that follows such checks is what later writers called the Malthusian catastrophe.
2.4.3 Questioning Malthus
Malthus has been questioned continuously for two centuries, and the objections fall into four groups. They are worth separating, because they are not equally strong and they do not all attack the same part of his argument.
Food output has grown faster than he allowed
The clearest objection is empirical. World population has multiplied many times over since 1798, yet food output has grown faster still over most of that period, and the long-run price of staple grains has not risen the way Malthus’s model implies. The rise in yields from improved crop varieties, fertilisers and irrigation — which Chapter 5 examines in detail — is the outstanding example.
Population pressure can itself drive innovation
The economist Ester Boserup argued that Malthus had the causal arrow backwards. In his model, food supply limits population. In hers, rising population pressure is what forces cultivators to adopt more intensive methods — shorter fallows, manuring, irrigation, double cropping — which they had no reason to adopt while land was plentiful, since the new methods demand far more labour. On this view, more people can mean more output rather than less food per head.
The scarcity may be one of distribution, not of supply
A third line of objection holds that famine in the modern world has rarely been a simple case of there being too little food in existence. Studies of major famines have found that food often remained available in the affected region while particular groups — landless labourers, artisans who lost their customers, families whose wages collapsed — could no longer command it. What failed was their ability to obtain food, not the total supply.
This argument does not deny that resources are finite. It denies that the number of mouths is the operative variable in most modern hunger, and it shifts attention to wages, entitlements, transport and relief policy. Friedrich Engels made an early version of the same point, arguing that a large population is also a large productive force, and that Malthus had compared numbers with output while ignoring how output is distributed.
Human beings respond, and birth rates fall
Finally, and most decisively, populations have not behaved as the model expects. Malthus assumed that people would go on reproducing near the biological maximum unless checked by disaster or moral restraint. In practice, birth rates have fallen substantially in country after country as incomes, health and education have risen — voluntarily, and without any catastrophe forcing the change. That regular pattern is the demographic transition, described in section 2.4.5.
What survives of the argument
None of this disposes of Malthus entirely, and it would be a mistake to teach that it does. His central proposition — that a finite environment cannot support unlimited growth in anything — remains valid, and is the basis of the concept of carrying capacity used throughout ecology. What has not held is his specific claim that food production cannot keep pace, and his assumption that fertility responds only to disaster.
Ecological economists make a modified version of his case today. They point out that yields have been raised partly by drawing down stocks that are not being replaced — groundwater, soil fertility, fossil energy — so that the ceiling has been lifted on borrowed terms rather than genuinely removed. Whether that constitutes a postponement of the Malthusian limit or an escape from it is a live disagreement, and the evidence at present supports neither side conclusively.
2.4.4 Population Growth versus Disparate Consumption
There is a second reason why counting people alone is a poor guide to environmental pressure. What loads an environment is not the number of individuals but the quantity of resources they use and the quantity of waste they return — and this varies enormously from person to person.
The relationship can be set out as an identity:
\[\text{Total impact} = \text{Population} \times \text{Consumption per person} \times \text{Impact per unit consumed}\]
Every term matters. A population can grow while total impact falls, if consumption per person or the impact of each unit consumed falls faster. A population can be stable while total impact rises sharply, if consumption per person rises.
An arithmetic illustration
Suppose Country X has 200 million people who each consume energy equivalent to 0.5 tonnes of oil a year, and Country Y has 40 million people who each consume 5 tonnes.
\[\text{Country X} = 200 \times 10^6 \times 0.5 = 100 \times 10^6 \text{ tonnes of oil equivalent per year}\] \[\text{Country Y} = 40 \times 10^6 \times 5 = 200 \times 10^6 \text{ tonnes of oil equivalent per year}\]
Country Y has one-fifth of the people and twice the total consumption. If Country X’s population grew by 10 per cent, its total would rise to 110 million tonnes — still well below Country Y’s. A discussion of the two countries that mentioned only their population sizes would point in exactly the wrong direction.
This is what is meant by disparate consumption: a large and persistent difference in resource use per person between groups. It exists amongst nations, where average consumption per person differs many times over between the richest and poorest countries. It also exists within nations, and often more sharply — a household with two cars, air conditioning and frequent flights and a household drawing water from a public tap live in the same national statistic and place utterly different loads on the environment.
Neither factor cancels the other. Rapid population growth genuinely strains schools, water supply, housing and cropland in the places where it occurs, and a country in that position has good reasons of its own to slow it. The point is only that the number of people and the resources consumed are separate quantities, and that environmental pressure is a product of both.
2.4.5 Demographic Transition
When demographers plotted birth and death rates for industrialised countries over the previous two centuries, they found something the Malthusian model did not predict: a regular sequence, repeated in country after country, in which both rates fell from high levels to low ones, with a period of rapid population growth in between.
The demographic transition is the shift of a population from high birth rates and high death rates to low birth rates and low death rates, accompanied by a temporary phase of rapid growth while the death rate has fallen and the birth rate has not yet followed. The description was set out by Warren Thompson in 1929 and developed by later demographers into the staged model used today.
Diagram
Fig. 2.4-2 – the demographic transition model; x-axis time divided into four labelled stages, y-axis birth rate and death rate in births or deaths per 1000 people per year, scale 0 to 40; birth rate curve and death rate curve drawn separately, both starting high, the death rate falling first in Stage 2 and the birth rate following in Stage 3, both low and close together in Stage 4; the gap between the two curves shaded and labelled “natural increase”; a separate line on a second y-axis showing total population size rising through Stages 2 and 3 and levelling in Stage 4
The gap between the two curves is the whole story. Growth is not caused by a rise in the birth rate — the birth rate never rises in this model. It is caused by the death rate falling first and the birth rate taking a generation or more to catch up.
Stage 1 — high birth rate, high death rate
Both rates are high and close together, so the population grows very slowly or not at all, and its size fluctuates with harvests and epidemics. Life expectancy is low and infant mortality is high. This was the condition of most human societies for most of history.
Stage 2 — death rate falls
The death rate drops sharply while the birth rate remains high. The fall is driven by better and more reliable food supply, clean water and sanitation, and later by immunisation and medical care. Infant and child mortality fall furthest, so a much larger proportion of children survive to adulthood. Population grows rapidly, and the age structure becomes strongly expanding.
Stage 3 — birth rate falls
The birth rate now declines, for the reasons set out in section 2.4.6. Growth continues but slows, because the gap between the two rates is narrowing. The age pyramid begins to lose its very broad base.
Stage 4 — low birth rate, low death rate
Both rates are low and close together, and the population is large but roughly stable. The age structure approaches the constant shape, with a substantial proportion of older people.
A proposed fifth stage
Some demographers add a fifth stage in which the birth rate falls below the death rate and the population declines and ages, as has occurred in several countries. Whether this is a genuine further stage of the same process, or a separate phenomenon, is debated — some populations in this condition have since seen fertility stabilise or rise slightly. The original model has four stages, and the fifth should be described as a proposed extension rather than an established part of it.
2.4.6 Factors Influencing Demographic Transition
The model describes two separate falls, and they have different causes. Keeping them apart is the key to understanding the whole transition.
What brings the death rate down
The fall in mortality in Stage 2 owes more to plain public provision than to advanced medicine, and the order in which the causes acted is instructive.
A more reliable food supply comes first — better yields, better storage and better transport, which together remove the local famines that had periodically cut populations back. Clean drinking water and the safe disposal of sewage come next, and their effect on infant and child mortality is very large, because they remove the water-borne infections that killed the youngest. Immunisation, antibiotics and organised health services follow, and lower mortality further at every age.
Two features of this fall shape everything after it. It happens quickly, often within a few decades. And it is concentrated among children, so it produces a surge of survivors who will reach reproductive age together.
What brings the birth rate down
The fall in fertility in Stage 3 is slower, because it depends on decisions made in millions of households rather than on services delivered to them. Several changes act together.
Child survival is the first. When most children survive, families no longer need to bear many children to be reasonably sure that some will reach adulthood. This adjustment is not immediate — it takes time for expectations to follow the facts — and that lag is precisely why Stage 2 lasts as long as it does.
Education, and especially the education of girls, is the most consistently observed influence. Educated women tend to marry later, to have their first child later, to space births further apart and to have fewer children overall, and these effects appear across very different societies and income levels.
Urbanisation changes the economics of a family. On a farm, children can contribute labour from a young age; in a city, they cost more to raise, are in school for longer, and contribute later. The child shifts from being an additional pair of hands to being a long investment.
Employment outside the home, particularly for women, raises the cost of an additional birth in time and earnings, and tends to postpone childbearing. A rising age at marriage acts in the same direction, by shortening the years available for childbearing.
Old-age security matters too. Where there is no pension or insurance, children are the only support in old age, and having several is a rational precaution. Where such support exists, that reason weakens.
Finally, the availability and acceptance of family planning services allow families to act on a decision they have already made. This is the point that is most often misread. Services do not, on their own, create the wish for smaller families; they make it achievable. Where the other conditions have changed, fertility falls even where services are limited, and where they have not, services alone achieve much less.
Values shift alongside all of these, and slowly. Attitudes to age at marriage, to the education of daughters and to the size of a desirable family change over a generation or two rather than in a year, which is one reason the timing of Stage 3 has differed so widely between countries with otherwise similar incomes.
2.5 Population Regulation
Left entirely to itself, with unlimited food and space and no enemies, a population would multiply without limit. No population does. Something always intervenes, and what intervenes is the subject of this section.
Two curves describe the two possibilities. One shows growth as it would proceed if nothing restrained it. The other shows growth as it actually proceeds when the habitat begins to push back.
2.5.1 Growth Without Regulation: The Exponential Curve
Exponential growth occurs when a population increases by a constant proportion of itself in each unit of time. Ten per cent added to ten individuals is one individual; ten per cent added to ten thousand is a thousand. The percentage stays the same and the absolute additions get larger every period, so the curve rises ever more steeply.
Plotted against time, this produces the J-shaped curve.
\[\frac{dN}{dt} = rN\]
Here \(N\) is the population size in number of individuals, \(t\) is time in years or other stated units, \(\frac{dN}{dt}\) is the rate of change of population size in individuals per unit time, and \(r\) is the intrinsic rate of natural increase per individual per unit time, met in section 2.3. Read the equation in words: the speed at which the population grows is proportional to how many individuals are already there. Nothing in it refers to resources, and that is precisely the point.
Diagram
Fig. 2.5-1 – exponential or J-shaped growth curve; x-axis time (years), y-axis population size N (number of individuals); curve beginning with a short flat lag phase, then rising with increasing steepness; the lag phase and the exponential (log) phase labelled; a dashed horizontal line marked K for carrying capacity crossed by the curve, with the abrupt fall beyond it labelled “population crash”
The two phases, and the crash
The J-curve has a short lag phase at the start, during which individuals are establishing themselves in the habitat and numbers change little. It is followed by the exponential or log phase, in which numbers rise rapidly.
What the J-curve does not have is a top. Because nothing in the model limits growth, the population continues past the carrying capacity of its habitat. Once it does, resources are exhausted, waste accumulates, and mortality rises abruptly — a sudden and severe fall in numbers called a population crash. Populations of insects, of annual plants and of algae in a nutrient-enriched pond behave in this way, rising through a favourable season and collapsing at its end.
So the J-curve is not a curve of long-term growth at all. It is a curve of growth over the interval during which resources are effectively unlimited, followed by a correction.
2.5.2 Simple Regulation: The Logistic Curve
In most habitats, resources become scarcer as numbers rise. Individuals compete for food, water, space and nesting sites; waste accumulates; disease spreads more easily in a crowded population. The sum of all these restraining influences is called environmental resistance, and it strengthens as the population grows.
Build environmental resistance into the growth equation and you get the logistic growth model, whose graph is the S-shaped, or sigmoid, curve.
\[\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)\]
The symbols are as before, with \(K\) the carrying capacity of the habitat in number of individuals. The added bracket is the whole difference between this model and the last one. It is a braking term. When \(N\) is very small compared with \(K\), the bracket is close to 1 and the population grows almost exponentially. As \(N\) approaches \(K\), the bracket approaches 0, and growth slows to nothing. When \(N\) equals \(K\), growth stops.
Diagram
Fig. 2.5-2 – logistic or S-shaped growth curve; x-axis time (years), y-axis population size N (number of individuals); curve rising slowly, then steeply, then levelling off; carrying capacity K marked as a horizontal dashed asymptote which the curve approaches but does not cross; five phases labelled along the curve in order — lag phase, positive acceleration phase, exponential (log) phase, negative acceleration phase, stationary (plateau) phase
The five phases
The S-curve is conventionally read in five phases.
The lag phase shows little or no growth while individuals adjust to the habitat and begin reproducing. The positive acceleration phase shows growth beginning and speeding up as the number of reproducing individuals rises. The exponential or log phase is the steepest part of the curve, where resources are still plentiful relative to numbers and growth is at its fastest. The negative acceleration phase shows growth slowing as environmental resistance takes hold. The stationary or plateau phase shows the population fluctuating around the carrying capacity, with the birth rate matched by the death rate, so that the growth rate is effectively zero.
Why the curve flattens
The flattening is not a decision made by the population. It is the arithmetical result of the braking term, and a short calculation makes it concrete.
Take a habitat with a carrying capacity \(K\) of 1000 individuals and a species with \(r = 0.1\) per individual per year. Compare the growth rate at three different population sizes.
At \(N = 100\):
\[\frac{dN}{dt} = 0.1 \times 100 \times \left(\frac{1000 - 100}{1000}\right) = 0.1 \times 100 \times 0.9 = 9 \text{ individuals per year}\]
At \(N = 500\):
\[\frac{dN}{dt} = 0.1 \times 500 \times \left(\frac{1000 - 500}{1000}\right) = 0.1 \times 500 \times 0.5 = 25 \text{ individuals per year}\]
At \(N = 900\):
\[\frac{dN}{dt} = 0.1 \times 900 \times \left(\frac{1000 - 900}{1000}\right) = 0.1 \times 900 \times 0.1 = 9 \text{ individuals per year}\]
The population is adding individuals fastest not when it is smallest and not when it is largest, but at about half the carrying capacity. That is the steepest point of the S-curve. Below it, there are too few individuals reproducing; above it, there are plenty of reproducers but too little left to go round. This single result explains the shape of the whole curve, and it is also why populations harvested for food or timber are, in principle, most productive when held at an intermediate size rather than a maximum one.
The logistic curve is the more realistic of the two, but it is still an idealisation. Real populations overshoot the carrying capacity and fall back, fluctuate around it rather than settling on it, and experience a carrying capacity that itself moves with the seasons. The model is used because it identifies the mechanism, not because any population traces it exactly.
2.5.3 Factors Regulating Population Size
Environmental resistance is a summary term. The factors that make it up act in different ways, and the syllabus identifies eight of them.
Before taking them one by one, one distinction is worth having. Some factors bite harder as the population gets denser — competition, predation, disease. These are called density-dependent factors, and they are the ones capable of regulating a population around a level, because their strength rises and falls with numbers. Others act with the same severity whatever the density, such as a cyclone or a frost. These are density-independent factors; they can reduce a population sharply but cannot hold it at a level, since they take no account of how many individuals are present.
Space
Every organism needs physical room — ground to root in, a place to nest, a stretch of river bed to feed on. As numbers rise, the space available per individual falls. Crowding limits reproduction directly, by leaving individuals without a site to breed in, and indirectly, by increasing contact and conflict. Space is one of the clearest density-dependent limits, and one of the least substitutable: food can be imported, room cannot.
Food and water
Food and water are usually the first resources to run short. Competition for them lowers the nutrition available to each individual, which reduces the number of offspring produced and raises mortality among the young and the old. Both the quantity and the quality matter: a habitat can hold enough plant matter to fill an animal but not enough of the right kind to sustain reproduction. Water is more sharply limiting than food in seasonally dry habitats, where the dry-season water supply, not the annual total, sets the ceiling.
Territories
In many birds and mammals, individuals or pairs defend an area against others of the same species for the exclusive use of its food and breeding sites. This behaviour, called territoriality, converts a continuous habitat into a fixed number of holdings. Once every territory is occupied, additional individuals cannot breed regardless of how much food the habitat contains in total; they are pushed into marginal areas where survival is poor. Territoriality therefore sets a ceiling on breeding numbers well before the food supply itself is exhausted.
Predators
Predation removes individuals directly, and it is density-dependent in a particular way: predators concentrate where prey is abundant and disperse or decline when it is scarce. This produces the coupled oscillation familiar from predator–prey studies, in which a rise in the prey population is followed by a rise in the predator population, which then drives the prey down, which in turn drives the predators down. Neither population is steady, but each is kept within limits by the other.
Weather and climate
Temperature, rainfall, humidity, wind and the length and intensity of sunlight determine what an organism can do and when. A late monsoon shortens the breeding season of a pond-breeding species; an unusually cold winter kills insects regardless of their numbers; the duration of sunlight controls photosynthesis in plants and triggers breeding cycles in many animals. Weather acts mainly in a density-independent way, and its effects can be severe, but it operates as a limit on numbers rather than as a regulator of them. Longer-term shifts in climate change the carrying capacity itself.
Parasites and diseases
Parasites live on or within a host and draw resources from it, slowing its growth and reducing its reproduction without necessarily killing it. Pathogens can kill outright. Both spread far more easily in a dense population, because transmission depends on contact, which is exactly why disease is a strongly density-dependent regulator. A population that has grown well beyond its usual density is unusually vulnerable, and epidemics in such populations can cause a crash.
Disasters
Floods, droughts, cyclones, forest fires, landslides and earthquakes can remove a large fraction of a population within days, and take no account whatever of how dense that population was. These are density-independent events. What determines their long-run importance is how quickly the population can rebuild afterwards — which returns to the strategies of section 2.3, since an r strategist can recover from a disaster in a season and a K strategist may need decades.
Self-regulation
Some populations reduce their own rate of increase as density rises, before any external resource actually runs out. Crowding has been observed to alter behaviour and physiology in ways that lower reproduction — increased aggression and stress responses, delayed maturity, reduced litter size, higher rates of dispersal out of the crowded area, and in some species the abandonment of breeding altogether in a bad year. Because these responses depend on the perception of crowding rather than on starvation, they can stabilise a population without the overshoot and crash that resource exhaustion would produce.
2.6 Human Population Control
The demographic transition describes a change that happened. It does not happen automatically, at the same speed, or by itself. Where governments have wanted to influence how quickly a population stabilises, four broad strategies have been used, and they work in very different ways.
The first acts directly on births. The other three act on the conditions under which families decide how many children to have — and the historical record suggests they are, in the long run, the more powerful of the two kinds.
2.6.1 Family Planning as a Strategy
Family planning is the ability of individuals and couples to decide freely how many children to have and when to have them, together with the information and health services that make such decisions possible.
Two words in that definition carry most of its weight. Voluntary distinguishes family planning from population control imposed on people. Spacing points to something often overlooked: family planning is not only about having fewer children but about the interval between births. A syllabus at this level does not require the methods themselves, and they are not described here; what matters is what the strategy is for and what it achieves.
The health case for spacing is well established. Births that follow closely on one another carry higher risk for both the mother and the infant, since the mother has not recovered nutritionally and the older child loses care and feeding at a vulnerable age. Better-spaced births are associated with lower maternal and infant mortality, and lower infant mortality in turn reduces the number of births parents feel they need — one of the self-reinforcing loops of the demographic transition.
Voluntary and coercive approaches
Family planning programmes have historically taken two rather different forms, and the distinction is important because their records differ.
International policy consensus since the 1990s has moved decisively towards the voluntary, rights-based approach, on both ethical and practical grounds: the sharpest and most durable fertility declines have occurred where services were available and where women had reason and freedom to use them, rather than where numerical targets were imposed.
2.6.2 Education
Education influences population growth through several channels at once, which is why its effect is larger than any single mechanism would suggest.
General schooling raises the age at which people marry and begin families, simply because years spent in education are years not spent raising children. A later start to childbearing shortens the reproductive span and lengthens the interval between generations, and both slow the growth of a population even if family sizes were unchanged.
Education also changes what parents want for their children. Where schooling is expected, each child represents a longer and more expensive commitment, and parents commonly respond by choosing fewer children and investing more in each — the same trade-off between number and investment that section 2.3 described in ecological terms.
Awareness of the advantages of a small family, of maternal and child health, and of the services available spreads through schools, health workers and mass media. The evidence here is stronger for sustained exposure than for single campaigns: information changes behaviour most reliably when it arrives repeatedly and from sources people already trust.
None of this is instantaneous. Education acts on a cohort as it grows up, so its effect on the birth rate appears roughly a generation after the investment is made — a lag that makes it politically unattractive and demographically powerful.
2.6.3 Economic Growth
Rising incomes are associated with falling fertility almost everywhere, and three mechanisms connect them.
Higher incomes and better nutrition and health care reduce child mortality, so fewer births are needed to raise a given number of children to adulthood. Employment outside agriculture and outside the home raises the opportunity cost of childbearing, particularly for women. And as savings, pensions and health services develop, grown children become less essential as the only available security in old age, which removes one of the oldest reasons for wanting a large family.
The direction of causation, however, is genuinely disputed. It is not clear whether higher incomes produce smaller families, or whether smaller families — by freeing women for paid work and allowing more investment per child — help produce higher incomes. Most likely both operate together, which makes the relationship difficult to use as a policy lever on its own.
There is also a complication that follows directly from section 2.4.4. Economic growth lowers the birth rate, but it raises consumption per person. A country that stabilises its population by growing richer may end up placing a larger demand on resources than it did with a larger and poorer population. Population control and resource conservation are related goals, but they are not the same goal, and growth does not serve both equally.
2.6.4 The Status of Women
Of all the strategies, the one with the most consistent evidence behind it across widely different societies is the improvement of women’s position — in education, in health, in employment and in the authority to make decisions about their own lives. The syllabus places its emphasis here, and the emphasis is well placed.
The mechanism is a chain rather than a single effect.
Diagram
Fig. 2.6-1 – flow: girls’ education → later age at marriage → later first birth and longer birth intervals → better maternal and child health → higher child survival → smaller desired family size → lower birth rate; a second arrow branching from “girls’ education” to “paid employment outside the home” and rejoining at “smaller desired family size”; a third arrow from “girls’ education” to “greater say in household decisions” also rejoining there
Women’s empowerment — the expansion of women’s ability to make and act on decisions about their own lives — affects population growth at every link in that chain. An educated woman marries later on average, and a later marriage postpones the first birth. She is more likely to be in paid work, which raises the cost of an additional child in time and earnings forgone. She is more likely to use health services for herself and her children, which improves survival. And she has more influence over decisions about the number and spacing of births, which matters because the preferred family size of women is, in survey after survey, smaller than the number of children actually born.
Raising the average age of marriage is, on its own, one of the most effective single measures available. It reduces the number of childbearing years, it reduces the risks associated with very early pregnancy, and it lengthens the gap between generations. Legal minimum ages set a floor, but schooling and employment are what actually raise the average.
None of this is an argument that women are responsible for population growth or for solving it. The decisions involved are household decisions taken under real constraints, and the constraints are what the strategies above are meant to loosen.
Strategies compared
The four strategies are usually presented as alternatives, but they do not work independently. Services without demand go unused; demand without services goes unmet; and both respond to education, child survival and income. The clearest fertility declines on record — in places as different from one another as several Indian states, parts of East and South-East Asia and much of Latin America — have come from combinations of these strategies rather than from any single one, and they have taken decades rather than years.
2.7 Threats to the Ecosystem
So far the chapter has treated populations as things that grow, level off and are regulated. The last two sections turn to what a growing and heavily consuming human population does to the systems it lives inside — and to what is being done about it.
2.7.1 Provisioning and Regulating Functions
An ecosystem does two rather different sorts of useful work, and the distinction between them explains why some kinds of damage are noticed immediately and others are not noticed until far too late.
The provisioning functions of an ecosystem are the tangible goods it supplies: food, fodder, fuelwood, timber, fibre, medicinal plants, fresh water, fish. These are the outputs people harvest, carry away and can put a price on. When a provisioning function fails, the failure is obvious — the well runs dry, the catch shrinks, the firewood has to be bought instead of gathered.
The regulating functions are the processes by which an ecosystem keeps conditions within the range that living things, including people, can tolerate. Forests and soils hold rainwater and release it slowly, so that a river runs through the dry season instead of alternating between flood and nothing. Wetlands filter sediment and break down organic waste. Vegetation holds soil against wind and rain. Insects and birds pollinate crops. Predators keep populations of crop pests within limits. Plants and soils absorb carbon dioxide.
Regulating functions are almost never bought or sold, so they carry no price, appear in no accounts, and are usually noticed only when they stop. A city discovers what a catchment forest was doing for its water supply in the year after the forest is cleared.
The threats that follow damage both kinds of function. What the syllabus asks you to notice is the second kind, because that is where the consequences are largest and the warning is weakest.
2.7.2 Habitat Destruction, Genetic Erosion and Loss of Diversity
Habitat destruction
Habitat destruction is the conversion of a natural habitat to another use, so that the species which lived there can no longer do so. Forest is cleared for cultivation, mining or settlement; grassland is ploughed; wetlands are drained and filled; mangroves are removed for aquaculture ponds and ports; hillsides are quarried. It is, by a wide margin, the largest single cause of species decline worldwide.
Outright clearance is only part of it. Habitat fragmentation — the breaking of a large continuous habitat into smaller isolated patches by roads, canals, railway lines, fields and settlements — leaves the total area reduced but, more importantly, cut into pieces. A small patch has proportionally more edge, and edge conditions are drier, windier and more exposed to fire and disturbance than the interior. Large animals cannot find enough range within one patch, and populations that cannot exchange individuals with neighbouring patches become small, isolated and vulnerable.
Diagram
Fig. 2.7-1 – habitat fragmentation in three panels; panel 1 a single continuous forest block; panel 2 the same block crossed by a road and a canal, leaving four large patches; panel 3 the same area reduced to several small scattered patches surrounded by fields and settlement; total forest area marked under each panel to show the loss, and the increasing proportion of edge shaded in each
Genetic erosion
Genetic erosion is the loss of genetic variety within a species — the disappearance of local varieties, breeds and populations that carry combinations of genes found nowhere else.
It happens in two ways. In the wild, when a population is reduced to a small number of individuals in a few isolated patches, the variety of genes it carries falls with it, and the species becomes less able to withstand disease or a change in conditions. In agriculture, it happens when many locally adapted traditional varieties are replaced by a small number of high-yielding ones. Many thousands of traditional rice varieties were once grown across India, each suited to a particular depth of water, soil, season or taste; the area now under cultivation is dominated by comparatively few varieties.
The consequence is a narrowing of options. The traits that plant breeders need in order to respond to a new pest, a new disease or a drier climate are found in exactly those old varieties and wild relatives, and once they are gone they cannot be reconstructed.
Loss of diversity
Loss of diversity is the reduction in the number and variety of species present in an ecosystem, the outcome to which the previous two threats lead.
Its effect on provisioning functions is direct: fewer species means fewer harvestable kinds of fish, plant, fruit and fodder. Its effect on regulating functions is subtler and more serious. A diverse system has redundancy — several species performing a similar function, so that the failure of one does not stop the process. As diversity falls, that cushion disappears, and the system becomes liable to change abruptly rather than gradually when it is disturbed. A single pollinator species is enough until the year it fails.
2.7.3 Expanding Agriculture and Impounded Water
Expanding agriculture
Feeding a growing population has been achieved partly by raising yields on existing land and partly by expanding agriculture onto land that was previously forest, grassland, wetland or slope. The second route is the one with ecological consequences.
Conversion removes the original habitat outright. It also replaces a mixed community with a monoculture — a single crop species grown over a large area — which supports far fewer other species and is more vulnerable to a pest that specialises on it. Cultivation on steep slopes and on shallow soils removes the vegetation that held them, and the soil follows. Where expansion is accompanied by heavy irrigation of poorly drained land, waterlogging and the accumulation of salts in the root zone can eventually put the land out of use altogether.
The consequences reach beyond the field. Fertiliser and pesticide carried off fields by rain enter streams and groundwater, damaging aquatic life and drinking water sources downstream. Pressure on land adjoining cultivation intensifies as common grazing land is converted.
Impounded water
To impound water is to hold it back behind a barrier — a dam, a barrage or an embankment — creating a reservoir. Impoundment supplies irrigation, drinking water, flood moderation and electricity, and these are substantial gains. It also transforms the river.
Upstream, a stretch of valley is submerged, and with it forest, farmland, and the settlements of the people living there, who must be resettled elsewhere. Downstream, the flow is regulated rather than seasonal, and the silt that the river formerly carried to its floodplain and delta is trapped behind the dam instead — so the floodplain loses the annual renewal of its fertility and the delta loses the sediment that maintained it against the sea. Fish that migrate up and down the river to breed are blocked by the barrier. In the reservoir itself, still water replaces flowing water, which favours an entirely different set of species and, where nutrient-rich runoff enters, can produce dense algal growth that strips the water of oxygen.
Diagram
Fig. 2.7-2 – a river before and after impoundment, in two panels; panel 1 a free-flowing river with a seasonal floodplain, migrating fish moving upstream and silt reaching the delta; panel 2 the same river with a dam, showing the submerged valley and reservoir upstream, silt trapped behind the wall, regulated flow and a shrunken floodplain downstream, and fish blocked at the barrier
2.7.4 Waste from Human Societies and Rising Consumption
Waste from human societies
Every settlement returns material to the environment: sewage, food and packaging waste, industrial effluent, ash, construction rubble, discarded electronics and plastics. Ecosystems have a real capacity to break down organic waste — this is one of their regulating functions — but that capacity is finite and specific.
Two things overwhelm it. The first is volume: organic waste discharged into a river faster than it can be broken down consumes the oxygen dissolved in the water, and fish and other oxygen-dependent life die. The second is the nature of the material. Plastics, heavy metals and many synthetic chemicals are not broken down at all by the organisms available; they persist, accumulate in soils and sediments, and pass up food chains, so that concentrations in the animals at the top can be far higher than in the water or soil they came from. The measurement of these effects is the subject of the next chapter.
Increasing human consumption
Underlying all of the above is the quantity taken out. Increasing human consumption raises demand for cropland, water, timber, fish, minerals, fossil fuels and manufactured goods, and it does so faster than population alone, because consumption per person is rising as well as the number of persons.
The connection to section 2.4.4 should now be plain. The pressure on any ecosystem is the product of how many people draw on it and how much each of them draws, and neither figure can be read off the other. A slow-growing population with fast-rising consumption per head can increase its demands on forests, fisheries and mineral stocks more quickly than a fast-growing population with low consumption.
The consequence, in the language of this section, is that provisioning functions are being drawn on at rates the regulating functions cannot sustain. A fishery harvested faster than the stock can reproduce, a groundwater table pumped faster than the monsoon can recharge it, and a forest cut faster than it regrows are all the same arithmetic — a stock being run down while the flow it depends on is treated as unlimited. Which is where conservation begins.
2.8 Conservation
The previous section described what is being lost. This one describes the attempt to hold on to it — an attempt that is scientific, legal and, unavoidably, political, because a forest is never empty of people and every rule about it takes something from someone.
2.8.1 What Conservation Means and Why It Matters
Conservation is the protection, careful management and wise use of natural resources and of the species and ecosystems that produce them, so that they continue to be available in the future.
Notice that the definition does not say “no use”. Conservation is not the same as leaving a resource untouched, which is preservation — keeping an area or a species entirely free from human interference. Conservation allows use, and asks that the rate of use be one the resource can sustain. Both approaches are practised, often in different zones of the same protected area.
Conservation is carried out in two fundamentally different places, and the whole of this section turns on the distinction.
In situ conservation is the conservation of species within their natural habitat, where the population continues to live, breed and evolve in the surroundings it is adapted to. The Latin phrase in situ means “in the original place”.
Ex situ conservation is the conservation of species outside their natural habitat, in facilities where individuals, seeds or tissues are maintained under human care. Ex situ means “out of place”.
Diagram
Fig. 2.8-1 – two-panel contrast; left panel labelled “in situ” showing a fenced-off forest tract with wild animals, natural vegetation and a boundary marker; right panel labelled “ex situ” showing a zoo enclosure, an aquarium tank, a botanical garden bed and a row of labelled seed jars in a cold store; an arrow from the right panel back to the left labelled “reintroduction”
Why conservation matters
Four arguments are usually given, and they are not equally strong for every species.
The first is that species have provisioning value: food, fibre, timber, fuel and, in particular, medicine, since a substantial proportion of drugs in use derive from compounds first found in wild organisms.
The second is the regulating value described in 2.7.1 — pollination, water regulation, soil retention, waste breakdown — which is lost when the species performing it disappear.
The third is the option value of genetic variety: traits that plant and animal breeders have not yet needed exist only in wild relatives and traditional varieties, and cannot be recovered once lost.
The fourth is ethical, and it does not depend on usefulness at all. On this view a species that has existed for millions of years has a claim to continue, independent of any benefit to people. This is a value position rather than a scientific finding, and you will find it argued in several of the schools of ecological thought.
The two are complements, not rivals. Ex situ collections act as insurance and as a source for reintroduction; in situ protection is where a species can actually go on being a wild species.
2.8.2 In Situ Conservation
India protects habitat through a network of legally designated areas, of which three types are prescribed here.
2.8.2.1 Wildlife Sanctuaries
A wildlife sanctuary is an area declared by law for the protection of wildlife, in which hunting is prohibited but certain regulated human activities may be permitted.
Its objective is the protection of particular species and their habitat — often a sanctuary is declared because of one flagship species, a heronry, a herd or a stretch of grassland. Its distinguishing feature is that limited human presence is allowed under permission: grazing of livestock, collection of certain forest produce, and existing rights of local communities may continue where they are judged compatible with protection. Boundaries can be altered by the state government by notification.
Advantages: because it accommodates existing use, a sanctuary can be declared over a larger area and with less displacement than a stricter category, and it can protect a species where an absolute exclusion of people would be neither fair nor enforceable.
Disadvantages: the same permitted activities can degrade the habitat if they expand; regulation is difficult to enforce over a large area with a small staff; and the weaker legal boundary makes a sanctuary easier to reduce or de-notify than a national park.
2.8.2.2 National Parks
A national park is an area of high ecological value declared by law for the strict protection of the whole ecosystem, in which human activity and private rights are not permitted.
Its objective is not one species but the entire community — flora, fauna, landscape and the processes linking them. Its features follow from that: grazing, cultivation, felling and collection of forest produce are prohibited, no rights are allowed to persist inside, and the boundaries, once fixed, cannot be altered except by resolution of the state legislature. Regulated tourism along fixed routes is normally the only public access.
Advantages: the strongest legal protection available, applied to an entire ecosystem, with a stability of boundary that allows long-term management and research.
Disadvantages: the exclusion of all rights means that people using the area must be relocated or lose access, which is the single largest source of conflict in Indian conservation; a strictly bounded park may be too small for wide-ranging species and, if surrounded by cultivation, becomes an island in the sense of 2.7.2; and animals moving out of it into surrounding farmland create crop and livestock losses that are borne by the neighbours of the park.
2.8.2.3 Biosphere Reserves
A biosphere reserve is a large area of representative ecosystem, internationally recognised under the Man and the Biosphere Programme of the United Nations Educational, Scientific and Cultural Organisation, managed to combine conservation with the sustainable use of resources by the people living in it.
It has three stated objectives — conservation of ecosystems, species and genetic variety; development that is ecologically and culturally sustainable for local communities; and logistic support for research, monitoring and education.
Its defining feature is zonation. The core zone is legally protected, undisturbed, and closed to all extraction; typically it is an existing national park or sanctuary. The buffer zone surrounds the core and permits research, education, restoration and limited, regulated use. The outermost transition zone contains settlements, cropland and other economic activity, and is where sustainable management practices are developed with the people who live there. Several biosphere reserves may contain, and do not replace, sanctuaries and national parks within them. The first in India was the Nilgiri Biosphere Reserve, designated in 1986.
Diagram
Fig. 2.8-2 – biosphere reserve zonation drawn as three concentric zones; innermost labelled core zone (legally protected, no extraction, research by permission only); middle labelled buffer zone (research, education, restoration, regulated use); outer labelled transition zone (settlements, cropping, grazing, sustainable use); a legend listing the permitted activity in each zone
Advantages: it conserves at the scale of a landscape rather than a fenced block; it treats the resident population as part of the system rather than as an obstacle; and the buffer and transition zones absorb some of the pressure that would otherwise fall directly on the core.
Disadvantages: the designation is a management framework rather than a separate legal category, so its strength depends on the laws already applying inside it; coordinating many departments and communities across a very large area is difficult; and where the promised development benefits in the transition zone do not materialise, local support for the arrangement weakens.
2.8.3 Ex Situ Conservation
Where a habitat has already gone, or a population has fallen too low to be secure in the wild, conservation moves indoors.
2.8.3.1 Zoos
A modern zoo maintains wild animals in captivity for conservation, captive breeding, research and public education, as well as display. In India, zoos are recognised and regulated by the Central Zoo Authority, constituted under the national wildlife law, which lays down standards of housing and care and coordinates breeding programmes.
Objectives: to maintain viable captive populations of threatened species, to breed them under managed conditions, to conduct research that is impossible in the field, and to build public familiarity with species most people will never see wild.
Advantages: an assured food supply, veterinary care and protection from predators and poaching; breeding can be planned to keep genetic variety as wide as the captive population allows; and a captive stock is insurance against a catastrophe in the wild.
Disadvantages: only a small number of species can be held, at high and permanent cost; captive populations start from few founders and lose genetic variety over generations; animals may lose the behaviour needed to survive in the wild, so releases often fail; and welfare in confinement is a serious and continuing concern.
2.8.3.2 Aquaria
Aquaria perform the same function for aquatic species, holding fish, corals, molluscs and aquatic plants under controlled conditions of temperature, salinity, light and water chemistry.
Objectives and advantages parallel those of zoos, with the addition that aquaria have proved important for breeding species whose reproduction is difficult to study in open water, and for holding stocks of ornamental and food fish whose wild populations are heavily exploited.
Disadvantages: maintaining water quality is technically demanding and expensive, many marine species will not breed in captivity at all, and a tank cannot reproduce the currents, depth and community relationships of a reef or an estuary.
2.8.3.3 Plant Collections
Plants are easier to conserve outside their habitat than animals, because seeds and tissues can be stored. Plant collections for conservation take four main forms: botanical gardens holding living plants; seed banks or gene banks holding seed under low temperature and low humidity; herbaria holding preserved specimens for reference; and tissue culture and cryopreservation for species whose seed cannot be dried and stored. India’s national collection of crop genetic resources is held by the National Bureau of Plant Genetic Resources.
Objectives: to conserve genetic variety, particularly the traditional varieties and wild relatives whose loss was described in 2.7.2, and to supply material for research, breeding and restoration.
Advantages: enormous numbers of varieties can be stored in a small space at modest cost; the material remains viable for long periods; and it is directly usable by plant breeders responding to a new pest or a changed climate.
Disadvantages: stored seed does not evolve, so it gradually falls behind the pests and conditions of the outside world; it must be periodically grown out and replaced, which is laborious and easily neglected; some species produce seed that cannot survive drying and freezing at all; and a power failure or a funding lapse can destroy a collection that took decades to assemble.
2.8.4 The Critical State of Indian Forests
India’s forests are under simultaneous pressure from a large rural population that depends on them directly, an expanding area of cultivation and settlement, and a growing demand for timber, minerals and infrastructure. Before that pressure can be discussed, it is worth knowing what the published numbers actually measure.
The Forest Survey of India assesses the country’s forest cover at fixed intervals using satellite imagery, and reports it in the India State of Forest Report. Forest cover in these assessments means all land above a minimum area with a tree canopy density above a stated threshold, irrespective of ownership, legal status or species. That definition is doing something important: it counts a plantation of a single exotic species, a mango orchard and an old-growth natural forest alike, provided the canopy is dense enough.
The consequence is that the national forest cover figure can hold steady or rise while natural forest is being replaced by plantation. A student reading such a figure should therefore ask three questions: what is being counted, at what canopy density, and how much of the change is dense forest as against open forest and plantation. The stated national policy goal — set out in the National Forest Policy of 1988 — is to bring about one-third of the country’s geographical area under forest and tree cover, and about two-thirds in the hills, in order to protect soil and water regimes.
The forests that remain are unevenly distributed and unevenly stocked. Much of what is recorded is open forest of low density; large tracts are degraded by heavy grazing, repeated lopping for fodder and fuelwood, and fire; and the areas richest in species are concentrated in a few regions, which is precisely where competing demands for mining and infrastructure are also concentrated.
2.8.5 Conflicts in Managing and Conserving Forests
Almost every difficulty in Indian forest conservation arises from a single fact: the forest is not empty. Several million people live inside or at the edge of forest areas and draw fuel, fodder, fruit, medicine, building material and grazing from them, and many are Scheduled Tribes whose settlements long predate the legal category of “forest land”. Conservation rules, whoever makes them, land on those people first.
People living in and around forests, and their rights
Under the forest administration inherited from the colonial period, large areas were declared state forest, and the customary use of those areas by the people living in them was reclassified as a concession granted by the department rather than a right. Communities that had used a forest for generations found themselves without recorded title, and were liable to be treated as encroachers on land they had never left.
This is the situation the Forest Rights Act of 2006 was passed to address.
Two positions are argued seriously and neither can be dismissed. One holds that secure recognised rights give communities a durable interest in protecting a forest they can no longer be evicted from, and that recorded community management has in many places reduced illegal felling. The other holds that where population pressure and market demand are high, recognised rights can accelerate conversion of forest to cultivation, and that core habitat of wide-ranging species needs areas free of resident use. The evidence supports both in different places, which is why the question is settled case by case rather than in principle.
Tourism
Tourism brings revenue and employment to areas with few other sources of income, and gives a protected area a visible constituency. It also brings vehicles, noise, waste, water demand and construction to the edge of habitat, concentrates visitors along the few routes where animals are reliably seen, and can push land prices beyond the reach of local residents. Whether it helps or harms depends almost entirely on the volume permitted and where the revenue goes.
Poaching
Poaching is the illegal killing or capture of wild animals. Its driver is demand — for skins, bones, horn, ivory, meat and live animals — and the price a rare species commands rises as it becomes rarer, which is the perverse feature of the trade. Poaching is capable of removing a population from a well-protected area quickly and quietly, as section 2.8.9 records.
Roads and development projects
Roads, transmission lines, canals and railway lines cut through forest do more damage than their area suggests. They fragment habitat in the sense of 2.7.2, they kill animals directly, and above all they open interior forest to access — for felling, grazing, hunting and settlement — that was previously impractical. Mining removes habitat outright and leaves overburden, dust and altered drainage behind.
Dams
Impoundment, described in 2.7.3, submerges valley forest, which is often the most productive and species-rich part of a forest tract, and displaces the people living in it. The compensatory plantation raised elsewhere in exchange rarely reproduces what was submerged, since a mature mixed forest cannot be recreated by planting seedlings on a different site.
Over-exploitation of forest resources
Over-exploitation is direct when the resource itself is removed faster than it regenerates — felling beyond the growth increment, lopping trees for fodder until they cannot recover, collecting fruit or seed so completely that no regeneration occurs, or grazing at densities that prevent seedlings from establishing.
It is indirect when the damage is a side effect of another activity: pollution from an upstream industry, fire lit to promote grass and escaping into the forest, an invasive species introduced with plantation stock, or the drying of a stream because the catchment above it was cleared. Indirect over-exploitation is harder to attribute and therefore harder to regulate.
2.8.6 The Role of the Forest Department and NGOs
Forests in India are managed by state forest departments, working under national forest and wildlife legislation. Their functions are wide: demarcating and protecting forest land; preparing and implementing working plans that determine what may be felled where and when; raising nurseries and plantations; controlling fire, grazing and encroachment; managing protected areas and wildlife; regulating the movement and sale of forest produce; and, increasingly, working with village institutions under joint management arrangements.
The department’s difficulties are structural rather than personal. It is expected to be a protector, a producer of revenue, a development agency and a regulator of the same people it is meant to work with — four roles that pull in different directions. Staff strength on the ground is thin relative to the area to be patrolled, and much of the frontline work is done by people with limited equipment in difficult terrain.
Non-governmental organisations occupy the space around this. Their contributions fall into recognisable types: field research and long-term monitoring, which supply the data that management decisions need; legal action and public advocacy; work with communities on nurseries, plantation, grazing management and alternative fuel; support for the recognition of forest rights; and independent scrutiny of official figures and of project clearances. Their strengths are flexibility, technical depth in a narrow area, and a willingness to publish inconvenient findings. Their limitations are equally clear: they have no statutory authority, their funding is short-term, their presence is concentrated where they happen to work, and coordination between many small organisations is weak. The conservation organisations named in the syllabus are examined in the chapter on schools of ecological thought.
2.8.7 Management Measures
Four approaches to managing forests are prescribed, and they differ in who manages, for whom, and to what end.
2.8.7.1 Scientific Forestry
Scientific forestry is the management of a forest as a crop, using systematic inventory, working plans, fixed rotations and a calculated annual yield, so that the volume removed does not exceed the volume grown.
Its scope is production forestry — timber, pulpwood and fuelwood raised on a planned cycle. The method entered India through the colonial forest administration of the nineteenth century, which introduced working plans, demarcation and sustained-yield felling on a large scale.
Advantages: it prevents the simple exhaustion of a forest by regulating removal against growth; it produces reliable timber supplies and revenue; and it introduced systematic measurement, mapping and record-keeping that management of any kind depends on.
Limitations: the method treats a forest primarily as a stock of timber volume. Species of little commercial value are removed or discouraged, which converts mixed forest towards even-aged stands of a few chosen species and reduces both diversity and the non-timber produce that local users depend on. A yield calculated as sustainable in cubic metres of wood may be nowhere near sustainable in terms of habitat, soil or the fodder and fruit that were never counted.
2.8.7.3 Joint Forestry Management
Joint Forestry Management (JFM) is an arrangement in which a village institution and the state forest department jointly protect and manage a defined patch of forest, sharing the responsibilities of protection and an agreed share of the produce.
Its scope extends chiefly to degraded forest, where the department’s own protection has proved inadequate and where regeneration depends on control of grazing and cutting. The approach was adopted nationally from 1990, and village-level committees were subsequently constituted across most states.
Diagram
Fig. 2.8-3 – flow: degraded forest patch identified → village committee constituted → written agreement with the forest department specifying duties and share of produce → community controls grazing, cutting and fire and the department supplies technical and nursery support → forest regenerates → produce shared according to the agreement
Advantages: protection by the people living next to a forest is continuous and cheap in a way that patrolling never can be; the community gains a legal share in the produce instead of an illegal one; and regeneration of degraded patches under such agreements has been substantial in several states.
Disadvantages: the department usually retains the final authority, so the partnership is unequal; benefits are often deferred for years while restrictions apply immediately, which falls hardest on the poorest households and on women, who bear most of the labour of fuel and fodder collection; committees can be captured by the more powerful families in a village; and the agreements have generally rested on executive orders rather than on secure legal right, which makes the community’s share vulnerable to being revised.
2.8.7.4 Ecotourism
Ecotourism is responsible travel to natural areas that conserves the environment, sustains the wellbeing of local people, and involves interpretation and education.
Its scope covers guided wildlife viewing, trekking and nature interpretation in and around protected areas, forests, wetlands and coasts, usually in small groups and with local guides and accommodation.
Advantages: it can make a protected area economically valuable to the people living beside it, which changes the incentive to convert or poach; it creates local employment as guides, drivers and hosts; and it builds a constituency of visitors who support protection.
Disadvantages: the label is applied loosely, and much of what is sold as ecotourism is ordinary tourism at the edge of a forest. Vehicle traffic, waste, water demand and construction accumulate at the gate of every popular reserve; visitors concentrate at the sites where animals are most reliably seen, which are often the most sensitive; revenue frequently accrues to outside operators rather than to residents; and once an area’s economy depends on visitor numbers, the pressure runs towards allowing more of them.
2.8.8 Wildlife Conservation and Management in India
India’s wildlife conservation rests on a legal framework, a network of protected areas and a set of species-focused programmes.
The protected area network built under this framework consists chiefly of wildlife sanctuaries and national parks, described above, together with two later categories — conservation reserves and community reserves — which allow protection of areas adjoining protected areas or held by communities without converting them into the stricter categories.
Alongside the area-based approach, India has run species-focused programmes, of which Project Tiger from 1973 and Project Elephant from 1992 are the largest. The logic of choosing a single large species is that it acts as an umbrella: an area secured for a tiger or an elephant, being large and intact, protects everything else living in it. The criticism of the same logic is that funding and attention concentrate on a few charismatic mammals, while species-poor in appeal but ecologically important groups — amphibians, invertebrates, grassland and wetland species — receive comparatively little.
Management of wildlife outside protected areas is now recognised as equally important, since large mammals move through cultivated and settled land, and human–wildlife conflict — crop damage, livestock loss and injury — is the point at which public support for conservation is most easily lost. Compensation schemes, barriers and early-warning arrangements exist, but the losses fall on households least able to absorb them.
2.8.9 Project Tiger
The tiger is the standard Indian case study in conservation because its record contains both a genuine recovery and a genuine failure, and the reasons for each are documented.
Diagram
Fig. 2.8-4 – timeline of tiger conservation in India, running left to right with dated markers: 1972 Wild Life (Protection) Act enacted → 1973 Project Tiger launched with nine reserves → expansion of the reserve network over following decades → 2005 tigers found eliminated from Sariska → 2006 National Tiger Conservation Authority constituted and camera-trap based estimation adopted → four-yearly national estimates thereafter
What the case shows
Three general lessons come out of it, and they apply well beyond tigers.
Protection of habitat, not of individual animals, is what allows a large K strategist to recover — and because such a species breeds slowly, as section 2.3 explained, recovery takes decades and can be undone in a single season of poaching.
Legal designation is not protection. Sariska was a designated reserve when its tigers were removed. What protects an area is staffing, patrolling, intelligence and the cooperation of the people around it.
And a conservation programme’s costs and benefits fall on different people. The benefit is national and, in part, global; the cost of relocation, of restricted access and of crop and livestock losses falls on the households living at the edge of the reserve. Whether that cost is fairly compensated is not a side issue in conservation. It is the condition on which the protection continues to hold.
2.9 Summary
Population dynamics
A population is a group of individuals of the same species living in a defined area at a given time, among whom interbreeding is possible. Population dynamics studies how its size and composition change, and why.
Population size, \(N\), is the total count of individuals. Population density is the number per unit area or volume, \(D = N/A\), and must always be quoted with its unit, such as individuals per hectare or persons per km². Density, not size, indicates the pressure on resources.
Only four processes change the number of individuals in a defined area: natality, mortality, immigration and emigration. Natality and immigration add; mortality and emigration subtract. Births and deaths change world totals; migration only redistributes individuals between areas.
Potential natality is the maximum reproductive output possible under ideal conditions; realised natality is what is actually achieved, and is always lower. Crude birth rate is births per 1000 people per year; infant mortality rate is deaths under one year per 1000 live births per year.
The four factors combine as \(N_t = N_0 + (B + I) - (D + E)\), for one defined area and one stated period. A stationary population is one in which credits and debits are equal, not one in which births and deaths have stopped.
Growth rate expresses change relative to starting size: \(\frac{N_t - N_0}{N_0} \times 100\) per cent per year. It separates into the rate of natural increase (birth rate minus death rate) and net migration (immigration minus emigration), which respond to different causes.
Age structure and population pyramids
Age structure is the proportion of a population in each age group, conventionally divided into pre-reproductive, reproductive and post-reproductive classes. The ratio between them determines the population’s reproductive status and allows its growth to be predicted.
A population pyramid plots age classes up the vertical axis and the number or percentage of people along the horizontal axis, males to the left and females to the right.
Three general shapes occur. An expanding pyramid has a broad base, high birth and death rates, and a rising total. A constant pyramid has near-vertical sides, low birth and death rates, and a steady total. A declining pyramid has a narrow base under a wider middle and top, and a falling, ageing total.
The dependency ratio is the number of people aged 0–14 and 65 and above per 100 people aged 15–64. The same ratio can arise from a young population or an ageing one, and the two need entirely different services.
Population momentum is the continued growth of a population for decades after family sizes fall, because a large cohort produced under earlier high birth rates is still entering its reproductive years while the older, smaller generations contribute few deaths.
Survivorship curves and life strategies
A cohort is a group of individuals born within the same period. A survivorship curve plots survivors of a cohort, per 1000 born, on a logarithmic vertical axis against relative age as a percentage of maximum life span. A straight line on that scale means a constant death rate at all ages.
Type I is convex: low mortality in early and middle life, concentrated in old age; few offspring, extensive parental care; large mammals and human populations. Type II is a straight diagonal: mortality constant at all ages; many songbirds, some reptiles, Hydra. Type III is concave: very heavy mortality at the start, low thereafter; very many offspring, little parental care; fishes, oysters and other marine invertebrates, many plants.
\(r\) is the intrinsic rate of natural increase, \(r = b - d\), per individual per unit time; \(K\) is the carrying capacity.
r strategists are small, mature early, have short generation times, produce many small offspring with little parental care, disperse widely, suit unstable habitats, stay well below \(K\), and typically show Type III survivorship. K strategists are large, mature late, live long, produce few well-provisioned offspring with extended care, suit stable saturated habitats, stay near \(K\), and typically show Type I survivorship. The two are ends of a continuum, not two boxes.
Because K strategists rebuild their numbers slowly, they are disproportionately the species at risk of extinction; r strategists recover quickly, which is why pests and weeds are hard to suppress.
Human populations
Carrying capacity, \(K\), is the maximum population an environment can sustain indefinitely, under stated conditions, without degrading the resource base. For humans it is not fixed, since irrigation, storage, sanitation, technology and trade alter it.
Malthus argued in 1798 that population grows geometrically while food supply grows arithmetically, so population must outrun subsistence, producing over-population. He classified the restraints as preventive checks, which lower the birth rate, and positive checks — famine, disease and war — which raise the death rate.
The model has been questioned on four grounds: yields per hectare rose far faster than arithmetically; Boserup argued that population pressure itself induces agricultural intensification; Engels and later Sen argued that shortage often reflects distribution and entitlement rather than total supply; and families voluntarily chose fewer children as incomes, education and child survival rose. Hardin and other neo-Malthusians retained the core point that a finite environment cannot support unlimited growth.
Environmental load is the number of people multiplied by consumption per person. Consumption of energy, water, metals and manufactured goods per person differs several-fold between and within nations, so a slowly growing high-consuming population may add more demand than a fast-growing low-consuming one. Population growth and disparate consumption must be assessed together.
Demographic transition, set out by Warren Thompson in 1929, is the long-term shift from high birth and death rates to low ones. Death rates fall first because they respond to food, water, sanitation and medicine; birth rates fall later because they depend on household decisions. Rapid population growth occurs in the gap.
Stage I: high birth and high fluctuating death rates, slow irregular growth. Stage II: death rates fall, births stay high, growth is rapid. Stage III: birth rates fall, growth slows. Stage IV: both rates low, population large and stable. A Stage V of below-replacement fertility and ageing is described by some writers but was not in the original model.
Death rates fall through food supply and storage, clean water, sanitation, immunisation and maternal and child health care. Birth rates fall through improved child survival, urbanisation, education — especially of girls, rising incomes and old-age security, access to and acceptance of family planning, and changing views of normal family size.
Population regulation
Exponential growth adds a constant proportion each period and gives the J-shaped curve, \(\frac{dN}{dt} = rN\). It has a lag phase and an exponential phase but no upper limit, so the population overshoots the carrying capacity and suffers a population crash.
Environmental resistance is the sum of restraining influences that strengthen as numbers rise. Building it in gives logistic growth, \(\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)\), and the S-shaped curve, with five phases: lag, positive acceleration, exponential, negative acceleration and stationary or plateau.
Growth is fastest at about half the carrying capacity, because below that too few individuals are reproducing and above it too little is left per individual. At \(K\), births equal deaths and the net growth rate is zero while both processes continue.
Density-dependent factors intensify as density rises and can hold a population near a level; density-independent factors act with equal severity at any density and limit numbers without regulating them.
Eight factors regulate population size. Space limits breeding sites and is largely unsubstitutable. Food and water limit reproduction and survival, with dry-season water often decisive. Territories fix the number of breeding holdings before food itself runs out. Predators remove individuals and oscillate with their prey. Weather and climate act mainly independently of density, and long-term climatic shifts move \(K\) itself. Parasites and diseases spread with crowding and are strongly density-dependent. Disasters remove large fractions regardless of density. Self-regulation reduces reproduction through crowding-induced changes in behaviour and physiology before resources are exhausted.
Human population control
Family planning is the voluntary decision by individuals and couples about the number and spacing of children, supported by information and health services. Adequate spacing lowers maternal and infant mortality, and lower infant mortality in turn reduces desired family size.
Voluntary, service-based programmes have produced more durable fertility decline than coercive, target-driven ones, which damage trust in health services and are difficult to sustain.
Education raises the age at marriage, shortens the reproductive span, raises the cost and expected investment per child, and spreads awareness — but acts with a lag of about a generation.
Economic growth lowers fertility through better child survival, higher opportunity cost of childbearing and old-age security, though the direction of causation is disputed; it also raises consumption per person, so it does not serve population stabilisation and resource conservation equally.
Women’s empowerment has the most consistent evidence behind it: girls’ education leads to later marriage, later first birth and longer intervals, better maternal and child health, paid employment, and greater say in household decisions, all of which lower the desired family size. Raising the average age of marriage is among the most effective single measures available.
Threats to the ecosystem
Provisioning functions are the goods an ecosystem supplies — food, fodder, fuelwood, timber, fibre, fresh water, fish, medicinal plants, genetic material. Regulating functions are the processes it maintains — flood moderation, groundwater recharge, water purification, pollination, pest control, soil formation, waste breakdown, carbon storage. Regulating functions are rarely priced and are usually noticed only after they fail.
Habitat destruction, the conversion of habitat to other uses, is the leading cause of species decline. Fragmentation compounds it by shrinking undisturbed interior, increasing edge, and isolating small populations — hardest on large, slow-breeding species.
Genetic erosion is the loss of variation within a species, through the disappearance of local varieties, breeds and wild relatives. It leaves crops uniformly vulnerable to new pests and diseases and removes the material from which resistant varieties would be bred.
Loss of biodiversity reduces the resilience of an ecosystem, since overlapping roles that once absorbed a disturbance are gone.
Expanding agriculture onto forest, wetland, grassland and slope causes erosion, loss of flood buffering and recharge, simplification of the crop and insect community, nutrient and pesticide runoff, and, with heavy irrigation and poor drainage, waterlogging and salinity.
Impounding water yields irrigation, drinking water, power and flood control, but submerges land, displaces people, blocks fish migration, traps sediment that formerly renewed downstream fields and deltas, replaces natural flow patterns, and can waterlog the command area. Benefits and costs fall on different people in different places.
Assimilative capacity is the rate at which an ecosystem can process waste without its own functioning being damaged. Sewage and organic effluent, industrial effluent, and persistent solid waste including plastics and electronic goods now exceed it in many places.
Increasing human consumption enlarges both ends of the flow — extraction from ecosystems and disposal into them — so population and consumption act on ecosystems as a single pressure.
Conservation
Conservation is the planned management, protection and sustainable use of natural resources and organisms, keeping use within the rate of renewal. It is defended on grounds of direct use, irreplaceable regulating functions, the genetic reservoir, and an ethical claim that species have a right to exist and that resources are held in trust.
In situ conservation protects species in their natural habitat and conserves whole ecosystems with evolution continuing; ex situ conservation maintains species outside it, in facilities where numbers are limited and genetic variation is easily lost. The two are complementary, with in situ the primary method.
A wildlife sanctuary, notified under the Wild Life (Protection) Act, 1972, prohibits hunting but may permit regulated grazing, collection of minor forest produce and rights of way; protection is weaker and boundaries more easily altered than in a national park.
A national park protects an entire ecosystem strictly, permitting no grazing, cultivation, felling or private rights, and its boundaries cannot be altered without a resolution of the state legislature. It gives the highest legal protection but excludes former users and may not match the range of the animals it protects.
A biosphere reserve, recognised under the Man and the Biosphere Programme of UNESCO, combines conservation, sustainable local development and research, and is zoned into a legally protected core, a buffer permitting compatible activity, and a transition zone of settlement and farming. Its size and its dependence on cooperation make management difficult.
Zoos maintain and breed threatened animals, provide stock for reintroduction and support research and education, but hold few individuals, lose genetic variation, and produce animals that may lack the behaviour needed in the wild. Aquaria do the same for aquatic species, at high technical cost, and cannot support species with complex migratory life cycles. Plant collections — botanical gardens, seed banks, field gene banks and tissue culture — store very many varieties cheaply and directly counter genetic erosion, but seed viability declines, some species cannot be dried and frozen, and stored material stops evolving.
Forest cover in India is reported biennially by the Forest Survey of India as land of one hectare or more with canopy density above ten per cent, irrespective of legal status or species, so plantations are counted alongside natural forest. Cover stands at a little over one-fifth of the geographical area, and forest and tree cover together at roughly one-quarter, against the National Forest Policy 1988 goal of one-third. Degradation matters more than outright clearance.
Conflict over forests arises because long-standing use by forest-dwelling communities, including many Scheduled Tribe communities, was not recorded as legal right when the state assumed ownership. The Forest Rights Act, 2006 provides for recognition of individual and community rights through the village assembly, and its implementation has been uneven.
Threats to forests are poaching, development projects such as roads, dams, transmission lines and mines, direct over-exploitation of produce beyond the rate of regeneration, indirect over-exploitation through grazing, fire, lopping and soil damage, and unregulated tourism.
The central difficulty is distributional: the benefits of a protected forest are diffuse and national, the costs concentrated on the few people living nearest, who are often among the poorest.
State forest departments protect, manage, regulate and restore forests through working plans, protected area management, control of felling and transit, fire control, enforcement and rights settlement, working with limited staff and budgets and combining regulatory and developmental roles. Non-governmental organisations add independent research and monitoring, community work, legal and policy input, public attention and outside funds, but depend on funders, cover ground patchily and hold no statutory power.
Scientific forestry manages forests on surveyed data and silviculture for a sustained yield, using inventories, working plans and rotation periods; in practice it has often converted mixed forest into single-species stands and counted timber while ignoring local produce and regulating functions.
Social forestry raises trees outside reserved forests to meet local fuelwood, fodder and small timber needs, in four forms — farm forestry, community forestry, extension forestry and agroforestry. Where commercial species were promoted, the produce went to industry rather than to villages, and landless households gained least.
Joint Forestry Management, set in motion by a central circular in 1990 after the National Forest Policy 1988, has village forest protection committees and the forest department jointly protect a defined patch and share the produce. Regeneration has often been rapid, but the department retains ownership and the final say, benefit sharing has been slow, and committees can be dominated by more powerful households.
Ecotourism is responsible travel to natural areas that conserves the environment, sustains local well-being and involves interpretation. It gives a protected area competing economic value and local employment, but the label is loosely applied, pressure concentrates in a few reserves and seasons, and revenue often leaves the community.
Wildlife is protected under the Wild Life (Protection) Act, 1972, through prohibition of hunting, regulation of trade, scheduled species listings and the protected area categories; supported by species programmes such as Project Tiger (1973) and Project Elephant (1992), and by international trade control under CITES. Corridors between protected areas and the management of human–wildlife conflict are now as important as the protected areas themselves.
Project Tiger, launched in 1973 with nine reserves, uses the tiger as an umbrella species and a core–buffer reserve design. It halted the slide towards extinction, created a large protected network conserving many other species and major catchments, and — after tigers were lost from a Rajasthan reserve, made public in 2004–05 — led to the statutory National Tiger Conservation Authority in 2006 and standardised camera-trap estimation. Poaching, small and isolated reserves, pressure on corridors, disputed village relocation and concentrated tourism remain unresolved.
2.10 Key Terms
Age structure — The proportion of a population falling into each age group, usually grouped as pre-reproductive, reproductive and post-reproductive.
Assimilative capacity — The rate at which an ecosystem can receive and process waste without its own functioning being damaged.
Biosphere reserve — A large protected area recognised under the Man and the Biosphere Programme, zoned into core, buffer and transition zones, combining conservation, sustainable local development and research.
Buffer zone — The middle zone of a biosphere reserve, surrounding the core, in which research, education, restoration and regulated resource use are permitted.
Carrying capacity (K) — The maximum population of a species that an environment can sustain indefinitely, under stated conditions, without degrading the resource base.
Census — A complete enumeration of every individual in a defined area at a stated time.
CITES — Convention on International Trade in Endangered Species of Wild Fauna and Flora; the international agreement regulating or prohibiting cross-border trade in listed species.
Cohort — A group of individuals of a population born, hatched or germinated within the same period, and therefore of roughly the same age.
Conservation — The planned management, protection and sustainable use of natural resources and living organisms, so that they remain available to present and future generations.
Constant population — A population whose successive cohorts are of similar size, shown by a pyramid with near-vertical sides, arising from low birth and low death rates.
Core zone — The innermost, legally protected zone of a biosphere reserve, closed to all human activity except non-invasive research.
Crude birth rate — The number of live births per 1000 people in the population per year.
Declining population — A population with fewer births each year than the year before, shown by a pyramid with a narrow base under a wider middle and top.
Demographic transition — The long-term shift of a population from high birth and death rates to low birth and death rates, with rapid growth in the intervening period.
Density-dependent factor — A factor whose effect on a population intensifies as density rises, and which can therefore hold numbers near a level.
Density-independent factor — A factor acting with the same severity whatever the population density, which limits numbers but cannot regulate them.
Dependency ratio — The number of people aged 0–14 and 65 and above per 100 people aged 15–64.
Disparate consumption — The very unequal use of resources by different populations and different groups, both among nations and within them.
Ecotourism — Responsible travel to natural areas that conserves the environment, sustains the well-being of local people, and involves interpretation and education.
Emigration — The permanent movement of individuals out of a population to another area, reducing its numbers.
Environmental resistance — The sum of all restraining influences — competition, crowding, disease, waste accumulation — that strengthen as a population grows.
Expanding population — A population in which each age class is larger than the one above it, shown by a broad-based pyramid, arising from high birth rates.
Exponential growth — Growth in which a population increases by a constant proportion of itself in each unit of time, giving the J-shaped curve.
Exponential (log) phase — The steepest part of a growth curve, in which numbers rise rapidly because resources are still plentiful relative to numbers.
Ex situ conservation — The protection of species outside their natural habitat, in zoos, aquaria, botanical gardens, seed banks and similar facilities.
Family planning — The voluntary decision by individuals and couples about the number and spacing of their children, supported by information and health services.
Genetic erosion — The loss of genetic variation within a species, through the disappearance of local varieties, breeds or wild populations.
Growth rate — The change in a population over a period expressed as a percentage of its size at the start of the period.
Habitat destruction — The conversion of a natural habitat to another use, so that the species dependent on it can no longer live there.
Habitat fragmentation — The breaking up of a continuous habitat into separated pieces, reducing interior area, increasing edge and isolating populations.
Immigration — The permanent movement of individuals into a population from outside, adding to its numbers.
Impounding water — Holding back a river behind a dam or barrage to form a reservoir.
Increasing human consumption — The rising quantity of materials and energy used per person, which enlarges both extraction from ecosystems and disposal into them.
Infant mortality rate — The number of deaths of children under one year of age per 1000 live births per year.
In situ conservation — The protection of species within their natural habitat, by conserving and managing the ecosystem in which they occur.
Intrinsic rate of natural increase (r) — The per capita rate at which a population would grow under unrestricted conditions, equal to the per capita birth rate minus the per capita death rate.
JFM — Joint Forestry Management; an arrangement in which a state forest department and a village institution jointly protect and manage a defined forest patch and share its produce.
J-shaped curve — The graph of exponential growth, rising ever more steeply with no upper limit, and ending in a population crash.
K strategist — A species that is large-bodied, matures late, lives long and produces few well-provisioned offspring with extended parental care, suited to stable, saturated habitats.
Lag phase — The initial phase of a growth curve in which individuals are establishing themselves and numbers change little.
Life expectancy at birth — The average number of years a newborn would live if current age-specific death rates continued unchanged.
Logarithmic scale — A scale on which equal distances represent equal multiplications rather than equal additions, used on the vertical axis of survivorship curves.
Logistic growth — Growth in which environmental resistance increases with population size, so that numbers approach the carrying capacity, giving the S-shaped curve.
Loss of biodiversity — The decline in the number and variety of species and of the ecosystems they compose, reducing the resilience of those ecosystems.
Mortality — The rate at which individuals are lost from a population by death, per unit time.
Natality — The rate at which new individuals are added to a population by birth, hatching or germination, per unit time.
National park — An area notified for the strict protection of an entire natural ecosystem, in which grazing, cultivation, felling and private rights are not permitted.
Negative acceleration phase — The phase of the logistic curve in which growth slows as environmental resistance takes hold.
Net migration — Immigration minus emigration for a defined area over a stated period.
Over-population — In the Malthusian sense, a condition in which the number of people exceeds what the available resources can feed.
Population — A group of individuals of the same species occupying a particular area at a particular time, among whom interbreeding is possible.
Population crash — A sudden and severe fall in numbers when a population has overshot the carrying capacity of its habitat.
Population density — The number of individuals of a species per unit area or per unit volume of habitat.
Population dynamics — The study of changes in the size and composition of a population over time, and of their causes.
Population momentum — The continued growth of a population for decades after family sizes fall, because a large cohort is still entering its reproductive years.
Population pyramid — A pair of back-to-back horizontal bar graphs showing the age structure of a population separately for each sex.
Population size (N) — The total number of individuals in a defined area at a given time.
Positive acceleration phase — The phase of the logistic curve in which growth begins and speeds up as the number of reproducing individuals rises.
Positive checks — In Malthus’s classification, the restraints that raise the death rate — famine, epidemic disease and war.
Post-reproductive group — Individuals of a population that have passed breeding age and no longer contribute offspring.
Potential natality — The maximum number of offspring a population could produce under ideal conditions.
Pre-reproductive group — Individuals of a population that have not yet reached breeding age.
Preventive checks — In Malthus’s classification, the restraints that lower the birth rate, such as delayed marriage.
Provisioning functions — The goods an ecosystem supplies directly, such as food, fodder, fuelwood, timber, fibre, fresh water and medicinal plants.
Rate of natural increase — The birth rate minus the death rate of a population, excluding migration.
Realised natality — The number of offspring actually produced under the conditions prevailing in the habitat, always lower than potential natality.
Regulating functions — The processes an ecosystem maintains, such as flood moderation, groundwater recharge, water purification, pollination, pest control, soil formation and carbon storage.
Relative age — Age expressed as a percentage of a species’ maximum life span, used on the horizontal axis of survivorship curves.
Reproductive group — Individuals of a population currently capable of breeding.
r strategist — A species that is small-bodied, matures early, has a short generation time and produces many small offspring with little parental care, suited to unstable habitats.
Scientific forestry — The management of forests on the basis of surveyed data and silvicultural science, to obtain a sustained yield without reducing productive capacity.
Sex ratio — The number of females per 1000 males in a population.
Social forestry — The raising of trees on land outside reserved forests to meet local fuelwood, fodder and small timber needs, in the forms of farm, community and extension forestry and agroforestry.
S-shaped curve — The sigmoid graph of logistic growth, rising through five phases and levelling off at the carrying capacity.
Stage V — A stage of below-replacement fertility and an ageing, declining population, added by some writers to the original four-stage demographic transition model.
Stationary (plateau) phase — The final phase of the logistic curve, in which births equal deaths and the net growth rate is effectively zero.
Survivorship curve — A graph of the number of individuals of a cohort surviving, plotted against age, showing the pattern of mortality across the life span.
Transition zone — The outermost zone of a biosphere reserve, where settlements, farming and other economic activity continue under cooperative management.
UNESCO — United Nations Educational, Scientific and Cultural Organization; the body under whose Man and the Biosphere Programme biosphere reserves are recognised.
Wildlife sanctuary — An area notified for the protection of wildlife, in which hunting is prohibited but certain regulated human activities may be permitted.
Women’s empowerment — The expansion of women’s ability to make and act on decisions about their own lives, in education, health, employment and the household.
2.11 Points to Ponder
A district’s population grows by 150 people in a year while 400 of its young adults leave to work elsewhere. Which figure tells you more about the district’s future, and why might a planner reading only the total reach the wrong conclusion?
Malthus assumed that food supply could not keep pace with population. Yields have risen far faster than he allowed — but they have risen partly by drawing on groundwater, soil fertility and fossil energy. Does that make his argument wrong, or only postponed?
If two countries place the same total demand on the earth’s resources, but one has five times the population of the other, is either of them entitled to ask the other to change first? On what grounds could such a claim be made or refused?
Every strategy for slowing population growth in this chapter works by changing the conditions under which families decide, rather than by deciding for them. Is that a practical finding about what works, or a principle that should hold even if coercion worked better?
A seed bank can hold a thousand rice varieties in a room, at a fraction of the cost of conserving the fields they came from. What is preserved in a living field that is not preserved in the bank, and how would you decide how much that difference is worth?
Protecting a forest produces benefits spread across a whole country and costs concentrated on a few thousand people living beside it. If the benefits genuinely exceed the costs, does that settle the question — or does it matter who receives which?
2.12 Objective Questions
- Population density is best expressed as:
- the total number of individuals in a habitat
- the number of individuals per unit area or volume
- the number of births per 1000 individuals per year
- the maximum number an environment can support [1 mark]
- In the relation \(N_t = N_0 + (B + I) - (D + E)\), the symbol \(E\) stands for:
- environmental resistance
- emigration
- exponential growth
- expected life span [1 mark]
- A survivorship curve that appears as a straight diagonal line on a logarithmic scale indicates:
- heavy mortality in early life
- heavy mortality in old age
- a constant rate of mortality at all ages
- zero mortality until the maximum life span [1 mark]
- Which of the following is a density-independent factor?
- Competition for territories
- Spread of an epidemic in a crowded population
- A cyclone striking a coastal habitat
- Predation increasing with prey abundance [1 mark]
- In a biosphere reserve, settlements and farming are permitted in the:
- core zone
- buffer zone
- transition zone
- none of the zones [1 mark]
- Assertion (A): A population may continue to grow for several decades after average family size has fallen. Reason (R): A large group of people born during a period of high birth rates is still entering its reproductive years.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true. [1 mark]
- Assertion (A): The logistic growth curve flattens as the population approaches the carrying capacity. Reason (R): Births and deaths both stop once the carrying capacity is reached.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true. [1 mark]
- Assertion (A): K strategists are disproportionately represented among species at risk of extinction. Reason (R): K strategists mature late and produce few offspring, so their populations rebuild slowly.
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true. [1 mark]
- Match the items in Column A with those in Column B:
| Column A | Column B |
|---|---|
| a. Warren Thompson | i. Wild Life (Protection) Act, 1972 |
| b. Thomas Malthus | ii. Demographic transition model |
| c. National park | iii. Man and the Biosphere Programme |
| d. Biosphere reserve | iv. Positive and preventive checks |
[1 mark]
- Match the conservation measure with its description:
| Column A | Column B |
|---|---|
| a. Seed bank | i. Trees grown by farmers on their own holdings |
| b. Farm forestry | ii. Village committee and forest department share produce |
| c. Joint Forestry Management | iii. Regulated visits to natural areas benefiting local people |
| d. Ecotourism | iv. Dried seed stored at low temperature and humidity |
[1 mark]
Fill in the blank: The maximum number of offspring a population could produce under ideal conditions is called __________ natality. [1 mark]
Fill in the blank: The sum of all restraining influences that strengthen as a population grows is called __________. [1 mark]
State whether true or false: The boundaries of a national park in India can be altered by the state government without a resolution of the state legislature. [1 mark]
State whether true or false: Forest cover, as reported for India, counts plantations and orchards along with natural forest. [1 mark]
Give the full form of: (i) JFM (ii) CITES [1 mark]
2.13 Check Your Concepts
Define carrying capacity, and state one reason why it is not a fixed quantity for human populations. [2 marks]
Define genetic erosion and give one example of it from Indian agriculture. [2 marks]
State the four processes that can change the size of a population in a defined area, and indicate which two increase it. [2 marks]
What is meant by the assimilative capacity of an ecosystem? [2 marks]
Name the three zones of a biosphere reserve and state what activity is permitted in each. [3 marks]
Differentiate between in situ and ex situ conservation on any three grounds. [3 marks]
Give reasons: a population that has just reached half its carrying capacity is adding individuals faster than one that has almost reached it. [3 marks]
Differentiate between a wildlife sanctuary and a national park on any three grounds. [3 marks]
Distinguish between direct and indirect over-exploitation of forest resources, with one example of each. [3 marks]
Draw the three general shapes of the survivorship curve on a single labelled graph, and explain what the shape of Type III tells you about the reproductive strategy of the species showing it. [4 marks]
Explain, with an example, why a factor such as a cyclone can limit a population but cannot regulate it. [4 marks]
Evaluate the claim that a country with a rapidly growing population necessarily places a greater load on the earth’s resources than one with a slowly growing population. [4 marks]
Describe the four types of social forestry, and state one reason why social forestry programmes have sometimes failed to reduce pressure on natural forests. [4 marks]
Describe the Malthusian model of population and resources, and discuss three grounds on which it has been questioned. [6 marks]
Describe the demographic transition model stage by stage, and explain why the population grows most rapidly in the second stage. [6 marks]
Compare r strategies and K strategies with respect to body size, number of offspring, parental care, habitat favoured and survivorship curve shown, and explain why the distinction matters for conservation. [6 marks]
Discuss the origin, aims and objectives of Project Tiger, and evaluate its record by giving two successes and two failures. [6 marks]
Discuss the causes and consequences of impounding water behind a large dam, with reference to both provisioning and regulating functions of the affected ecosystems. [6 marks]
2.14 Passage-Based Questions
- A small town has recorded its own vital statistics for many years. In the most recent year it registered 640 births and 290 deaths. Over the same year, 95 people moved into the town permanently and 410 left, most of them young adults who had found work in a larger city. The town’s schools report falling enrolment in the primary classes, while the municipal hospital reports rising demand at its outpatient department for older patients.
- Calculate the town’s rate of natural increase and its net migration for the year, and state whether the population rose or fell. [3 marks]
- Explain how the same year’s data can show a positive natural increase alongside falling primary enrolment. [2 marks]
- Predict what shape the town’s population pyramid will move towards if this pattern continues for two decades, and give one consequence for the dependency ratio. [3 marks]
- A new highway is built through a large block of forest, and within a few years two mining leases and a line of small settlements have grown up along it. Surveys taken before the road and ten years afterwards show that the total area under tree cover has fallen by about a tenth, but that the number of large mammal species recorded has fallen by considerably more. The forest department also reports that a river flowing out of the block now carries much more silt after heavy rain than it did before.
- Name and explain the process that accounts for the loss of large mammal species being greater than the loss of tree cover. [3 marks]
- Explain why large mammals in particular are affected in this way, using the idea of life strategies. [2 marks]
- Identify the ecosystem function whose failure is indicated by the increased silt load, and state one downstream consequence. [3 marks]
- A degraded forest patch adjoining a village is brought under a joint management arrangement. A village committee agrees to protect the patch from grazing and fire, and the forest department supplies seedlings and technical help. Members are permitted to collect specified minor forest produce and are promised a share of the eventual harvest. Five years later the canopy has closed noticeably and fodder grass has returned. However, several landless households complain that they have lost the grazing they previously depended on, and that the promised share has not yet been received.
- Identify the management measure described, and state two of its advantages illustrated by the passage. [3 marks]
- Explain why protection alone was enough to bring about visible regeneration in five years. [2 marks]
- Explain the distributional problem the landless households have raised, and state why it is a common weakness of such arrangements. [3 marks]
2.15 Practice With Data
A grassland of 40 hectares supports 1 800 individuals of a grasshopper species at the start of the season and 4 500 at the end of it. Calculate the population density in individuals per hectare at each of the two times, and the percentage growth over the season. [3 marks]
A species has an intrinsic rate of natural increase \(r = 0.2\) per individual per year in a habitat with a carrying capacity \(K = 2\,000\) individuals. Using the logistic equation, calculate the growth rate in individuals per year when \(N = 200\), when \(N = 1\,000\) and when \(N = 1\,800\). State at which of the three the population is growing fastest, and explain why. [4 marks]
A population of 60 000 people is distributed as follows: 21 600 aged 0–14 years, 34 200 aged 15–64 years and 4 200 aged 65 years and above. Calculate the dependency ratio, state the shape of pyramid this age structure would produce, and name one service the district should be planning to expand. [4 marks]
Two districts each report a crude birth rate of 24 births per 1000 people per year. District P reports a crude death rate of 9 deaths per 1000 per year and District Q reports 16 deaths per 1000 per year. Calculate the rate of natural increase for each, and state which of the two is likely to be at an earlier stage of the demographic transition, giving one reason. [4 marks]
A survivorship study follows a cohort of 1 000 hatchlings. At 10 per cent of the maximum life span, 60 individuals remain alive; at 50 per cent, 40 remain; at 90 per cent, 25 remain. State which type of survivorship curve this cohort shows, explain how the figures support your answer, and state what the vertical axis of such a graph should be scaled in. [4 marks]
2.8.7.2 Social Forestry
Social forestry is the raising of trees on land outside the reserved forest — on farms, village common land, roadsides and urban space — in order to meet local needs for fuelwood, fodder, small timber and fruit, and thereby reduce pressure on natural forest. The term came into use in India through the recommendations of the National Commission on Agriculture in 1976.
Table 2.11 – Types of social forestry | Type | Where trees are raised | Chief purpose | |—|—|—| | Farm forestry | On private agricultural holdings, on bunds and field margins | Fuelwood, fodder, small timber and income for the farm household | | Community forestry | On village common land and other public land | Fuelwood and fodder for the village as a whole, managed collectively | | Extension forestry | Along roads, canals, railway lines and on wasteland | Extending tree cover to land not otherwise used; shelter and soil protection | | Agroforestry | On cultivated land, with trees grown together with crops or livestock | Combining tree produce with food or fodder production on the same land | | Urban forestry | In and around towns and cities — parks, avenues, green belts | Shade, dust and noise reduction, recreation and amenity |
Advantages: it produces fuel and fodder close to where they are used, which reduces both the collection pressure on natural forest and the labour of collection; it puts otherwise unproductive land to use; and it gives the growers a direct stake in the trees.
Disadvantages: the record of the large programmes of the 1970s and 1980s is mixed. Farm forestry was taken up most readily by larger landholders and, in several regions, was planted with fast-growing species sold to industry rather than the fuelwood and fodder species the programme was designed to supply. Where cropland was diverted to trees, employment in agriculture fell. Community woodlots on common land often failed because the question of who would protect the trees and who would receive the produce was never settled.