Chapter 3: Population and Conservation Ecology
In 1944 a small United States Coast Guard party released twenty-nine reindeer on St Matthew Island, a treeless island in the Bering Sea. The animals were meant to be an emergency food supply for the men stationed there. The station closed within a year or two, and the reindeer were left alone on an island with deep beds of lichen, no wolves, no bears and no hunters.
A biologist, David Klein, visited the island in the 1950s and again in the 1960s. In 1957 he counted about 1,350 animals. Six years later there were roughly 6,000 — a herd that had multiplied more than two hundredfold from the original twenty-nine, on an island of about 330 square kilometres. The animals were noticeably smaller and lighter than before, and the lichen mats that carried them through winter had been cropped down to bare rock in many places. When Klein returned in 1966, only a few dozen animals remained. Within a few more years the herd was gone.
Nothing hunted these reindeer. No disease swept through them. What ended the herd was the island itself — a food supply that could not keep pace with the number of mouths.
Diagram
Fig. 3.0-1 – illustration of a windswept treeless island in the Bering Sea with a scattered herd of reindeer grazing on low lichen-covered ground; a small abandoned wooden station building in the background; no people shown
This chapter will answer the following questions.
3.1 Population Dynamics
3.1.1 What a Population Is
The reindeer on St Matthew Island were not simply a scattering of animals. They were a population — a group of individuals of the same species, living in the same area at the same time, and able to breed with one another. Ecology studies populations rather than single animals because the behaviour of the group over time is what determines whether a species persists or disappears.
A population is never a fixed thing. It gains members and loses them constantly, and its size on any given day is only a snapshot of a moving quantity. The study of how and why that number changes is called population dynamics.
Size alone can be misleading. Two thousand deer in a small valley and two thousand deer spread over an entire district are very different situations, so ecologists usually work with population density — the number of individuals per unit area:
\[D = \frac{N}{A}\]
where \(D\) is population density, \(N\) is the number of individuals, and \(A\) is the area in square kilometres (or hectares, for small plots). For a plant population, density is often counted per square metre using sample plots.
Why density matters more than number
Density decides how often individuals meet. It sets how hard they compete for food, how quickly a disease spreads between them, and how easily a predator finds them. Almost every process discussed in this chapter works through density, not through the raw head-count.
3.1.2 Factors Causing Population Change
Only four processes can change the size of a population. Two of them add individuals, and two remove them.
Natality is the addition of new individuals by birth, hatching or germination. It is usually expressed as a rate — the number of births per thousand individuals per year. For human populations this measure is the crude birth rate (CBR), “crude” because it is calculated over the whole population, including men, children and the elderly, who do not give birth.
Mortality is the loss of individuals by death, again expressed per thousand per year, and for human populations called the crude death rate (CDR).
The remaining two processes move individuals across the boundary of the area being studied. Immigration is the arrival of individuals of the same species from outside, and emigration is the departure of individuals to some other area. Neither creates nor destroys life; both redistribute it.
Diagram
Fig. 3.1-1 – a rectangle labelled “population, size N”; two inward arrows entering it labelled “natality (births)” and “immigration (arrivals)”; two outward arrows leaving it labelled “mortality (deaths)” and “emigration (departures)”; caption line: population grows when the inward arrows outweigh the outward arrows
Notice in Fig. 3.1-1 that the four factors are not four separate causes acting independently. They are two pairs of opposites, and only the balance between them matters.
3.1.3 The Relation Between the Factors
Because births and immigration add, while deaths and emigration subtract, the population at the end of a period can be written directly from the population at the start:
\[N_{t+1} = N_t + (B + I) - (D + E)\]
where \(N_t\) is the number of individuals at the start of the period, \(N_{t+1}\) the number at the end, \(B\) the number of births, \(I\) the number of immigrants, \(D\) the number of deaths and \(E\) the number of emigrants, all counted over the same period, usually one year.
Three outcomes are possible. If \((B + I)\) exceeds \((D + E)\), the population grows. If the two sides are equal, the population is stationary — individuals are still being born and still dying, but the total does not move. If \((D + E)\) exceeds \((B + I)\), the population declines.
The difference between births and deaths alone is called the rate of natural increase, because it excludes movement. For a country as a whole, migration is usually small compared with births and deaths, so natural increase dominates. For a single city, a mining town or a forest-fringe village, migration can easily be the largest term of the four.
Working through a real balance
Suppose a small town has a population of 40,000 on 1 January. During the year there are 720 births and 400 deaths; 150 people move into the town and 270 move away.
The natural increase is \(720 - 400 = 320\) persons. The net migration is \(150 - 270 = -120\) persons — a loss, because more people left than arrived. The population on 31 December is therefore
\[N_{t+1} = 40{,}000 + (720 + 150) - (400 + 270) = 40{,}200\]
The annual growth rate is
\[\frac{200}{40{,}000} \times 100 = 0.5\% \text{ per year}\]
Expressed per thousand, the crude birth rate is \(\frac{720}{40{,}000} \times 1000 = 18\) per 1000 per year, and the crude death rate is \(\frac{400}{40{,}000} \times 1000 = 10\) per 1000 per year. Natural increase is thus 8 per 1000, or 0.8 per cent — noticeably larger than the 0.5 per cent the town actually grew by. The gap is the emigration, and it tells a story the growth figure alone hides: this is a town where people are being born faster than they die, but which younger people are steadily leaving.
3.1.4 Age Structure and Its Significance
Two populations of exactly the same size can have completely different futures, because what matters is not how many individuals there are but how old they are.
The age structure of a population is the proportion of individuals in each age group. Ecologists usually collapse these into three broad classes: the pre-reproductive group, too young to breed; the reproductive group, currently capable of breeding; and the post-reproductive group, past breeding age.
A population with a very large pre-reproductive group is loaded with future parents. Even if each of them has only a small family, the sheer number of them entering the reproductive class will push births up for decades. A population with a small pre-reproductive group and a heavy post-reproductive group is the reverse: births will fall as the reproductive class empties out.
This inertia has a name. Population momentum is the tendency of a population to keep growing for some time after fertility has fallen, simply because a large cohort of young people is still moving into the child-bearing ages. It explains a fact that puzzles many students — that a country’s family sizes can shrink while its total population continues to rise.
The dependency ratio
Age structure also matters economically. The dependency ratio compares the number of people in the non-working age groups, conventionally those below 15 and above 64, with the number in the working ages between them. A high ratio means a small working group supporting many dependants. A low ratio — sometimes called a demographic dividend — means a large working-age share, and a chance to raise savings and output, provided those workers can actually find employment.
3.1.5 Population Pyramids and What They Predict
An age structure is easiest to read as a picture. A population pyramid is a pair of horizontal bar graphs placed back to back: age classes are stacked on the vertical axis from youngest at the bottom to oldest at the top, with males shown to the left of the centre line and females to the right. The length of each bar gives the number, or the percentage, of the total population in that age and sex group.
Three general shapes recur, and each carries a different prediction.
An expanding pyramid is broad at the base and tapers steeply upwards. Each younger age class is larger than the one above it, which means high birth rates and, usually, high death rates at every age. Such a population will grow, and will keep growing for a considerable time because of momentum.
A constant pyramid has sides that are almost vertical up to middle age, narrowing only in the oldest classes. The number entering the population roughly matches the number leaving it, and the total is close to stationary.
A declining pyramid is narrower at the base than in the middle, giving it an urn-like shape. Fewer children are being born than there are adults to replace, and unless immigration makes up the difference the population will shrink and grow older.
Diagram
Fig. 3.1-2 – three population pyramids drawn side by side and labelled expanding, constant and declining; in each, the vertical axis carries age classes in five-year bands from 0–4 at the bottom to 80+ at the top, males plotted leftwards and females rightwards from a central line, horizontal axis showing percentage of total population; the first pyramid broad-based and triangular, the second with near-vertical sides, the third narrow-based and urn-shaped
Reading Fig. 3.1-2 is a skill worth practising. Look first at the base relative to the class immediately above it: that single comparison tells you the direction of change. Then look at the middle, where a sudden narrowing can mark a past famine, epidemic, war or a wave of out-migration. Then look at the two sides against each other, since a marked imbalance between males and females in the young or working ages points either to sex-selective processes or to labour migration.
The figures in Table 3.1 are illustrative rather than drawn from a particular country, but the contrast is a real one. In Population A, close to two-fifths of everyone alive has not yet reached child-bearing age, so births will stay high for at least a generation and the pressure will fall on schools, on housing and on the creation of new jobs. In Population B, the elderly outnumber the elderly of Population A almost five to one in percentage terms, and the pressure falls instead on pensions, on hospitals and on finding enough workers. Both populations happen to have a similar working-age share — which is why the working-age figure alone tells you very little without the two classes on either side of it.
3.1.6 Survivorship Curves
Age structure describes a population at one moment. A survivorship curve describes what happens to a single group of individuals born at the same time — a cohort — as it ages, by plotting the number still alive against age.
The vertical axis is conventionally drawn on a logarithmic scale, usually as survivors per 1000 born. The reason is practical. Survival falls by factors, not by fixed amounts — a species may lose nine-tenths of its young in the first weeks — and a logarithmic scale turns a constant proportional loss into a straight line, which makes the three patterns instantly distinguishable. If a logarithm is unfamiliar, think of it this way: on such a scale, equal distances up the axis represent equal multiplications, so 10, 100 and 1000 are equally spaced.
Three general shapes are recognised.
Type I is a convex curve. Mortality is low through infancy and adult life, and rises sharply only in old age, so the curve stays high and then drops steeply near the end. Species showing this pattern produce few offspring and invest heavily in each — large mammals, including humans in societies with good nutrition and medical care, and elephants.
Type II is a diagonal straight line on the logarithmic scale. A roughly constant proportion of survivors dies in each age interval, so the risk of dying does not depend much on age. Many birds, several rodents and hydra approximate this pattern.
Type III is a concave curve. Mortality is enormous in the earliest stages and falls sharply for the few that survive them, so the curve plunges at the left and then flattens. Species producing very large numbers of small, unprotected offspring show this shape — most fish, oysters, many insects, and trees that release thousands of seeds.
Diagram
Fig. 3.1-3 – three survivorship curves plotted on one graph; x-axis relative age as percentage of maximum lifespan, 0 to 100; y-axis number of survivors per 1000 born on a logarithmic scale from 1 to 1000; Type I drawn convex staying high then falling steeply at the right, Type II a straight diagonal line, Type III concave falling steeply at the left then flattening; each curve labelled with its type and one example species
Real species rarely match a type exactly. Many fall between two of them, and some change shape when conditions change. The value of the three curves is that they let you infer a species’ reproductive strategy from its pattern of dying, and that inference is where this section is heading.
3.1.7 r Strategies and K Strategies
Behind those curves lies a trade-off that no species escapes. The energy available to an organism can be spent on producing many offspring or on caring for a few, but not on both. Ecologists describe the two ends of this trade-off as r strategies and K strategies, borrowing the symbols from the growth equations you will meet in section 3.3: \(r\) stands for the intrinsic rate of natural increase, and \(K\) for the number of individuals a habitat can support in the long run.
r strategists are species selected for a high rate of increase. They mature early, are usually small-bodied and short-lived, breed once or a few times, and produce very large numbers of small offspring with little or no parental care. Most of those offspring die, giving Type III survivorship. These species do well in disturbed, unpredictable or newly opened habitats — a freshly ploughed field, a burnt patch of grassland, a puddle after rain — where the advantage goes to whoever colonises fastest. Their populations tend to boom and crash rather than settle.
K strategists are species selected for competitive ability near the ceiling of what the habitat can support. They mature late, are usually larger and longer-lived, breed repeatedly, and produce few, large offspring with substantial parental care. Most offspring survive, giving Type I survivorship. These species do well in stable, crowded, predictable habitats — the interior of an old forest, a coral reef — where the advantage goes to whoever competes best for a scarce resource rather than to whoever arrives first.
This distinction has direct consequences for conservation, which is why it appears in a chapter that ends with forests and tiger reserves. A species at the K end recovers slowly from any loss, because it cannot breed fast enough to replace a sudden decline. A poached population of large mammals therefore needs decades of protection to rebuild, while a heavily fished r-strategist species may rebound within a few seasons once fishing stops — provided its habitat is intact.
3.2 Human Populations: Carrying Capacity, Malthus and Demographic Transition
3.2.1 Carrying Capacity
Every habitat has a ceiling. A pond can support only so many fish, a grassland only so many grazing animals, a district only so many wells drawing on the same aquifer. That ceiling is set not by the resource present on one good day but by the resource the habitat can go on supplying, season after season, without being run down.
Ecologists call this ceiling the carrying capacity of the environment, and give it the symbol \(K\). It is measured in the same unit as population size — number of individuals — and it applies to a named species in a named area.
Two words in that definition carry the weight. “Indefinitely” rules out the numbers a habitat can feed for a single good season. “Without degradation” rules out numbers that are only reached by consuming the resource faster than it renews itself.
This is why a population can rise well above its carrying capacity for a while, and why doing so is dangerous rather than impressive. When more individuals feed on a resource than it can regenerate, the resource itself shrinks — and a smaller resource base means a lower carrying capacity than before. The population has not merely overshot the ceiling; it has pulled the ceiling down. Recovery, if it happens at all, starts from a poorer environment.
Carrying capacity is not a fixed number engraved on a habitat. It rises when the resource base improves, through good rainfall or the growth of new vegetation, and falls when it deteriorates, through drought, erosion, salinity or overgrazing. For human beings it also shifts with technology and trade, a point taken up in the next two subsections.
3.2.2 The Malthusian View of Over-population and Resources
The oldest and most influential argument about human numbers was made by Thomas Malthus, an English clergyman and political economist, in his essay on population first published in 1798.
Malthus began from two claims he regarded as obvious. First, population, when unchecked, increases in a geometric progression — 1, 2, 4, 8, 16 — because each generation multiplies rather than adds. Second, the food supply can at best be increased in an arithmetic progression — 1, 2, 3, 4, 5 — because land is finite and each additional acre brought into cultivation is poorer than the last.
If those two claims hold, the conclusion follows unavoidably. Whatever the starting gap between people and food, a multiplying quantity must eventually overtake an adding one. Human numbers will therefore always press against the limit of subsistence, and the natural condition of most people will be scarcity.
Diagram
Fig. 3.2-1 – two lines on one graph; x-axis time in generations (0 to 6), y-axis units of population and units of food supply; a straight rising line labelled “food supply, arithmetic increase” passing through 1, 2, 3, 4, 5, 6; a steeply curving line labelled “population, geometric increase” passing through 1, 2, 4, 8, 16, 32; the point where the curve crosses the straight line marked “subsistence limit reached”
Malthus called the state in which numbers exceed the means of subsistence over-population. Note carefully what this term does and does not mean. It is not a statement about density, and not a judgement about any place being crowded. It is a relation between a population and its available resources, so a thinly settled dry region can be over-populated in this sense while a dense river valley is not.
Malthus then asked what actually prevents the gap from widening for ever, and answered that two kinds of check operate. Positive checks raise the death rate — famine, epidemic disease, and the ordinary attrition of poverty. Preventive checks lower the birth rate, and for Malthus these meant late marriage and restraint. His expectation was that where preventive checks were weak, positive checks would do the work instead.
3.2.3 Questioning Malthus
Two centuries later, the world’s population is many times larger than in 1798 and food produced per person is higher, not lower. Any serious account of population must explain why the prediction failed, and the answers fall into four groups.
The first concerns technology. Output per hectare has risen enormously through irrigation, mechanisation, chemical fertilisers and bred varieties. Food supply, in other words, has not behaved arithmetically at all.
The second reverses Malthus’s direction of cause. The economist Ester Boserup argued that population pressure is itself a driver of agricultural innovation: when land grows scarce relative to people, farmers shorten fallows, adopt more intensive methods, invest more labour per hectare and raise yields. On this view, more people can mean more food rather than less, at least within limits, because necessity forces the change.
The third concerns distribution. Modern famines have generally occurred in places where food was physically present in the region but where particular groups could not buy or obtain it, because wages collapsed, prices rose or entitlements failed. The economist Amartya Sen developed this argument in detail. If that is right, hunger is often a failure of access rather than of aggregate supply, and Malthus’s exclusive attention to totals misses the mechanism.
The fourth concerns fertility itself. Malthus expected people to have more children as they grew richer. Across almost every country that has industrialised and urbanised, the opposite has happened, and birth rates have fallen without famine and without coercion. This is the pattern described in section 3.2.5.
None of this makes the Malthusian argument worthless, and it would be a mistake to treat the debate as closed. The core insight — that a resource with a fixed renewal rate cannot support an indefinitely multiplying demand — remains exactly right, and applies with force to groundwater in over-drawn basins, to marine fisheries, to firewood around expanding settlements and to grazing on common land. Garrett Hardin’s much-discussed argument about commonly held resources, published in 1968, is a modern restatement of the same worry: where a resource is open to all and the cost of over-use is shared while the benefit is private, users have an individual incentive to take more than the system can bear. Critics of that argument point out that many commons around the world have in fact been managed sustainably for centuries by their user communities, through rules those communities made and enforced themselves.
The disagreement, then, is not about whether limits exist. It is about whether the binding limit is aggregate food, as Malthus held, or the distribution of access, the rate of technological change, and the institutions governing particular resources.
3.2.4 Population Growth and Disparate Consumption of Resources
There is a second reason the sheer count of people is a poor guide to environmental pressure. Two people do not make equal demands on the earth. What matters is population multiplied by per capita consumption — the average quantity of resources used, and waste generated, by each person in a year.
Consider two countries. Country X has a population of 30 million and consumes 60 million tonnes of a particular resource each year. Country Y has a population of 300 million and consumes 150 million tonnes.
Country Y’s total consumption is two and a half times Country X’s, so on a national ranking Y looks like the larger problem. But per person,
\[\text{Country X} = \frac{60}{30} = 2.0 \text{ tonnes per person per year}\]
\[\text{Country Y} = \frac{150}{300} = 0.5 \text{ tonnes per person per year}\]
Each resident of X uses four times as much as each resident of Y. If X’s population grew by a single million, the additional demand would be 2 million tonnes; the same million added to Y would add 0.5 million tonnes. The environmental effect of one more person depends entirely on which population that person joins.
Diagram
Fig. 3.2-3 – paired bar chart; left panel “total consumption, million tonnes per year” with two bars, Country X at 60 and Country Y at 150; right panel “consumption per person, tonnes per year” with two bars, Country X at 2.0 and Country Y at 0.5; both panels drawn to the same width so the reversal between the two panels is visible at a glance
This is what the syllabus means by disparate consumption of resources among nations. Countries with high incomes and small populations often account for a share of global resource use and waste far larger than their share of world population, while countries with large populations and low average incomes account for a share far smaller.
The same disparity exists inside nations, and this is easily overlooked. Within any country, an urban household with piped water, two vehicles, air conditioning and frequent replacement of appliances consumes many times what a rural household without regular electricity consumes. Averages at the national level conceal this completely. When a country’s per capita figure rises, it may mean that everyone is consuming somewhat more, or that a small section is consuming a great deal more — and the environmental and equity implications of those two cases are entirely different.
3.2.5 Demographic Transition
The most useful description of how human populations actually change is not a prediction at all but a summary of what has been observed. Across country after country, as societies have industrialised and urbanised, death rates and then birth rates have fallen, in that order, and total population has risen steeply in between. This sequence is called the demographic transition.
The model is usually drawn in four stages.
In the first stage, high stationary, both birth and death rates are high, roughly 35 to 45 per 1000 per year, and both fluctuate with harvests and epidemics. Natural increase is small and irregular, so population is nearly stationary over the long run. This was the condition of most of the world before industrialisation.
In the second stage, early expanding, the death rate falls sharply while the birth rate stays high. Nothing about family behaviour has changed yet, but far more of the children born now survive. The gap between the two curves is at its widest, so population grows fastest in this stage. Almost all the growth in world population over the past two centuries has come from societies passing through it.
In the third stage, late expanding, the birth rate begins to fall, slowly at first and then rapidly, while the death rate continues to decline gently and then levels off at a low value. The gap narrows, so the population is still growing, but growth is decelerating.
In the fourth stage, low stationary, both rates are low — commonly 10 to 15 per 1000 per year — and close together. Natural increase is small once more, and the population is nearly stationary, but now at a much larger size and with a much older age structure than in the first stage.
Some demographers add a fifth stage, in which the birth rate falls below the death rate and the population declines without migration. Several countries are now in this condition, which is why the four-stage model is best read as a description of what has happened rather than a law about what must happen.
Diagram
Fig. 3.2-2 – demographic transition model; x-axis time (not to scale), left y-axis birth rate and death rate in births or deaths per 1000 population per year, scale 0 to 50, right y-axis total population size; birth rate drawn as a solid line staying high through stages 1 and 2 then falling steeply through stage 3 and levelling low in stage 4; death rate drawn as a dashed line high in stage 1, falling steeply through stage 2, levelling low from stage 3; the area between the two lines shaded and labelled “natural increase”; a third line showing total population rising slowly, then steeply, then flattening; four vertical divisions labelled Stage 1 high stationary, Stage 2 early expanding, Stage 3 late expanding, Stage 4 low stationary
Read Table 3.3 across the rows rather than down the columns, and one point stands out. Rapid population growth is not a permanent condition of any society; it is a stage — the interval between the fall in deaths and the fall in births. The longer that interval lasts, the larger the population becomes before it stabilises.
3.2.6 Factors Influencing Demographic Transition
The transition is not automatic, and it does not run on a timetable. It happens because particular things change in people’s lives, and the two declines have quite different causes.
Why death rates fall
The fall in mortality is largely a public-health and food-supply story, and much of it precedes any large rise in incomes. Clean drinking water and the separation of sewage from water supply remove the great water-borne killers. Immunisation removes several of the childhood infections that once carried off a large fraction of every cohort. Antibiotics and oral rehydration treat what would otherwise be fatal. More reliable food supply, helped by transport that can move grain into a district with a failed harvest, removes the famine mortality that used to punctuate the first stage.
The single most important indicator here is the infant mortality rate (IMR) — deaths of children under one year of age per 1000 live births. It falls earlier and further than any other component of mortality, and its fall is what drives Stage 2.
Why birth rates fall
The fall in fertility is slower because it depends on decisions inside millions of households, and those decisions change only when the circumstances behind them change.
Child survival matters first. Where many children die young, families have more children partly to ensure that some survive. When infant mortality falls and stays low, that reason weakens — but only after families have seen it fall for long enough to trust it. This lag is precisely why births fall after deaths rather than alongside them.
The economic position of children matters next. On a farm, children contribute labour from an early age. In a town, children attend school for many years, and the cost of raising each one — schooling, healthcare, housing space — rises sharply while their contribution to household income falls. Urbanisation therefore tends to lower fertility.
Education matters, and female education matters most consistently of all. Across a very wide range of countries, the number of children a woman has falls as her years of schooling rise. Education raises the age of marriage, improves knowledge of and access to health services, and widens the alternatives available to her outside the household.
Women’s employment outside the home works in the same direction, both by raising the opportunity cost of leaving work to raise children and by increasing women’s say in household decisions.
Old-age security matters too. Where there is no pension and no institutional support, adult children are the only reliable provision for old age, and that is a powerful reason to have several. Where such support exists, the reason weakens.
Finally, access to health services that allow families to plan their size is a necessary condition. A family that wants fewer children can only have fewer if the means to space and limit births are available and reachable. Where the desire has changed but the services have not, fertility falls slowly and unevenly.
A useful summary measure of all this is the total fertility rate (TFR) — the average number of children a woman would bear over her lifetime at current age-specific fertility rates. A TFR of about 2.1 is called replacement level fertility: enough for a couple to replace themselves, with a small allowance for children who do not survive to adulthood. Below that level, a population will eventually decline once momentum works itself out — though, as section 3.1.4 explained, “eventually” can mean several decades.
3.3 Population Regulation
3.3.1 Growth Without Regulation: The Exponential Curve
Imagine a population placed in an environment with unlimited food and space, no predators and no disease. Nothing removes individuals except ordinary ageing, and nothing restrains reproduction. What shape would its growth take?
Each individual contributes a fixed surplus of births over deaths per unit time. That surplus is the intrinsic rate of natural increase, written \(r\), measured per individual per year and characteristic of the species. The number of individuals added to the whole population in a year is therefore \(r\) multiplied by the number present:
\[\frac{dN}{dt} = rN\]
where \(N\) is the number of individuals, \(t\) is time in years, \(r\) is the intrinsic rate of natural increase per individual per year, and \(\frac{dN}{dt}\) is the number of individuals added per year. You are not required to solve this expression; what matters is reading what it says. The rate of addition is proportional to the number already present.
That single feature produces the whole shape of the curve. Take a species with \(r = 0.02\) per individual per year. When the population is 5,000, it adds \(0.02 \times 5000 = 100\) individuals a year. When it has reached 50,000, the same \(r\) adds \(0.02 \times 50{,}000 = 1000\) individuals a year — ten times as many, from an unchanged birth and death behaviour. The population is not reproducing any faster per individual; there are simply more individuals doing it.
This is exponential growth, and plotted against time it gives a curve that rises slowly at first, then bends upward and climbs almost vertically. Because of that shape it is called the J-shaped curve.
Diagram
Fig. 3.3-1 – exponential growth curve, J-shaped; x-axis time in years, y-axis population size N in number of individuals; curve starting low and nearly flat, bending upward and rising almost vertically at the right; no upper limit line drawn; a note on the curve marking that the slope at any point is proportional to N
The early flatness of the J-curve deceives observers constantly. Look at Fig. 3.3-1 and notice that the population takes a long time to become visibly large, and then becomes very large very quickly. Nothing changed at the moment of the upturn. The rate was the same throughout; only the base it acted on had grown.
Exponential growth is real but temporary. It is seen when a species colonises a new area with abundant resources — bacteria in fresh culture medium, an insect pest in a newly planted field, a species introduced onto an island. It cannot continue, because no environment supplies resources without limit. A J-curve in nature always ends: either it bends over into the pattern described next, or it collapses.
3.3.2 Simple Regulation: The Logistic Curve
Real populations grow in real environments, where food is finite, space runs out and waste accumulates. As numbers rise, each individual gets less. Births per individual fall, deaths per individual rise, and the surplus \(r\) shrinks. The sum of all these limiting influences is called environmental resistance.
The simplest way to represent this is to let the growth rate depend on how much of the environment’s capacity is still unused. If the carrying capacity is \(K\) and the population is \(N\), the unused fraction is \(\frac{K-N}{K}\), and the growth equation becomes
\[\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)\]
with \(N\), \(t\) and \(r\) as before, and \(K\) the carrying capacity in number of individuals. Read the bracketed term slowly, because it does all the work. When \(N\) is very small, \(\frac{K-N}{K}\) is close to 1, the bracket has almost no effect, and growth is nearly exponential. When \(N\) is half of \(K\), the bracket is 0.5 and growth runs at half the unrestrained rate — this is where the population adds individuals fastest in absolute numbers. When \(N\) approaches \(K\), the bracket approaches 0 and growth almost stops. If \(N\) were ever to exceed \(K\), the bracket would turn negative and the population would decline.
This is logistic growth, and its curve is the S-shaped curve or sigmoid curve, with three recognisable phases. The lag phase is the slow start, when numbers are too few for growth to be visible. The exponential phase is the steep middle section, where resources are still ample relative to numbers. The plateau phase is the flattening at the top, where the population levels off near the carrying capacity and fluctuates gently around it.
Diagram
Fig. 3.3-2 – logistic growth curve, S-shaped; x-axis time in years, y-axis population size N in number of individuals; carrying capacity K drawn as a horizontal dashed asymptote near the top; the curve rising slowly, then steeply, then flattening towards the dashed line without crossing it; the three phases labelled lag phase, exponential phase and plateau phase; the point of steepest slope marked at N equal to half of K
The logistic model is a simplification, and its assumptions are worth noticing. It treats carrying capacity as a fixed number, when in reality \(K\) shifts from year to year with rainfall and vegetation. It assumes the population responds instantly to its own density, when in fact many species respond with a delay — an animal born in a good season breeds a year later, when conditions may have changed. Where such a delay is long, the population regularly runs past the ceiling before the brakes take effect.
That failure has a name. Overshoot is the rise of a population above the carrying capacity of its habitat, and it is followed either by a decline back towards the ceiling or, where the resource base itself has been damaged, by a crash to a level far below the original one.
Diagram
Fig. 3.3-3 – overshoot and crash curve; x-axis time in years, y-axis population size N in number of individuals; carrying capacity K drawn as a horizontal dashed line; the curve rising steeply past the dashed line, peaking well above it, then falling abruptly to near zero; a second dashed line drawn lower than the first and labelled “reduced carrying capacity after damage to the resource base”
Compare Fig. 3.3-3 with Fig. 3.3-2 and the difference between a regulated and an unregulated population becomes visible in a single glance. Both populations faced the same ceiling. One approached it gradually and settled; the other ran through it.
3.3.3 Factors Regulating Population Size
What actually applies the brakes? Environmental resistance is not one force but many, and the syllabus lists them individually. It is useful to notice as you read them that some act more strongly as the population grows denser, while others strike with the same severity whatever the density.
Space. Every species needs room — for nesting, rooting, hunting or shelter. As numbers rise, the space available per individual falls, and beyond a point individuals cannot establish themselves at all. In plants this shows as self-thinning, where seedlings germinate in thousands and only those with room to spread their roots and leaves survive.
Food and water. These are the most direct limits. As density rises, each individual obtains a smaller share, growth slows, adults become lighter, females produce fewer or weaker young, and the weakest die first. The reindeer of St Matthew Island were visibly smaller before the herd declined, which is the classic signature of a food supply stretched too thin.
Territories. Many animals, particularly birds and mammals, defend an exclusive area against others of their own species. Territoriality converts a gradual squeeze into a sharp cut-off: once every viable territory in a habitat is occupied, additional individuals are excluded altogether. They do not share the space; they are pushed into poorer ground where they usually fail to breed. Territoriality is one of the most effective natural limits on breeding numbers.
Predators. A predator’s population usually rises in response to an abundant prey population, and falls when prey become scarce, producing the linked oscillations seen in many predator–prey systems. Predation typically removes the young, the old and the sick, and takes a larger proportion of prey when prey are dense and easy to find.
Weather and climate. An unusually cold winter, a failed monsoon, a heatwave or a prolonged drought can kill a large fraction of a population regardless of how many individuals there are. Weather sets the outer bounds within which the other factors operate, and for many insect populations it is the dominant control.
Parasites and diseases. Infection spreads by contact, so its effect strengthens sharply with density. A dispersed population may carry a pathogen at low levels indefinitely; the same pathogen in a crowded population can produce an epidemic. This is why disease is often the factor that finally halts a population that has been growing for a long time.
Disasters. Floods, forest fires, cyclones, landslides and volcanic events remove individuals in large numbers and without reference to density. A population of a hundred and a population of a hundred thousand in the same valley face the same flood.
Self-regulation. Some species reduce their own reproduction before external resources are exhausted. Under crowding, animals may show delayed maturity, smaller litters, reabsorption of embryos, abandonment of young, increased aggression and increased dispersal away from the crowded area. These responses are triggered by density itself, often through hormonal changes brought on by stress. Self-regulation slows growth in advance of scarcity rather than in response to it, and where it operates it can hold a population below the point at which the habitat would be damaged.
Table 3.5 explains something the logistic curve alone cannot. A population is steered towards its carrying capacity by density-dependent factors, and knocked about around that path by density-independent ones. Which of the two dominates varies with the species: large, long-lived animals in stable habitats are mostly regulated by density-dependent factors, while small, short-lived species in harsh or variable environments are largely at the mercy of weather.
3.4 Human Population Control
3.4.1 Why Population Control Is Attempted
Section 3.2 established that rapid growth is a stage rather than a permanent condition — the interval between the fall in death rates and the fall in birth rates. But that interval can last a long time, and while it lasts a country adds people faster than it can build schools, hospitals, housing and water supply for them. Every year of delay in completing the transition leaves the population larger when it finally stabilises.
Population control refers to the deliberate attempt to influence the rate at which a population grows, usually by lowering fertility. It is worth being clear about what it is not. It is not the same as the natural regulation described in section 3.3, which happens to a population whether or not anyone intends it; and it is not, in any defensible form, a matter of deciding how many people ought to exist. In modern practice it means enabling families to have the number of children they themselves want, at the time they want them — which in most societies turns out to be fewer than they currently have.
The case for such policies rests on three observations. Growth in Stage 2 is fast enough to outrun the expansion of public services. Population momentum means that a delay of a decade in beginning the fertility decline adds substantially to the eventual total. And a larger population raises total demand on a fixed resource base — water in a catchment, land in a district — even when per-person consumption stays low.
The case against treating numbers as the central problem is equally worth stating. As section 3.2.4 showed, environmental pressure is population multiplied by consumption per person, and in many settings the second term is doing more of the work than the first. Fertility has fallen substantially in most parts of the world without any policy directed at it, simply as living conditions changed. And programmes that have relied on numerical targets and pressure on health workers have a poor record: they have damaged people’s trust in public health services, deterred families from using them for other purposes, and produced less durable fertility decline than programmes built on education and health care. This is why the international consensus since the 1990s has been that population policy works through voluntary, informed choice or it does not work well at all.
3.4.2 Education, Economic Growth and Family Planning
The strategies that have actually accompanied falling fertility are the same conditions that section 3.2.6 identified as driving the second half of the demographic transition. This is not a coincidence: population policy succeeds when it accelerates a transition that is already under way for other reasons.
Education
Schooling lowers fertility through several channels at once. It delays marriage, because a girl in school is not being married. It raises the value of a person’s time, so the cost of a large family rises. It improves the ability to use health services and to act on information. And it changes expectations about what children themselves will need — parents who expect to educate each child tend to plan for fewer.
The effect is strongest for girls, and it is one of the most consistent relationships in demography. Across countries, across states within a country and across districts within a state, the number of children per woman falls as female schooling rises.
Diagram
Fig. 3.4-1 – scatter graph with a fitted downward-sloping line; x-axis female literacy rate as a percentage, scale 0 to 100; y-axis total fertility rate in children per woman, scale 0 to 6; points scattered along a clear downward trend; a horizontal dashed line drawn at 2.1 on the y-axis and labelled “replacement level”
The figures in Table 3.6 are illustrative rather than drawn from named districts, but the pattern they show is real and repeatedly observed. Read across the rows and notice that the third column and the fourth move together: as the mean age at marriage rises by about six years between the first district and the last, fertility falls by more than half. Part of the effect of education is simply that it removes years from the child-bearing span, and part is that it changes what happens within the remaining years.
Notice also what the table does not prove. Districts with high female literacy usually also have better health services, higher incomes and more urban employment, and all of these lower fertility as well. A correlation of this kind shows that the factors travel together; it does not by itself establish which of them is doing the work.
Economic growth
Rising income lowers fertility indirectly, and mainly by changing the reasons for having children. Where incomes rise, child survival improves, so fewer births are needed to end up with a surviving family. Employment moves out of household farming and into work where children cannot contribute, so children stop being a source of labour and become a cost. Provision for old age becomes possible through savings, pensions or institutional support, so adult children stop being the only security available. Growth also pays for the schools and health centres through which the other strategies operate.
The relationship is not automatic. Growth that leaves incomes concentrated in a small section of the population, without spreading schooling and health care, has a much weaker effect on fertility than growth of the same size that is broadly shared.
Family planning services
Family planning means enabling couples to decide freely how many children to have and when to have them, and providing the health services and information that make that decision effective. None of the above translates into smaller families unless families have the means to act on their intentions. The essential requirements are that services be available within reach, that they be affordable, that they be delivered without pressure or coercion, and that couples receive accurate information and choose freely. Where the desire for smaller families has changed but services have not reached people, fertility falls slowly and unevenly; where services exist but the underlying conditions have not changed, uptake stays low. The two work together or not at all.
3.4.3 The Status of Women and Women’s Empowerment
Everything in the previous subsection converges on a single point, and this is where the syllabus places its emphasis. The most consistent predictor of fertility decline is the position of women — their schooling, their health, their age at marriage, their access to paid work, their control over household resources, and their say in decisions about their own lives.
Women’s empowerment means the expansion of women’s ability to make and act on choices that affect them, including choices about marriage, childbearing, education and work.
Why does it matter so much for population? Because childbearing is not a decision made by a household in the abstract. It is borne physically by women, and its costs in health, in time and in forgone opportunity fall on them. Where women have little say, the number of children reflects other people’s preferences and the family’s demand for labour and security. Where women’s own preferences carry weight, family sizes fall — usually towards the number women say they want, which surveys repeatedly find to be smaller than the number they have.
The specific mechanisms are worth naming, because “empowerment” is otherwise a vague word.
Schooling raises the age at marriage and with it the age at first birth, which shortens the child-bearing span and improves outcomes for both mother and child.
Paid work outside the household raises the opportunity cost of continuous childbearing and increases a woman’s bargaining position within the family, including her ability to spend on her children’s health and education.
Health care during pregnancy and childbirth reduces maternal mortality, which in turn makes small families a safer proposition rather than a risky one.
Legal rights — to inherit property, to hold land and accounts in one’s own name, to marry only at or above the legal age — change the conditions under which all the above operate.
Representation in local decision-making changes what a village or ward spends on: studies of local government in India have found that where women hold decision-making positions, allocations to water supply, sanitation and schooling tend to rise.
There is an important consequence for how such policies should be judged. Every one of these measures is worth pursuing for its own sake, quite apart from its effect on birth rates. A girl’s schooling, a woman’s health care and a woman’s legal rights are not instruments of population policy; they are entitlements. That they also happen to bring fertility down is a strong argument for pursuing them, but it is not the reason they are owed.
3.5 Threats to the Ecosystem
3.5.1 What the Ecosystem Provides and Regulates
Before asking what threatens an ecosystem, it is worth being precise about what is at risk. An ecosystem is a community of living organisms together with the physical environment they occupy, exchanging energy and materials. What human beings obtain from it is usually grouped under the term ecosystem services, and two groups of these matter for this section.
Provisioning functions are the tangible things an ecosystem supplies: food, fodder, fuelwood, timber, fibre, medicinal plants, freshwater, and the genetic material from which new crop varieties are bred. These are easy to see and easy to count, because most of them are traded.
Regulating functions are the processes that keep conditions within the range organisms can tolerate: the moderation of floods by soil and vegetation, the recharge of groundwater, the holding of soil against erosion, pollination of crops by insects and birds, the breakdown of organic waste, the cycling of nitrogen and phosphorus, the absorption of carbon dioxide, and the buffering of local temperature. These are invisible until they fail, and almost none of them is traded — which is precisely why they are so easily lost.
The threats that follow are grouped by cause, but each of them should be read against Table 3.7 with a single question in mind: is this damaging what the ecosystem supplies, or what it regulates, or both?
3.5.2 Habitat Destruction
Habitat destruction is the conversion of a natural habitat to some other use, so that the species living there can no longer complete their life cycles. It is the largest single cause of species loss worldwide.
The causes are almost all conversions of land for human use: clearing forest for cropland, draining wetlands for building, quarrying and mining, laying roads, canals and pipelines, and the outward spread of towns. In each case a habitat is not damaged so much as replaced.
Total destruction is only part of the problem. Habitat fragmentation — the breaking of a continuous habitat into smaller separated patches by roads, railway lines, canals, plantations and fields — often does as much harm as outright clearance, even though a satellite image may show that most of the vegetation is still standing.
Fragmentation harms in three ways. Each patch is too small to hold a viable population of large or wide-ranging species, which need extensive territory. The patches are isolated, so animals cannot move between them to breed, and a population lost from one patch is not replaced from another. And the proportion of edge to interior rises sharply, which matters because the edge of a forest is hotter, drier, windier and more exposed to fire, weeds and human disturbance than its interior. Species that require deep-forest conditions therefore disappear from a fragment long before the last tree is cut.
Diagram
Fig. 3.5-1 – habitat fragmentation, drawn as three panels in sequence; panel 1 a single continuous shaded forest block; panel 2 the same block crossed by a road and a canal, dividing it into four unequal patches; panel 3 the four patches reduced further with cultivated land between them; in each panel the interior area shaded dark and a band of “edge habitat” shaded light along every boundary, with a note that total forest area falls modestly from panel 1 to panel 3 while interior area falls steeply
3.5.3 Genetic Erosion
A species is not a uniform stock. It is a gene pool — the total collection of genes and their variants present in all its individuals — and that variation is what allows a population to respond to a new disease, a hotter season or a changed food supply. Some individuals carry the combinations that survive the change, and the population continues through them.
Genetic erosion is the loss of genetic variation from a population or a species. It occurs when populations become small and isolated, when local varieties are replaced by a few uniform ones, and when the individuals that die carry variants found nowhere else.
Two consequences follow, and both are slow. A small, isolated population breeds within itself, and closely related parents produce offspring with reduced vigour, lower fertility and weaker resistance to infection. And the species as a whole loses the raw material of adaptation, so that a change it could once have survived becomes fatal.
Genetic erosion is closely tied to fragmentation, because fragments hold small populations that cannot exchange individuals. It also occurs in agriculture, where hundreds of locally adapted varieties of a crop have been replaced across large areas by a handful of uniform high-yielding ones — a change that raises output while narrowing the base from which future varieties must be bred.
3.5.4 Loss of Diversity
Biodiversity is the variety of life at three levels: the variety of genes within a species, the variety of species within a community, and the variety of ecosystems within a region. Loss at any level is a loss of diversity, though the species level is what people usually mean by the phrase.
Diversity is not spread evenly over the earth. It rises towards the equator and falls towards the poles, and it is higher where temperature and rainfall are high and stable, where the terrain offers many different conditions within a small area, and where habitats have remained undisturbed for long periods. This is why tropical forests, coral reefs and the wet slopes of mountain ranges hold a disproportionate share of the world’s species — and why damage concentrated in those places removes more diversity per hectare than damage elsewhere.
The consequences run straight into Table 3.7. A community with fewer species captures and cycles nutrients less completely, produces less biomass on average, breaks down organic matter more slowly, and purifies water less effectively. It is also less stable over time, because a diverse community contains several species capable of performing the same function, so the failure of one is compensated by another. A simplified community has no such reserve: when its single pollinator, or its single decomposer, is lost, the function stops. The regulating side of the table degrades first, and quietly.
3.5.5 Expanding Agriculture
Agriculture occupies more of the earth’s habitable land than any other human activity, and its expansion has been the largest single driver of habitat destruction. But the threat is not only about area; it is also about what happens on the land already farmed.
Farmland carries two kinds of diversity. Planned diversity is what the farmer deliberately grows — the crops, the trees on the bunds, the livestock. Associated diversity is everything that arrives uninvited — soil organisms, pollinating insects, birds, weeds, herbivores and pathogens. Traditional mixed farming maintained a good deal of both.
Monoculture — the growing of a single crop variety over a large area, season after season — reverses this. It removes planned diversity by definition, and it eliminates associated diversity deliberately, through herbicides and pesticides. The gains are real: uniform fields can be sown, treated and harvested by machine, and yields per hectare rise.
The costs run through both columns of Table 3.7. A genetically uniform crop over a large area is an ideal target for a pest or pathogen that can overcome its single resistance, so outbreaks spread rapidly and pesticide use climbs. Continuous cropping of one plant draws the same nutrients from the same soil depth year after year. Pesticides applied broadly kill pollinators and the natural enemies of pests along with the pests themselves, removing a regulating function the farmer was previously getting free. And the replacement of many local varieties by a few uniform ones is genetic erosion of the kind described above, taking place on the world’s most important species.
3.5.6 Impounding Water
To impound water is to hold back a flowing river behind a barrier and store it in a reservoir. Dams and barrages are built for irrigation, for drinking water, for flood control and for hydroelectric power, and they deliver those benefits substantially. They also alter a river ecosystem more comprehensively than almost any other single intervention.
Upstream, a stretch of flowing-water habitat is converted into a standing-water one. The species adapted to fast, oxygenated, gravel-bottomed water are replaced by species adapted to still water, and a large area of land, often including forest and farmland, is submerged.
The barrier itself blocks movement. Fish that migrate upstream to breed cannot reach their spawning grounds, and populations dependent on that migration decline sharply even though the water itself is unpolluted.
Downstream, the river receives less water and receives it on a schedule set by the dam’s operators rather than by rainfall. It also receives much less sediment, because silt settles in the reservoir instead of travelling on. That silt was doing two jobs: renewing the fertility of floodplain soils, and replenishing deltas and coasts against erosion. Both jobs stop. Meanwhile the reservoir slowly fills with the trapped sediment, reducing the storage it was built to provide.
Two further effects deserve mention. Where reservoir water is applied to land without adequate drainage, the water table rises and salts accumulate in the root zone, producing waterlogging and salinity that can take land out of cultivation. And in warm, nutrient-rich reservoirs, algal growth can be heavy, reducing dissolved oxygen when the algae die and decay.
Diagram
Fig. 3.5-2 – flow: catchment rainfall → river flowing with sediment and migrating fish → dam and reservoir (submerged land upstream, sediment settling, fish migration blocked) → regulated release downstream (reduced flow, sediment-poor water) → floodplain (loss of soil renewal) → delta and coast (erosion); each stage labelled with the function lost at that stage
3.5.7 Waste from Human Societies
Every settlement produces waste, and an ecosystem can absorb a certain quantity of it. Soil organisms break down organic matter, rivers dilute and oxidise a load of sewage, and the atmosphere disperses smoke. Waste becomes a threat when its quantity exceeds that capacity, or when its composition is something no organism can break down.
Organic waste — sewage, food waste, agricultural residue — is biodegradable, but only if the receiving system is not overloaded. When a large volume of sewage enters a river, the bacteria that decompose it consume dissolved oxygen faster than the river can replace it. Fish and other oxygen-dependent life die, not from the sewage itself but from the oxygen it consumes. Where sewage and fertiliser runoff supply large quantities of nitrogen and phosphorus, the resulting algal growth blooms and then decays, consuming oxygen in the same way. This nutrient enrichment of a water body, and the oxygen depletion that follows it, is called eutrophication.
Non-biodegradable waste behaves differently: it accumulates. Plastics, glass, construction rubble and metals persist in soil and water for decades or centuries. Plastic in particular breaks into smaller and smaller fragments without breaking down chemically, and is ingested by animals from soil invertebrates to cattle to marine species.
Toxic waste is a third category, and the most serious per unit of volume. Heavy metals, pesticide residues and industrial effluents are not merely persistent; they are absorbed by organisms and passed along food chains, becoming more concentrated at each level. Electronic waste, dismantled for its recoverable metals, releases lead, cadmium and mercury into the soil and air of the places where that work is done.
The threat here is to both columns of Table 3.7 at once. Provisioning fails when a river’s fish die or a shallow aquifer becomes unfit to drink. Regulating fails when the decomposer community itself is poisoned, so the system loses its capacity to process the next load.
3.5.8 Increasing Human Consumption
The threats above are not seven independent problems. They are the outlets through which a single rising quantity finds expression: the total demand human societies place on the biosphere, which is population multiplied by consumption per person.
The second term has been rising faster than the first for several decades. More of what people use is produced far from where they live, so the environmental cost of a purchase is borne in a place the purchaser never sees. Products are replaced more frequently than they are repaired, so each unit of use requires more extraction. And the material intensity of ordinary life — the metals in a phone, the water in a cotton shirt, the fuel behind a delivery — has risen even where the visible quantity of goods has not.
The consequence is that demand can outrun regeneration even where populations are stable or falling. Fish are taken faster than stocks reproduce. Groundwater is pumped faster than aquifers recharge. Timber is cut faster than forests regrow. Carbon dioxide is released faster than forests and oceans absorb it. Each of these is the overshoot described in section 3.3.2, applied not to one species on one island but to particular resources across whole regions — and it produces the same result, a reduced capacity to supply, arrived at through a period of apparent abundance.
This is where a chapter on populations turns necessarily into a chapter on conservation. If the resource base can be drawn down faster than it renews, then protecting some part of it deliberately, and managing the rest, is not a sentimental preference but the only alternative to running the ceiling down.
3.6 Conservation
3.6.1 What Conservation Means
Section 3.5 described what damages an ecosystem. This section deals with the deliberate response to that damage.
Conservation is the protection, careful management and sustainable use of natural resources and of the species and habitats that make them up, so that they continue to be available in the future. It is not the same as preservation, which means leaving something entirely untouched. Conservation accepts that people use forests, water, soil and wildlife, and asks how that use can be kept within what the resource can sustain — the carrying-capacity reasoning of section 3.2.1 applied to management.
Conservation of a species can be attempted in two quite different places: where the species lives, or somewhere else that has been made safe for it. This is the basic division of the whole field.
In situ conservation — the Latin in situ means “in the original place” — is the conservation of species within their natural habitats, by protecting and managing the habitat itself.
Ex situ conservation — ex situ means “out of the original place” — is the conservation of species outside their natural habitats, in facilities where they can be maintained and bred.
Diagram
Fig. 3.6-1 – classification tree; top box “Conservation” branching into two boxes, “In situ (in the natural habitat)” and “Ex situ (outside the natural habitat)”; under In situ, three boxes: wildlife sanctuaries, national parks, biosphere reserves; under Ex situ, three boxes: zoos, aquaria, plant collections including botanical gardens and seed banks
3.6.2 The Importance of Conservation
Why conserve at all? Four kinds of reason are usually given, and it is worth separating them, because they justify different amounts of effort in different places.
The first is that ecosystems supply the provisioning and regulating functions set out in section 3.5.1, and those functions have no cheap substitute. A forested catchment that recharges groundwater and holds soil is doing work that would otherwise have to be done by dams, canals and desilting — at very much higher cost, and less well.
The second is the value of genetic material. Wild relatives of crops and the many locally adapted varieties still grown by farmers are the stock from which resistance to future pests and tolerance of future climates must be bred. Once a variant is gone it cannot be recovered, so conserving it is insurance against a risk whose form is not yet known.
The third is livelihood. Large numbers of households obtain fuelwood, fodder, fruit, honey, medicinal plants, bamboo, fibre and grazing from forests and common lands, and these are the households least able to buy substitutes when the resource disappears. Conservation that ignores this simply shifts the cost onto them, which is why it so often fails.
The fourth is ethical and cultural. Many people hold that species have a claim to exist independent of their usefulness, and many communities attach religious or cultural value to particular groves, trees, hills and animals. These reasons are not measurable in the way the first three are, but they are among the strongest motives that have actually protected habitats over long periods.
3.6.3 In-Situ Conservation
In situ conservation is the primary strategy, for a straightforward reason: a species conserved in its habitat continues to interact with the other species around it, continues to be subject to natural selection, and continues to occupy its place in the flow of energy and nutrients. It also protects thousands of species that no one has separately decided to protect. Its instrument is the protected area, and the three most important kinds are described below.
3.6.3.1 Wildlife Sanctuaries
A wildlife sanctuary is an area set aside for the protection of wildlife, where the killing, hunting and capture of animals is prohibited but certain regulated human activities are permitted.
Its objectives are to protect species and their habitat while allowing the area to remain in some use by people who depend on it. Its characteristic feature is precisely this flexibility: grazing, collection of specified minor forest produce, and rights of way may be allowed under regulation, and some private landholding may continue inside the boundary. Sanctuaries are usually declared for a particular species or group of species, though everything within the boundary gains protection.
The advantages follow from the flexibility. A sanctuary can be declared over a large area without extinguishing existing rights, so it is quicker to create and produces less conflict with residents than a stricter category would. It protects the habitat as a whole, and it is compatible with the continued presence of people who have long lived there.
The disadvantages follow from the same source. Continued grazing, extraction and movement of people and livestock keep disturbance high, and can degrade the habitat gradually even though no animal is being hunted. Protection depends heavily on how well the regulations are enforced, and enforcement over a large, populated area is difficult.
3.6.3.2 National Parks
A national park is an area strictly reserved for the protection of wildlife and its habitat, in which grazing, cultivation, forestry, hunting and the extraction of forest produce are not permitted.
Its objective is the protection of an entire ecosystem in as undisturbed a state as management allows, along with its scenery and its scientific and educational value. Its defining features are exclusivity of purpose and stringency of protection: no private rights ordinarily survive inside a national park, no exploitation of resources is allowed, and entry is regulated, usually restricted to defined routes for visitors and researchers.
The advantage is a genuinely low level of disturbance, which is what species sensitive to human presence require, and a firm legal basis for excluding damaging activity. Ecological research is easier where the confounding effects of extraction have been removed.
The disadvantage is the human cost of that stringency, and it is not a small one. Extinguishing rights of use, and in some cases relocating settlements, removes access to fuelwood, fodder, grazing and minor forest produce from households that had relied on them for generations. Where relocation and compensation are inadequate, the result is lasting hostility between the park administration and the surrounding population — and a protected area whose neighbours have no stake in its protection is difficult to protect. National parks are also usually smaller than sanctuaries and can be too small for wide-ranging animals, and their strict boundaries mean that animals leaving the park enter areas with no protection at all.
3.6.3.3 Biosphere Reserves
A biosphere reserve is a large protected area designated to conserve the whole biological diversity of a representative ecosystem, together with the traditional life and knowledge of the communities living in it, and to support research, monitoring and education.
Biosphere reserves originate in the Man and the Biosphere Programme of the United Nations Educational, Scientific and Cultural Organisation, launched in 1971, which sought a form of protection that treated people as part of the system rather than as intruders in it. Their distinguishing feature is zonation into three concentric zones.
The core zone is legally protected and undisturbed, with no human activity permitted other than approved research. It holds the reserve’s most sensitive habitat and its endemic species.
The buffer zone surrounds the core, and permits activities that are compatible with conservation — research, education, training, restoration work, and limited resource use that does not damage the core.
The transition zone, sometimes called the manipulation zone, is the outermost and largest zone, where settlements, cropping, forestry, grazing and other economic activity continue, managed cooperatively by the reserve authorities and the local communities.
Diagram
Fig. 3.6-2 – zonation of a terrestrial biosphere reserve, drawn as three concentric regions; innermost region labelled “core zone — legally protected, no human activity except approved research”; middle ring labelled “buffer zone — research, education, restoration, limited compatible use”; outer ring labelled “transition zone — settlements, cropping, forestry, grazing, managed cooperatively”; small symbols for a settlement and cultivated fields placed only in the transition zone and a research station in the buffer zone
The advantages are considerable. The zonation allows strict protection and continued human use to exist within one designated area, so the choice between the two is not forced. It conserves diversity at all three levels — ecosystem, species and genetic — over an area large enough to be viable, and it builds in research and long-term monitoring, which most protected areas lack.
The disadvantages are those of any arrangement requiring cooperation. Zonation exists on paper and must be maintained on the ground, which needs sustained staff and funding. The boundaries of the outer zones often cut across administrative and revenue boundaries, so several agencies must coordinate. And if the communities in the transition zone gain no benefit from the arrangement, the buffer zone is steadily encroached and the model collapses into an ordinary protected area with an unusually complicated map.
3.6.4 Ex-Situ Conservation
Ex situ conservation is used where in situ protection is not enough: when a population has fallen so low that it cannot survive the ordinary hazards of the wild, when its habitat has already been destroyed, or when material must be stored for breeding and research.
3.6.4.1 Zoos
A zoo maintains and breeds animals in captivity. Its conservation objectives are to keep populations of threatened species alive under controlled conditions, to breed them under planned programmes that maintain genetic variation, to supply animals for release into the wild where a habitat is available, and to educate visitors.
Its features are controlled feeding, veterinary care, protection from predators and poachers, and — in modern practice — studbooks that record ancestry so that mating can be planned to avoid close inbreeding, and exchanges of animals between institutions for the same purpose.
The advantages are that a species can be saved from immediate extinction, that reproduction can be assisted where it is failing, that animals are available for study at close quarters, and that a large public sees living animals and learns something of their situation.
The disadvantages are serious enough that zoos are treated as a last resort rather than a strategy. The number of animals kept is small, so genetic variation is lost over generations however carefully breeding is planned. Behaviour changes in captivity, and animals raised in enclosures often lack the skills to find food and avoid predators, which is why releases so frequently fail. Costs per animal are very high. And the habitat problem is untouched: a species bred successfully in captivity has nowhere to be released to if the forest it came from has been cleared.
3.6.4.2 Aquaria
An aquarium performs the same functions for aquatic species — maintaining and breeding fish, amphibians, corals and aquatic plants in tanks under controlled conditions of temperature, salinity, oxygen and light.
Its particular features are the precision with which water quality must be managed and the fact that many aquatic species can be bred in large numbers once the conditions are right, which makes restocking a practical possibility.
The advantages are the ability to conserve species whose water bodies have been polluted or dammed, to breed and restock depleted stocks, and to display an environment that most people never see, which does more for public understanding of aquatic systems than any description.
The disadvantages mirror those of zoos, with an additional one: aquatic species are extremely sensitive to water chemistry, so failure of a system can destroy a collection quickly. Coral and many marine species remain difficult to keep at all, and stocking aquaria has itself been a source of pressure on wild populations where specimens are collected rather than bred.
3.6.4.3 Plant Collections
Plants are conserved ex situ in botanical gardens, arboreta, field gene banks and seed banks. The objectives are to maintain living collections of species and varieties, to store genetic material for breeding, to support research and taxonomy, and to hold material for the restoration of degraded habitats.
Their features vary by method. A botanical garden maintains living plants, which is essential for species whose seeds cannot be dried and stored. A field gene bank grows many varieties of a crop side by side, which preserves whole plants but occupies land and remains exposed to weather and disease. A seed bank stores dried seed at low temperature and low moisture, which is by far the most economical method — an enormous number of varieties can be kept in a single room, and seed of many crop species remains viable for decades.
The advantages are compactness, low cost per variety, and safety from the hazards of the field. A seed bank is the most efficient form of conservation yet devised for the species it suits.
The disadvantages are important. Seeds of many tropical species — including several large-seeded trees and crops such as coconut and cocoa — cannot survive drying and freezing, so they must be kept as living plants instead. Stored seed loses viability over time and must be periodically germinated, grown and re-harvested, which requires continuous funding and skilled staff. Most fundamentally, a stored variety stops evolving: it is a snapshot of a population at the moment of collection, while the pests and climate it will eventually face continue to change.
3.6.5 The Critical State of Indian Forests
India’s forests carry an unusually heavy load. They hold a large share of the country’s biodiversity, they are the catchments of most of its rivers, and they supply fuelwood, fodder, grazing and minor produce to a very large rural population, while also being the source of commercial timber and the land most readily available for mines, roads, dams and industry.
Forest cover is measured by satellite and reported periodically by the Forest Survey of India. Cover is classified by the density of the tree canopy, which matters because a hectare of open forest does not do the work of a hectare of dense forest.
Read Table 3.10 alongside any figure for total forest cover, because the total conceals the composition. Cover recorded as forest may consist largely of open forest and plantations, while very dense forest — the class that actually delivers interior habitat and the strongest regulating functions — is a much smaller share. A national figure that is stable over a decade can therefore hide a shift from dense to open forest, which is degradation without deforestation.
The current position can be summarised in four points, each of which is well established even where the precise figures move between assessments.
Total forest cover is a little over one-fifth of India’s geographical area. The National Forest Policy of 1988 set a goal of one-third of the total land area under forest and tree cover, with two-thirds in the hills and mountains to guard against erosion and landslides. The gap between goal and position has persisted for decades.
Quality is the more serious problem than quantity. A substantial part of the recorded area is open or degraded forest, and much of the increase reported in recent assessments comes from plantations and trees outside forests rather than from recovery of natural forest, which is not ecologically equivalent.
Pressure is concentrated rather than uniform. Forests near dense settlement carry heavy demands for fuelwood, fodder and grazing; forests over mineral deposits face mining; forests in river valleys face submergence.
Distribution is uneven. The north-eastern states, the Western Ghats and central India hold the densest forest, and the north-eastern states in particular have recorded losses associated with shifting cultivation cycles and infrastructure, while some drier states have gained cover through plantation.
3.6.6 Conflicts in Managing and Conserving Forests
Forest conservation is difficult less because the ecology is complicated than because several legitimate claims on the same land conflict. This subsection sets out the four principal conflicts.
3.6.6.1 People Living In and Around Forests, and Tribal Rights
A large number of people live inside or immediately around India’s forests, and a significant proportion of them belong to Scheduled Tribes. Their households have long drawn fuelwood, fodder, grazing, bamboo, honey, resin, leaves and medicinal plants from these forests, and in many areas have cultivated within them.
The conflict has a historical origin that is worth understanding, because it explains the shape of the present dispute. Colonial forest legislation from the second half of the nineteenth century vested large areas in the state and classified them as reserved or protected forest. Existing use by resident communities, which had rested on custom rather than on written title, was reclassified as a privilege granted by the state, or as an offence. After independence this legal structure was largely retained, and further areas were brought under it as protected areas were declared. The result was that people who had used a forest for generations held no recognised right to it, and could be excluded from it by administrative decision.
The consequences have been of two kinds. Households lost secure access to resources they depended on, without compensation, and enforcement of forest rules became a persistent source of friction. And conservation itself was weakened, because a rule that residents regard as illegitimate is expensive to enforce and easy to evade.
The debate on this subject is genuine, and both positions rest on evidence. One holds that secure rights improve conservation, because communities with a recognised stake protect a resource they can no longer be arbitrarily excluded from, and points to areas where community management has restored degraded forest. The other holds that recognising rights inside the most sensitive habitats increases pressure on them and complicates protection of species that tolerate very little disturbance. The book’s task is to make both arguments clear, not to declare one of them correct.
3.6.6.2 Poaching
Poaching is the illegal hunting or capture of wildlife. It threatens species selectively — those whose parts command high prices, such as tiger and leopard skin and bone, elephant ivory, rhinoceros horn, musk, pangolin scales and certain birds and reptiles.
Two features make it difficult to control. The demand is usually distant from the supply, often in another country, so the price paid at the final market is enormous compared with the amount received by the person who kills the animal — which leaves plenty of margin to finance an organised chain of transport and concealment. And the animals most in demand are precisely the K strategists of section 3.1.7: large, slow-breeding species whose populations recover very slowly from any loss, so a level of offtake that a fast-breeding species would absorb is enough to drive them towards extinction.
Enforcement is expensive because protected areas are large, staff are few, and patrolling is difficult terrain. Where the surrounding population is hostile to the protected area, information about poaching does not reach the staff, which is why relations with neighbouring villages have a direct bearing on enforcement.
3.6.6.3 Development Projects: Roads and Dams
Forest land is often the cheapest land available to the state, because it carries no private title, and it is therefore where roads, railways, transmission lines, mines and reservoirs tend to be sited.
Roads and railways take a narrow strip but have effects out of proportion to the area, for the reasons given in section 3.5.2: they fragment the block, create long edges, kill animals crossing, and — most importantly — open the interior to access, so felling, grazing and settlement follow the road inwards.
Dams submerge a block of valley outright, and the effects on the river system were described in section 3.5.6. For forests specifically, submergence takes the most productive land in the catchment, and the resettlement of displaced households frequently takes place on other forest land, so the loss occurs twice.
Mining removes vegetation and soil together and alters drainage and groundwater over an area larger than the pit. Restoration of a mined area to forest is possible but slow, and is often incomplete.
None of this establishes that such projects should not be built. It establishes that the forest land they use is not free, that the cost includes the regulating functions lost, and that these costs are rarely counted in the appraisal of the project.
3.6.6.4 Over-exploitation of Forest Resources
Over-exploitation is extraction beyond the rate at which the forest regenerates, and it takes two forms that are worth separating.
Direct over-exploitation is the removal of the forest product itself: felling of timber beyond the annual increment, cutting of fuelwood faster than the trees regrow, lopping of branches for fodder to the point where the tree declines, over-grazing that removes seedlings so that no new generation establishes, and collection of a minor forest produce — a medicinal root, a resin, a fruit — at a rate that prevents the plant from reproducing.
Indirect over-exploitation is damage to the forest caused by using it for something else: fires set to promote fresh grass for grazing, which kill regeneration; the removal of leaf litter for fuel or fodder, which starves the soil of organic matter; compaction of soil by livestock, which stops rainwater soaking in; and the introduction of livestock diseases to wild herbivores.
The distinction matters for management. Direct over-exploitation can be addressed by regulating offtake, and its effects are visible. Indirect over-exploitation leaves the tree count unchanged while the forest’s capacity to renew itself declines, and it is generally noticed only when a generation of seedlings fails to appear.
3.6.7 The Role of the Forest Department and NGOs
Two kinds of body carry most of the work of forest management, and their strengths are almost exactly complementary.
The Forest Department of each state is the custodian of forest land. Its functions are to survey and demarcate forest boundaries, to prepare and implement working plans that govern what may be extracted and where, to carry out afforestation and regeneration, to protect forest from fire, encroachment, illicit felling and poaching, to manage protected areas and their staff, to regulate the movement and sale of forest produce, and to administer the legal provisions of forest and wildlife law, including prosecution of offences.
Its strengths are legal authority, a permanent field establishment down to the level of the individual beat, technical expertise in silviculture and mapping, and continuity across decades. Its recognised weaknesses are chronic shortages of field staff and funds relative to the area to be covered, a regulatory relationship with local residents inherited from colonial forest law, and organisational objectives — set as targets for area planted or offences booked — that do not always match ecological outcomes.
Non-governmental organisations work alongside and sometimes in tension with the department. Their functions include independent monitoring of forest and wildlife status, scientific research and long-term field studies, mobilising and training village institutions for forest protection, legal action on behalf of affected communities or of the forest itself, supporting livelihoods that reduce pressure on the forest, and raising public awareness.
Their strengths are flexibility, the ability to work closely and continuously with a few villages, technical and legal specialisation, and independence from the administration whose decisions they may need to question. Their limitations are equally clear: they have no legal authority over land, their funding is short-term and project-bound, their coverage is patchy, and they can rarely sustain an effort over the decades that forest management requires.
Effective conservation has generally depended on the two working together — statutory authority and permanence from the department, local engagement and independent scrutiny from outside it.
3.6.8 Management Measures
Protection alone cannot meet the demands placed on forests, because the demand for fuelwood, fodder, timber and grazing does not disappear when a boundary is drawn. Four management approaches have been used, each with a different answer to the question of who the forest is being managed for.
3.6.8.1 Scientific Forestry and Its Limitations
Scientific forestry is the management of forests by systematic survey, classification and working plans, so that the volume harvested each year does not exceed the volume the forest grows in that year — the principle of sustained yield.
Its scope covers the inventory of growing stock, division of the forest into compartments, prescription of what may be felled where and when, planned regeneration of felled areas, protection from fire and grazing, and record-keeping over rotations that may run to many decades.
Its origins in India lie in the second half of the nineteenth century, when the colonial administration, faced with heavy demand for railway sleepers and shipbuilding timber, adopted forestry methods developed in Germany and France and recruited foresters trained there. Dietrich Brandis, a German botanist, became the first Inspector General of Forests in India in the 1860s, and the system of reserved forests, working plans and a trained forest service dates from this period.
The advantages are real. Sustained-yield management prevents the sheer exhaustion of a timber resource, it produces reliable long-term data on what a forest contains, and it introduced systematic regeneration where felling had previously been followed by nothing.
The disadvantages are equally real and are the reason the approach has been so heavily criticised. Its objective was the sustained yield of a few commercially valuable timber species, not the maintenance of ecological diversity, so mixed natural forest was frequently converted to even-aged single-species stands — which is to say, to a monoculture with the vulnerabilities described in section 3.5.5. It treated the many non-timber products on which local households depended as of no account, and it treated the people using the forest as a source of damage to be excluded. It also, by design, valued the forest by the timber it could yield, so functions such as soil retention, water regulation and habitat did not enter the calculation at all.
3.6.8.3 Joint Forestry Management
Joint Forestry Management (JFM) is an arrangement under which a state Forest Department and an organised village institution manage a defined patch of degraded forest together: the village protects it from fire, grazing, encroachment and illicit felling, and in return receives the usufruct — the fuelwood, fodder, grass and minor forest produce it yields — along with an agreed share of the revenue when timber is finally harvested.
Its scope is deliberately limited to degraded forest, on the reasoning that a badly degraded patch has little standing value to lose and much to gain from protection, and that the village’s share of a recovering forest is worth more to it than the small quantities it could take from a degraded one.
The approach grew from an experiment at Arabari in West Bengal, begun in the early 1970s by a forest officer, A. K. Banerjee, who found that a degraded sal forest recovered rapidly once neighbouring villages were given a stake in its protection and a share of its produce. A national policy circular in 1990 extended the approach across the country, and most states adopted some form of it.
The advantages are that protection costs fall sharply, because the people best placed to observe and prevent damage have an interest in doing so; that degraded forest regenerates naturally, which is far cheaper than replanting; that villagers obtain a legal and predictable supply of the produce they need; and that the relationship between the department and the village changes from policing to partnership.
The disadvantages have been documented as carefully as the successes. The arrangement usually rests on an administrative order rather than on a right, so the village’s share can be altered by the state and cannot be enforced against it, which weakens the incentive to invest effort over a long rotation. Benefits inside the village are often captured by the better-off households, while those who lose most from the closure of grazing — usually the poorest, and often the women who collected fuelwood there — are compensated least. The share of final timber revenue can be small and arrives only after many years. And where the committee exists on paper because the department needed one, no genuine joint management takes place at all.
Diagram
Fig. 3.6-3 – flow: degraded forest patch identified → agreement between Forest Department and village forest protection committee → village provides protection from fire, grazing, encroachment and illicit felling; department provides technical support and legal backing → natural regeneration of the patch → village receives fuelwood, fodder, grass and minor forest produce, plus an agreed share of final timber revenue → improved forest condition and reduced protection cost
3.6.8.4 Ecotourism
Ecotourism is responsible travel to natural areas, undertaken in a way that conserves the environment and improves the well-being of the local people. The definition in this form is associated with The International Ecotourism Society, established in 1990.
Its scope covers wildlife viewing, trekking, birdwatching and nature interpretation in and around protected areas, together with the local accommodation, guiding and transport that support them. What distinguishes it from ordinary tourism in a scenic place is the intention that the visit should generate income for conservation and for local households, and that its environmental impact should be limited by design.
The advantages, where the design holds, are considerable. The forest acquires a continuing economic value that competes with the value of clearing it, so conservation gains a constituency it did not have. Local households obtain income as guides, drivers, cooks and hosts, which reduces dependence on extraction from the forest. Visitor fees can fund protection directly. And people who have watched wild animals in their habitat generally support their protection afterwards.
The disadvantages are equally well documented, and most of them arise when volume outruns design. Vehicles, noise and crowding disturb the animals that visitors came to see, particularly at water sources and during breeding. Construction of lodges and roads fragments habitat and increases demand for water and fuel in exactly the places least able to supply it, and waste disposal becomes a problem where no system exists. Where facilities are owned from outside the region, most of the revenue leaves it and local residents obtain only the lowest-paid seasonal work, so the promised local benefit does not materialise. Communities can be excluded from areas they formerly used so that visitors may see them undisturbed, which transfers a cost to the residents and a benefit to the visitors. And demand for souvenirs made from wildlife can create a market where none existed.
Ecotourism is therefore best understood not as a form of conservation in itself but as a mechanism that can finance conservation, and only where visitor numbers are capped, revenue is genuinely shared, and the areas most sensitive to disturbance are kept closed.
3.6.9 Project Tiger: A Case Study in Conservation
The tiger is a useful subject for a case study because it concentrates most of this chapter’s themes in one species. It is a K strategist with a low reproductive rate, so it recovers slowly. It needs a very large range, so it cannot be conserved in a small area. It is at the top of its food chain, so protecting it requires protecting the prey species and therefore the vegetation those prey depend on — which is why it is described as an umbrella species, one whose protection shelters a whole community. And it has been hunted both for sport and for trade, so its decline was rapid and documented.
Three lessons from Project Tiger apply well beyond it.
The first is that protecting a single species means protecting a system. Nothing in the programme worked until the prey base and the grassland and water it depended on were restored, and the areas protected for one predator now shelter a very large number of other species.
The second is that measurement determines management. For years the population was estimated by a method that could be, and was, reported inaccurately; the change to camera traps and statistical estimation altered what managers could see and therefore what they could act on. An indicator that cannot be verified is worse than no indicator, because it produces confidence.
The third is that the relationship with people at the boundary is not a side issue. Where relocation was handled well and where the surrounding population obtained some benefit, protection held; where it was not, the reserve had a hostile perimeter, and no patrolling budget compensates for that.
3.7 Summary
Population dynamics
A population is a group of individuals of one species occupying an area at a given time and able to interbreed. Population density, \(D = N/A\), expresses size relative to area and determines competition, disease transmission and predation risk.
Only four processes change population size: natality and immigration add individuals; mortality and emigration remove them. The population at the end of a period is \(N_{t+1} = N_t + (B + I) - (D + E)\).
The difference between births and deaths alone is the rate of natural increase. Crude birth rate and crude death rate are expressed per 1000 population per year; dividing such a rate by 10 converts it to a percentage.
Age structure is the distribution of individuals among age groups, grouped as pre-reproductive, reproductive and post-reproductive. Because of population momentum, a population keeps growing for years after fertility falls, since a large young cohort is still entering the breeding ages.
A population pyramid plots age classes vertically with males left and females right. Three shapes recur: expanding (broad base, growing), constant (near-vertical sides, stationary) and declining (narrow base, shrinking and ageing).
A survivorship curve plots survivors of a cohort per 1000 born, on a logarithmic scale, against relative age. Type I is convex (low mortality until old age, as in large mammals), Type II is a straight diagonal (constant mortality rate, as in many birds), Type III is concave (very heavy early mortality, as in most fish and trees).
r strategists are small, short-lived and early-maturing, produce many small offspring with little care, show Type III survivorship and thrive in disturbed habitats. K strategists are larger, longer-lived and late-maturing, produce few well-cared-for offspring, show Type I survivorship and thrive in stable, crowded habitats. The two are ends of a continuum, not separate categories.
Human populations
Carrying capacity, \(K\), is the maximum number of individuals an environment can support indefinitely without degradation of the resource base. A population that overshoots it does not merely exceed the ceiling; by damaging the resource it lowers the ceiling.
Malthus argued that population increases geometrically while food supply increases only arithmetically, so numbers press permanently against subsistence and are held down by positive checks, which raise the death rate, and preventive checks, which lower the birth rate. Over-population in his sense is a relation between numbers and resources, not a statement about density.
The prediction failed for four reasons: yields rose far faster than he assumed; population pressure can itself induce agricultural innovation, as Boserup argued; modern famine has generally been a failure of access rather than of aggregate supply; and fertility has fallen rather than risen as incomes have risen. The underlying insight — that a resource with a fixed renewal rate cannot support indefinitely multiplying demand — still holds for aquifers, fisheries and forests.
Environmental pressure is population multiplied by per capita consumption. Total national consumption and consumption per person can rank countries in opposite orders, and the same disparity exists within countries, where national averages conceal it.
Demographic transition is the observed shift from high birth and death rates to low ones, through an intermediate phase of rapid growth. Its four stages are high stationary, early expanding (death rate falls, growth fastest), late expanding (birth rate falls, growth decelerates) and low stationary.
Death rates fall mainly through clean water, sanitation, immunisation, treatment and more reliable food supply, with the infant mortality rate falling earliest and furthest. Birth rates fall later, through improved child survival, urbanisation, the rising cost of raising each child, education — especially of girls — women’s employment, old-age security, and access to health services that let families plan.
Total fertility rate is the average number of children a woman would bear at current age-specific rates; about 2.1 is replacement level.
Human population control
Population growth has been addressed through family planning services, education, economic growth and improvement in the status of women. These work on the same household decision from different directions and are most effective together.
Programmes built on numerical targets, incentives and disincentives have generally performed worse than those built on access and choice, and have damaged trust in health services. International policy shifted towards a rights-based approach at the Cairo conference of 1994.
Female education is the most consistent predictor of lower fertility across countries and income levels, acting through later marriage, better use of health services, wider employment alternatives and higher expected spending per child. The relationship is steepest in the first years of schooling and flattens near replacement level.
Threats to the ecosystem
Ecosystem services divide into provisioning functions — food, fodder, fuelwood, timber, fibre, fresh water — and regulating functions — soil retention, groundwater recharge, flood moderation, pollination, nutrient cycling, air purification. Provisioning failures are noticed at once; regulating failures appear elsewhere and later.
Habitat destruction is the largest single threat to biodiversity. Fragmentation is especially damaging because it converts interior habitat to edge, cuts off movement between patches and leaves populations small enough for a single event to remove them.
Genetic erosion is the loss of variation from a gene pool, through small isolated populations breeding within themselves and through the replacement of many local varieties by a few uniform ones. It removes the raw material of future adaptation.
Loss of diversity reduces the completeness of nutrient cycling, biomass production, decomposition and water purification, and removes the redundancy that allows one species to compensate for the failure of another.
Expanding agriculture converts habitat and simplifies what remains. Monoculture raises yield per hectare while creating an uninterrupted food supply for specialised pests, increasing pesticide use, and removing the pollinators and natural enemies that were previously working free.
Impounding water yields irrigation, drinking water and power, and in exchange traps sediment in the reservoir, blocks migratory fish, removes the downstream flood pulse that maintains floodplains, submerges land and displaces households.
Waste becomes pollution when it exceeds the rate at which an ecosystem can process it. Organic loading strips dissolved oxygen from water; nutrient enrichment causes eutrophication; non-biodegradable plastics and heavy metals persist and accumulate. Dilution lowers concentration at a point but does not remove a persistent substance from the system.
Increasing consumption per person drives the other threats, raising extraction at one end and waste at the other, and has grown faster than population in recent decades.
Conservation
Conservation is the planned management and sustainable use of natural resources, species and habitats. In situ conservation protects species in their natural habitat; ex situ conservation maintains them outside it.
A wildlife sanctuary prohibits hunting but permits regulated human use, so it is easier to declare and carries continuing disturbance. A national park excludes extraction and settlement, giving low disturbance at the cost of lost access for surrounding households. A biosphere reserve is zoned into an undisturbed core, a buffer permitting compatible use, and a transition zone where economic activity continues, and it conserves ecosystem, species and genetic diversity along with the communities present.
Zoos, aquaria and plant collections rescue species whose wild populations or habitats are gone, and supply material for breeding and research. All of them hold small numbers, so genetic variation is steadily lost, behaviour changes in captivity, release to the wild often fails, and stored material stops evolving. Ex situ conservation is insurance for in situ programmes rather than a substitute for them.
Indian forest cover is a little over one-fifth of the geographical area, against the National Forest Policy 1988 goal of one-third overall and two-thirds in the hills. Cover is classified by canopy density — very dense at 70 per cent and above, moderately dense at 40 to 70 per cent, open at 10 to 40 per cent — and a stable total can conceal a shift from dense to open forest, which is degradation without deforestation.
Forest conflicts arise from competing legitimate claims: the rights of forest-dwelling communities, whose customary use was reclassified by colonial-era forest law and is now addressed by the Forest Rights Act 2006; poaching, which selectively targets slow-breeding species with distant high-value markets; roads, dams and mines sited on forest land because it carries no private title; and over-exploitation, both direct through excess extraction and indirect through fire, litter removal and soil compaction.
The Forest Department brings legal authority, a permanent field establishment and technical continuity, but is short of staff and funds and inherits a regulatory relationship with residents. Non-governmental organisations bring flexibility, close local work, research and independent scrutiny, but lack legal authority, permanence and coverage.
Scientific forestry manages forest for sustained yield through working plans, preventing exhaustion of the timber resource, but historically converted mixed forest to single-species stands and disregarded non-timber uses and local users. Social forestry raises trees outside forest land — as farm forestry, community forestry, extension forestry and agroforestry — to supply fuelwood and fodder locally, but has often been captured by market species, poor aftercare and unequal distribution of the benefit.
Joint Forestry Management gives an organised village institution the usufruct of a degraded forest patch and a share of final revenue in return for protecting it. It lowers protection costs and regenerates forest cheaply, but rests on administrative order rather than right, and its benefits are often unequally shared within the village.
Ecotourism can give a forest a continuing economic value and fund protection, but disturbs wildlife, drives construction and waste where volume outruns design, and delivers little locally where facilities are owned from outside the region.
Project Tiger, launched in 1973 with nine reserves on a core-and-buffer pattern, prevented the extinction of the tiger in India and protected large areas of habitat and many other species. Its record also shows the loss of an entire reserve population to poaching, unreliable early monitoring that was later replaced by camera-trap estimation, slow and contested relocation, reserves left isolated by conversion of the forest between them, and recovery concentrated in a limited number of well-protected reserves.
3.8 Key Terms
Age structure — the distribution of the individuals of a population among different age groups at a given time.
Agroforestry — the deliberate growing of trees together with crops or livestock on the same unit of land.
Aquarium — a facility in which aquatic species are maintained and bred in tanks under controlled conditions, as a form of ex situ conservation.
Biodiversity — the variety of life in an area at three levels: genetic, species and ecosystem.
Biosphere reserve — a large protected area zoned into core, buffer and transition zones, conserving ecosystem, species and genetic diversity together with the communities living in it.
Buffer zone — the middle zone of a biosphere reserve, surrounding the core, where research, education, restoration and limited compatible use are permitted.
Carrying capacity (K) — the maximum number of individuals of a species that an environment can support indefinitely without degradation of its resource base.
CBR — Crude Birth Rate; the number of live births per 1000 population per year.
CDR — Crude Death Rate; the number of deaths per 1000 population per year.
Community forestry — the raising of trees on community or village common land, tended collectively, with the produce intended for the village as a whole.
Conservation — the planned management and sustainable use of natural resources, species and habitats, so that they remain available in the future.
Constant pyramid — a population pyramid with near-vertical sides, indicating a roughly stationary population.
Core zone — the innermost, legally protected and undisturbed zone of a biosphere reserve, where no activity other than approved research is permitted.
Declining pyramid — an urn-shaped population pyramid, narrower at the base than in the middle, indicating a shrinking and ageing population.
Demographic transition — the historically observed shift of a population from high birth and death rates to low ones, through an intermediate phase of rapid growth.
Dependency ratio — the number of people in the non-working age groups relative to the number in the working ages.
Direct over-exploitation — removal of a forest product itself at a rate faster than the forest can regenerate it.
Doubling time — the time a population takes to double in size, approximated by dividing 70 by the annual percentage growth rate.
Ecosystem services — the benefits people obtain from ecosystems, comprising the goods they provide and the natural processes they regulate.
Ecotourism — responsible travel to natural areas in a way that conserves the environment and improves the well-being of local people.
Emigration — the departure of individuals from a population to another area.
Environmental resistance — the sum of the factors that slow population growth as numbers rise towards the carrying capacity.
Eutrophication — nutrient enrichment of a water body, causing dense algal growth whose decay depletes the dissolved oxygen.
Ex situ conservation — the conservation of species outside their natural habitat, in zoos, aquaria, botanical gardens and seed banks.
Expanding pyramid — a broad-based, triangular population pyramid, indicating a growing population.
Exponential growth — growth in which the number added in each interval is proportional to the number already present, giving a J-shaped curve.
Exponential phase — the steep middle portion of a logistic curve, where resources are still plentiful relative to numbers.
Extension forestry — planting of trees along roadsides, canal and river banks, railway margins and on public wasteland.
Family planning — enabling couples to decide freely how many children to have and when, with the services and information to act on that decision.
Farm forestry — the planting of trees by individual farmers on their own holdings, for household use or for sale.
Forest Department — the state agency that is custodian of forest land, responsible for working plans, regeneration, protection, protected areas and forest law.
Gene pool — the total collection of genes and their variants present in a population or species.
Genetic erosion — the loss of genetic variation within a species or population, reducing the range of inherited characteristics available to it.
Habitat destruction — the conversion of a natural habitat to another use, so that the species living there can no longer complete their life cycles.
Habitat fragmentation — the division of a continuous habitat into smaller separated patches, converting interior habitat to edge and isolating populations.
Immigration — the arrival of individuals of the same species into a population from another area.
IMR — Infant Mortality Rate; deaths of children under one year of age per 1000 live births.
Indirect over-exploitation — damage to a forest’s capacity to renew itself through activities such as burning, litter removal, grazing and soil compaction.
In situ conservation — the conservation of species in their natural habitat, by protecting and managing the ecosystem in which they occur.
Intrinsic rate of natural increase (r) — the per capita difference between birth and death rates under ideal conditions, per individual per year.
JFM — Joint Forestry Management; management of a degraded forest patch jointly by the Forest Department and a village institution, which protects it in return for usufruct and a share of revenue.
J-shaped curve — the curve produced by exponential growth, rising ever more steeply with no upper limit.
K strategists — species that mature late, live long, produce few well-cared-for offspring and compete well in stable, crowded habitats.
Lag phase — the early portion of a logistic curve, where numbers are small and increase slowly in absolute terms.
Logistic growth — growth that slows as the population approaches carrying capacity, giving an S-shaped curve.
Monoculture — the cultivation of a single crop species, often a single variety, over a large area.
Mortality — the loss of individuals from a population by death.
Natality — the addition of individuals to a population by birth, hatching or germination.
National park — an area strictly reserved for wildlife and its habitat, where grazing, cultivation, forestry, hunting and extraction of produce are not permitted.
Over-population — a condition in which the numbers of a population exceed the resources available to sustain it.
Overshoot — the rise of a population above the carrying capacity of its habitat, usually followed by a decline.
Per capita consumption — the average quantity of resources used, and waste generated, by each person in a year.
Plateau phase — the flattening upper portion of a logistic curve, where births and deaths are in approximate balance near carrying capacity.
Poaching — the illegal hunting or capture of wildlife.
Population — a group of individuals of one species occupying a particular area at a particular time and capable of interbreeding.
Population control — deliberate measures intended to slow the rate of growth of a human population.
Population density — the number of individuals of a population per unit area, \(D = N/A\).
Population dynamics — the study of how and why the size of a population changes over time.
Population momentum — the tendency of a population to keep growing after fertility has fallen, because a large young cohort is still entering the reproductive ages.
Population pyramid — a paired bar diagram showing the proportion of a population in each age class, males to the left and females to the right.
Positive checks — in the Malthusian model, influences that raise the death rate, such as famine and epidemic disease.
Post-reproductive group — the individuals of a population past breeding age.
Pre-reproductive group — the individuals of a population too young to breed.
Preventive checks — in the Malthusian model, influences that lower the birth rate, such as later marriage.
Provisioning functions — the material goods an ecosystem supplies, such as food, fodder, fuelwood, timber, fibre and fresh water.
r strategists — species that mature early, live briefly, produce many small offspring with little care, and colonise disturbed habitats rapidly.
Rate of natural increase — the difference between the birth rate and the death rate of a population, excluding migration.
Regulating functions — the processes by which an ecosystem maintains conditions, such as soil retention, groundwater recharge, flood moderation, pollination, nutrient cycling and air purification.
Replacement level fertility — a total fertility rate of about 2.1 children per woman, at which a generation exactly replaces itself.
Scientific forestry — management of forests by survey, classification and working plans, so that annual harvest does not exceed annual growth.
Self-regulation — the reduction of a population’s own reproduction in response to crowding, before external resources are exhausted.
Social forestry — the raising of trees outside reserved forest, on farms, common land, roadsides and wasteland, to meet local needs for fuelwood, fodder and small timber.
S-shaped curve — the curve produced by logistic growth, rising and then flattening against the carrying capacity.
Survivorship curve — a graph of the number of survivors of a cohort at each age, plotted on a logarithmic scale against relative age.
Territoriality — the defence by an individual of an exclusive area against others of its own species.
TFR — Total Fertility Rate; the average number of children a woman would bear in her lifetime at current age-specific fertility rates.
Transition zone — the outermost zone of a biosphere reserve, where settlements, cropping, forestry and grazing continue under cooperative management.
Type I survivorship — a convex curve; low mortality until old age, characteristic of large, well-cared-for species.
Type II survivorship — a straight diagonal line on a logarithmic scale; a constant proportion of survivors dies in each age interval.
Type III survivorship — a concave curve; very heavy mortality in the earliest stages, characteristic of species producing large numbers of small offspring.
Wildlife sanctuary — an area where hunting and capture of wildlife are prohibited but certain regulated human activities are permitted.
Women’s empowerment — the improvement of women’s education, health, employment, property rights and authority over decisions affecting their own lives.
Zoo — a facility in which animals are maintained and bred in captivity, as a form of ex situ conservation.
3.9 Points to Ponder
If environmental pressure is population multiplied by consumption per person, which of the two terms should a country with a large population and low average consumption work on first — and what would you need to know about that country before answering?
A protected area records rising numbers of a threatened species while the villages along its boundary lose access to grazing and fuelwood. On what basis could you decide whether this arrangement is working?
A seed bank can hold thousands of varieties in a single cold room, at a fraction of the cost of maintaining them as living plants in the field. What is lost in the transfer, and could a seed bank ever substitute for a farmer still growing the variety?
Malthus was wrong about food and right about limits. Which resources, if any, does his argument still describe accurately today, and what distinguishes them from the resources it fails to describe?
Fertility falls when girls stay in school longer, and school enrolment is far easier for a government to measure than the number of children a family intends to have. Does that make education a population policy, or does describing it that way misstate what education is for?
A national forest-cover figure holds steady for a decade while dense forest is replaced by open forest and plantation. What would you need to add to that figure before it told you whether the country’s forests had improved or deteriorated?
3.10 Objective Questions
- Which of the following pairs of processes adds individuals to a population?
- Natality and mortality
- Natality and immigration
- Mortality and emigration
- Immigration and emigration [1 mark]
- A survivorship curve that appears as a straight diagonal line on a logarithmic scale indicates:
- Very heavy mortality in early life
- Low mortality until old age
- A constant proportion of survivors dying in each age interval
- No mortality until the maximum lifespan is reached [1 mark]
- In the logistic growth equation, the population adds individuals fastest when N is:
- Close to zero
- About one-quarter of K
- About one-half of K
- Equal to K [1 mark]
- Which stage of the demographic transition shows the fastest population growth?
- High stationary
- Early expanding
- Late expanding
- Low stationary [1 mark]
- Which of the following is a density-independent factor?
- Territoriality
- A cyclone
- Parasites and diseases
- Competition for food [1 mark]
- Forest recorded as having a tree canopy density of 45 per cent is classified as:
- Very dense forest
- Moderately dense forest
- Open forest
- Scrub [1 mark]
- In which zone of a biosphere reserve are settlements and cropping permitted?
- Core zone
- Buffer zone
- Transition zone
- In none of the three zones [1 mark]
- Assertion (A): A population may continue to grow for several decades after its total fertility rate has fallen below replacement level. Reason (R): A large cohort of young people is still entering the reproductive age groups.
- 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): A district’s forest area may fall by only a small percentage while several forest-interior bird species disappear from it. Reason (R): Fragmentation converts interior habitat into edge habitat without removing much total area.
- 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): Ex situ conservation is preferred over in situ conservation wherever a species is threatened. Reason (R): Captive populations are protected from predators, poachers and disease.
- 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. Type I survivorship | i. Trees on farm boundaries planted by individual farmers |
| b. Farm forestry | ii. Undisturbed zone of a biosphere reserve |
| c. Core zone | iii. Low mortality until old age |
| d. Positive checks | iv. Famine and epidemic disease |
[1 mark]
Fill in the blank: The number of individuals of a species that an environment can support indefinitely without degradation of its resource base is called its ____________. [1 mark]
State whether true or false: Dilution of an effluent in a large river removes the persistent substances it contains from the river system. [1 mark]
Give the full forms of: (i) TFR (ii) IMR (iii) JFM (iv) CBR [1 mark]
3.11 Check Your Concepts
Define carrying capacity. State one reason why the carrying capacity of a habitat may fall over time. [2 marks]
Name the four processes that change the size of a population, and state which two of them are excluded when the rate of natural increase is calculated. [2 marks]
What is meant by population momentum? Give one consequence of it for a country whose fertility has recently fallen. [2 marks]
Give the full form of JFM and state one function it performs for the village institution that takes part in it. [2 marks]
Differentiate between direct and indirect over-exploitation of forest resources, with one example of each. [3 marks]
Differentiate between a wildlife sanctuary and a national park with respect to permitted human activity, typical size and level of disturbance. [3 marks]
Give reasons why the fall in birth rates during the demographic transition occurs later than the fall in death rates. [3 marks]
Explain, with one example each, the difference between a provisioning function and a regulating function of an ecosystem, and state why the loss of a regulating function is usually noticed later. [3 marks]
Draw and label a logistic growth curve. Mark the carrying capacity and the three phases, and explain what causes the curve to flatten. [4 marks]
Explain, with an example, why a species with a Type I survivorship curve recovers more slowly from poaching than a species with a Type III curve. [4 marks]
Describe the three zones of a biosphere reserve and explain what each contributes to conservation. [4 marks]
Evaluate the claim that social forestry relieved the pressure on India’s natural forests. Refer to at least two of its four types and to two documented limitations of the programme. [4 marks]
Describe the Malthusian model of population and resources. State the evidence on which it rested, and give three reasons why its central prediction has not been borne out. [6 marks]
Compare in situ and ex situ conservation with respect to what is conserved, continuing evolution, cost and principal limitation. State the circumstances in which ex situ conservation becomes necessary. [6 marks]
Describe Project Tiger under the headings origin, aims and objectives, successes and failures. [6 marks]
Discuss the causes and consequences of impounding water behind a large dam, referring to the effects upstream, downstream and on the people of the submerged area. [6 marks]
3.12 Passage-Based Questions
- A hill district records a total fertility rate of 1.8 children per woman, well below replacement level, and its crude birth rate has been falling for fifteen years. Yet the district’s total population has risen in each of those fifteen years. The district’s schools report falling enrolment in the earliest classes, while its hospitals report rising demand from patients above 60. Migration into the district is negligible; a steady stream of young adults leaves it each year for work.
- Explain why the district’s population has continued to rise despite a below-replacement fertility rate. [2 marks]
- Sketch the shape of the population pyramid you would expect this district to show in twenty years, and name the shape. [2 marks]
- Identify the two factors from the population balance equation that are pulling in opposite directions here, and state which will dominate in the long run. [2 marks]
- A stretch of moist forest is crossed by a new four-lane highway and, five years later, by a canal running at right angles to it. Records show that the total forest area in the block has fallen by 6 per cent over that period. Field surveys report that two ground-nesting bird species have disappeared from the block, that a large herbivore is now regularly seen on cultivated land beyond the block, and that seedlings of several deep-shade tree species are no longer establishing.
- Name the process at work and explain why the ecological effect is greater than a 6 per cent loss of area suggests. [3 marks]
- Account for the appearance of the large herbivore on cultivated land. [2 marks]
- Suggest one management measure that would address the underlying problem rather than its symptoms, and state its limitation. [2 marks]
- A village bordering a degraded forest patch enters into an agreement with the state Forest Department. The village committee undertakes to prevent grazing, fire and unauthorised felling on 80 hectares; in return it receives the fuelwood, grass and minor produce the patch yields, and an agreed share of revenue when timber is finally harvested. Six years later the patch has regenerated naturally and yields more grass than before. The committee’s records show, however, that three households which had grazed cattle on the patch have lost that grazing and have not received any alternative, and that most of the committee’s decisions are taken by a small group of larger landholders.
- Name the arrangement described and state the principle on which it rests. [2 marks]
- Explain why natural regeneration succeeded here where replanting programmes often do not. [2 marks]
- Identify the equity problem the records reveal, and explain why it also threatens the long-term success of the arrangement. [3 marks]
3.13 Practice With Data
- A town has a population of 25,000 on 1 January. During the year there are 500 births and 300 deaths; 240 people move in and 90 move out.
- Calculate the population on 31 December. [1 mark]
- Calculate the crude birth rate and crude death rate, in births or deaths per 1000 population per year. [2 marks]
- Calculate the annual growth rate as a percentage, and state whether natural increase or migration contributed more to it. [2 marks] [5 marks]
- A population follows logistic growth with an intrinsic rate of natural increase of 0.2 per individual per year and a carrying capacity of 2000 individuals.
- Calculate the number of individuals added per year when N is 400, when N is 1000 and when N is 1800. [3 marks]
- State at which of the three values growth is fastest, and explain in one sentence why. [2 marks] [5 marks]
- Study the table and answer the questions that follow.
| Age group (years) | Country P (% of total population) | Country Q (% of total population) |
|---|---|---|
| 0–14 | 41 | 14 |
| 15–64 | 55 | 64 |
| 65 and above | 4 | 22 |
- Name the shape of population pyramid each country would show. [2 marks]
- State which country will face greater pressure on its schools over the next fifteen years, and give a reason. [2 marks]
- Both countries have a similar working-age share. Explain why this figure alone does not tell you which country’s working population is under greater strain. [2 marks] [6 marks]
- A country’s recorded forest cover remains at 21.5 per cent of its geographical area across two successive assessments ten years apart. Over the same period, very dense forest falls from 3.1 to 2.6 per cent of the geographical area, moderately dense forest falls from 9.4 to 8.9 per cent, and open forest rises from 9.0 to 10.0 per cent.
- Calculate the change in each of the three density classes, in percentage points of geographical area. [2 marks]
- State what has happened to the country’s forests over the decade, and explain why the unchanged total conceals it. [2 marks]
- Name one regulating function that would be weakened by the change you have identified. [1 mark] [5 marks]
- A population is growing at 2.5 per cent per year.
- Estimate its doubling time in years. [1 mark]
- A second population of the same size is growing at 1.0 per cent per year. Estimate its doubling time, and state how many years earlier the first population reaches double its size. [2 marks] [3 marks]
3.6.8.2 Social Forestry and Its Types
Social forestry is the raising of trees on land outside the reserved forest — on farmland, community land, roadsides, canal banks, railway margins and wasteland — in order to meet local needs for fuelwood, fodder, small timber and minor produce, and thereby to relieve the pressure on natural forests.
The term entered Indian policy in the 1970s on the recommendation of the National Commission on Agriculture, and the programme spread widely in the following decade. Its scope is deliberately outside the forest: the object is not to manage the forest better but to supply from elsewhere the goods that were being taken from it. Four types are usually distinguished.
Farm forestry is the planting of trees by individual farmers on their own holdings — on field boundaries, as windbreaks, on land unsuited to crops, or as a plantation crop. It may be for the household’s own fuel, fodder and small timber, or for sale.
Community forestry is the raising of trees on community or village common land, tended collectively, with the produce intended for the village as a whole rather than for individual owners.
Extension forestry, sometimes called urban forestry when it occurs in cities, is planting along the linear strips of public land — roadsides, canal and river banks, railway lines — and on degraded public wasteland, extending tree cover into areas that were never forest.
Agroforestry is the deliberate combination of trees with crops or livestock on the same unit of land, so that the tree provides fodder, fuel, timber, shade or nitrogen fixation while the ground beneath it remains productive.
The advantages are substantial where the programme works as intended. Supply of fuelwood and fodder close to where it is needed reduces both the pressure on natural forest and the hours households spend collecting; tree cover on degraded land reduces erosion and can improve soil over time; farmers gain an additional source of income; and the trees are on land where planting requires no displacement of anyone.
The disadvantages emerged clearly from experience. Where farmers planted for the market rather than for household needs, fast-growing species suited to pulp and pole markets displaced the multipurpose species that supplied fodder and fuel, so the fuelwood shortage the programme was meant to relieve was not relieved. Species selection was often poor: extensive planting of a single fast-growing exotic species drew heavily on soil water in dry regions and supported very little local wildlife, and the ecological consequences of large uniform blocks were repeated here. Common land planted under community schemes sometimes ended up serving those best placed to claim the produce rather than the households that had grazed it, so the poorest lost a grazing resource and gained nothing. And plantations frequently received no care after the planting season, so survival rates were low even where the area planted looked impressive in the record.
Table 3.11 – scientific forestry vs social forestry | Feature | Scientific forestry | Social forestry | |—|—|—| | Where practised | Inside reserved and protected forest | Outside forest land — farms, common land, roadsides, wasteland | | Primary objective | Sustained yield of commercial timber | Local supply of fuelwood, fodder and small timber | | Managed by | Forest Department, through working plans | Farmers, village bodies and public agencies, with department support | | Species preferred | Commercially valuable timber species, often in even-aged stands | Fast-growing and multipurpose species; choice varies with purpose | | Chief advantage | Prevents exhaustion of the timber resource; systematic records and regeneration | Reduces pressure on natural forest; income and fuel close to the user | | Chief criticism | Converts mixed forest to monoculture; ignores non-timber uses and local users | Market-driven species choice may not meet local fuel and fodder needs; poor aftercare; common land may be captured |