Chapter 13: Organisms and Populations
In 1977, a small research submersible named Alvin was lowered into the Pacific Ocean near the Galápagos Islands. The scientists inside expected to find bare volcanic rock. The deep sea, after all, is pitch dark. No sunlight reaches it, and without sunlight there can be no photosynthesis — so, by every rule they had been taught, there should have been almost nothing alive down there.
What their lights actually picked out, more than two kilometres below the surface, astonished them. Around cracks in the sea floor, where superheated mineral-rich water gushed out of the earth, there were dense crowds of living things: giant tube worms taller than a person, clams, crabs, and mats of bacteria. An entire community was flourishing in total darkness, under crushing pressure, beside water hot enough to melt lead.
That single dive changed how biologists think about life. It showed that organisms do not merely survive their surroundings — they are shaped by them, adapted to them, and organised by them into populations and communities that persist for generations.
This chapter is about exactly that relationship. It asks how an organism copes with the temperature, water, light and soil around it, how individuals of the same kind form populations, how those populations grow, and how different species living together help, harm, or simply tolerate one another.
Diagram
Fig. 13.0 – Illustration of a deep-sea hydrothermal vent community: dark ocean floor, vent chimney releasing hot mineral-rich water, giant tube worms, clams and crabs clustered around it, with the submersible Alvin and its beam of light above; label vent, tube worms, bacterial mat, submersible.
Keep these questions in mind as you read. Each one of them will be answered somewhere in this chapter.
13.1 Organisms and the Environment Around
Levels of Organisation in the Living World
The natural world around us shows an amazing diversity of forms and an equally amazing complexity of relations. A single mango tree in your neighbourhood is not just a tree — it is food for caterpillars, a perch for birds, a support for an orchid, and a home for fungi in the soil below it.
To make sense of all this, biologists study the living world as a series of levels, each one built out of the level below it. The sequence runs: macromolecules, cells, tissues, organs, individual organism, population, communities, ecosystems and biomes.
This ladder matters because every level has properties the level below it does not have. A single cell has no heartbeat; an organ system does. A single individual has a birth and a death; a population has a birth rate and a death rate. As we climb, new features appear.
What Ecology Studies
You have already studied in school that ecology is the study of the interactions among organisms, and between organisms and their physical (abiotic) environment. In other words, ecology asks two questions at once: who lives with whom, and who lives with what.
The term ecology was first used by Reiter, but it was E. Haeckel who gave substance to the term and introduced it in the sense we use today. From that point on, the relationship between an organism and its surroundings became a subject of study in its own right, and not merely a background detail.
The Ecological Hierarchy
Grouping organisms ecologically simply means arranging them in an ecological hierarchy. There are four sequential levels of ecological (biological) organisation, each more complex than the one before it: organism, population, community and biome.
The individual organism is the basic unit of the ecological hierarchy — the smallest complete living thing that has to face the environment on its own. Organisms of the same kind inhabiting a geographical area constitute a population. Several populations of different species in a particular area constitute a community, whose members interact with one another in many ways.
A biome constitutes a large regional terrestrial unit delimited by a specific climatic zone, having a major vegetation zone (plant communities) and the associated fauna. There are six major groups of terrestrial biomes. The ordered sequence of these levels is shown in FC 13.1.
Flowchart
FC 13.1 – Ecological hierarchy in increasing order of complexity: Individual organism → Population (same kind of organisms in an area) → Community (several populations of different species interacting) → Biome (large regional unit with a characteristic climate, vegetation and fauna)
Notice that each step upward is defined by adding something: one organism becomes a population when you add more of its own kind; a population becomes a community when you add other species; a community becomes a biome when you add a whole climatic region. In this chapter we shall explore only the first two levels — the organism and the population.
Ecology at the Level of the Organism
Ecology at the level of the organism is basically the study of animal or plant physiology. It helps us understand how organisms are adapted to their environments — not only for their survival, but also for their propagation (multiplication).
That last point deserves attention. Merely staying alive is not enough in nature. An organism that survives a harsh summer but produces no offspring leaves nothing behind, and its traits disappear. So adaptation is always measured against two demands: stay alive, and leave descendants.
Climate, Seasons and the Making of Biomes
You have studied in earlier classes that the rotation of the earth around the Sun, and the tilt of its axis, cause the seasons. These seasons, together with the annual variation in precipitation in the form of rain and snow, give rise to the formation of the major biomes of the earth — desert, rain forest, grassland, tundra, and so on.
So a biome is not a random patch of land. It is the predictable outcome of two master variables: how warm a place is on average, and how much rain or snow it receives. The major biomes of the earth, plotted against these two variables, are shown in Fig. 13.1.
Diagram
Fig. 13.1 – Graph showing distribution of major biomes with respect to mean annual temperature (Y-axis, –15 °C to 30 °C) and mean annual precipitation/rainfall (X-axis, 50 to 450 cm); overlapping zones labelled Desert, Grassland, Tropical forest, Temperate forest, Coniferous forest, and Arctic and alpine tundra.
From Biome to Habitat
Regional and local variations within each biome lead to the formation of a variety of habitats. A single grassland biome may contain a rocky slope, a shaded gully, a termite mound and a temporary pond, and each of these offers a different set of conditions to live in.
On the Earth, life exists even in extreme and harsh habitats — the scorching deserts of Rajasthan, the perpetually rain-soaked forests of the North Eastern states, and the high mountain tops of the Himalayas. Wherever conditions vary, some organism has been shaped by evolution to exploit them.
Here we must remember something important: it is not only the physico-chemical (abiotic) components that make up the habitat of an organism. The habitat also includes biotic components — the plants, pathogens, parasites, and predators of that organism.
We assume that over a period of time, the organism has, through natural selection, evolved adaptations that optimise its survival and reproduction in its habitat. In other words, an organism’s body and behaviour are a kind of long record of the habitat that shaped them.
Habitat and Niche
Habitat is a place, or the set of environmental conditions around an organism, to which it must adapt in order to survive and prosper. The term niche is used to denote the functional role played by an organism in its environment (J. Grinnell 1917). A niche includes various aspects of the life of an organism, such as its diet and its shelter.
A habitat defines the physical space of an organism along with the other living and non-living factors present there, while a niche describes how that organism is linked with its physical and biological environment. In colloquial language, the habitat is the postal address of an organism, while the niche is its profession.
Factors like sunlight, average rainfall, annual temperatures, the type of soil present, and other abiotic (topographic) factors affect the presence of organisms in a habitat. These factors together help determine which particular type of species will be found in that environment.
Pond, river and ocean are examples of habitats, since many organisms are found living in the same place. Habitats can be arboreal (on trees), terrestrial (on land), aerial (in air) or aquatic (in water). The immediate surrounding of an organism is sometimes referred to as its microhabitat — an important concept to remember when working with sedentary or weakly motile organisms, for whom a few centimetres of moss or bark may be the entire world.
What a Niche Really Describes
A niche is described as the position of a species in the environment: What does it do to survive? How does it fulfil its needs of shelter and food? A niche deals with the flow of energy from one organism to another, and this is why it is important to understand what an organism eats and how it interacts with other organisms.
As soon as a niche is left vacant, other organisms move in and fill that position — nature does not leave a job undone for long. The niche is specific to each species, which means that no two species can share the same niche.
If a species creates its own unique niche in an ecosystem, this helps in reducing competition for resources among species. Take the example of birds living in the same habitat. Some birds eat only insects, some eat only fruits, and some eat both and anything else they come across. We can therefore conclude that these birds, though sharing one habitat, differ in their niches because of their different eating habits.
Three Types of Niches
Ecologists recognise three types of niches, based on what aspect of the organism’s role is being described. Their basis is compared in FC 13.2.
Flowchart
FC 13.2 – Types of niche: Niche → (a) Spatial or habitat niche (based on physical space occupied) → (b) Trophic niche (based on trophic level in a food chain) → (c) Multidimensional or hypervolume niche (based on many biotic + abiotic factors together)
a. Spatial or habitat niche: It deals with the physical space occupied by the organism — for instance, the canopy of a tree as against its trunk or its roots.
b. Trophic niche: It is based on the trophic level of an organism in a food chain — that is, on where the organism draws its energy from, whether as a producer, a herbivore, or a carnivore.
c. Multidimensional or hypervolume niche: This considers a number of environmental factors together (both biotic and abiotic). Because each factor is an independent “dimension”, the resulting space is a hypervolume — not something that can be perceived by the human mind. This space is called the hypervolume niche. Alternatively, it is the position of an organism in the environmental gradient.
Fundamental and Realized Niche
For every species, there is a fundamental niche and a realized niche. The fundamental niche is the niche a species would occupy in the absence of all competitors — the full range of conditions and resources it is capable of using. This situation is highly improbable in nature, because competitors are almost always present.
Hence, the realized niche is a more realistic approach. It is the niche a species actually occupies in the presence of competition for the resources available in the habitat. A species is therefore usually pushed into a smaller working space than its biology would allow.
Looking Ahead
We have seen that each habitat type is regulated by a number of abiotic or physico-chemical factors, which decide which organisms can be present at all. The key abiotic factors that influence any habitat are ambient temperature, availability of water, light and type of soil. We take these up one by one in the next section.
13.2 Major Abiotic Factors
Every habitat is regulated by a set of physico-chemical, non-living conditions. Four of these matter more than the rest, because they decide whether an organism’s cells can work at all: ambient temperature, availability of water, light, and the type of soil. We take them up one by one.
Temperature
Temperature is the most ecologically relevant environmental factor. Average temperature on land varies from subzero levels in polar areas and at high altitudes, upwards up to 50 °C in tropical deserts in summer. Temperature also varies seasonally, so the same place can offer very different conditions in January and in May.
There is a clear geographical pattern to it. Temperature decreases progressively from the equator towards the poles, and also from the plains to the mountain tops. This is why a climb up a Himalayan slope takes you through the same sequence of vegetation types you would meet on a long journey northwards.
Some habitats are extreme even by these standards. Hot springs run at 80 to 100 °C, and deep-sea hydrothermal vents have average temperatures usually around 400 °C — the water there stays liquid only because of the enormous pressure at that depth. Yet organisms live in both.
Why should a few degrees matter so much to a living body? Because ambient temperature affects the enzyme kinetics of the cell. Enzymes are protein catalysts, and each works fastest within a narrow band of temperature. Below that band reactions slow down; above it, the enzyme’s shape is destroyed and it stops working altogether. Since every reaction in the body is enzyme-driven, temperature therefore controls the entire metabolism, activity and other physiology of the organism, as summarised in Fig. 13.2.
Diagram
Fig. 13.2 – Graph of enzyme activity (Y-axis) against temperature (X-axis): activity rises with temperature, reaches a peak at the optimum, then falls steeply as the enzyme denatures; label optimum temperature, low-temperature slow zone, and denaturation zone.
Organisms differ greatly in the range of temperature they can handle. Only a few can tolerate and thrive in a wide range of temperatures — these are called eurythermal organisms. A vast majority, however, are restricted to a narrow range of temperatures, and these are called stenothermal.
Water
Availability of water is an important factor affecting organisms. As we know, life on earth originated in water, and the body of every organism is still mostly water. Its availability is so limited in deserts that only special adaptations make survival there possible. The productivity and distribution of plants are also heavily dependent on water — which is precisely why rainfall was one of the two axes that decided the biomes in Fig. 13.1.
Organisms living in water bodies such as oceans, lakes and rivers have their own water-related problems. For them the difficulty is not scarcity but chemistry: for aquatic organisms, the chemical composition and pH of the water are important.
The dissolved salt concentration is measured as salinity, in parts per thousand (ppt). It is less than 5 ppt in the fresh waters of streams, lakes and rivers, and 30–35 ppt in the seas and oceans. It may go up to 100 ppt in some hypersaline lagoons — water so salty that most life is simply excluded.
Just as with temperature, organisms differ in the range they can tolerate. Some are tolerant of a wide range of salinities and are called euryhaline; others are restricted to a narrow range and are called stenohaline. Many fresh water animals cannot live for long in sea water, and vice versa, because of the osmotic problems they would face.
Light
Plants use light for photosynthesis, and photosynthesis is the only source of energy for the entire ecosystem — every animal, every decomposer and every predator ultimately eats sunlight that a plant captured first. Photosynthesis can occur only in the presence of sunlight.
But light is not equally available everywhere, even in a forest. Many species of small plants (herbs and shrubs) growing on the forest floor are adapted to perform photosynthesis optimally under very low light conditions, because they are constantly overshadowed by tall trees. Their whole physiology is tuned to the leftovers of light that filter through the canopy.
For animals too, light matters — but as information rather than as food. Diurnal and seasonal variations in light intensity and in duration (the photoperiod) act as clues for timing their foraging, reproductive and migratory activities. A bird does not wait to feel the cold; it reads the shortening day and leaves before winter arrives.
The availability of light on land is closely linked with that of temperature, since the sun is the source of both. This is why the two factors so often change together, and why organisms can use one as a signal for the other.
Soil
The nature and properties of soil depend on the climate and on the weathering process that broke down the parent rock in the first place. Soil is therefore not a passive floor; it is a product of the same climate that shapes the biome above it.
Various characteristics of the soil, such as its composition and grain size, determine the percolation and water-holding capacity of the soil. A coarse, sandy soil lets water drain straight through, while a fine-grained soil holds it. These characteristics, along with pH, mineral composition and topography, determine the vegetation of an area. Vegetation, in turn, dictates the type of animals found there. The chain is shown in FC 13.3.
Flowchart
FC 13.3 – Soil controls life: climate + weathering → soil composition and grain size → percolation and water-holding capacity (together with pH, minerals, topography) → type of vegetation → type of animals supported
Responses to a Changing Environment
The abiotic factors that determine the type of habitat also show considerable diurnal (day-to-night) and seasonal variations. No habitat holds still. Plants and animals must adapt to these changes in order to survive and flourish in the habitat.
During the course of their evolution, many species have evolved a relatively constant ‘internal’ environment that permits all biochemical reactions and physiological functions to proceed at the optimum rate, and allows the species to flourish. Organisms try to maintain the constancy of this internal environment — homeostasis — despite variations in the external environmental conditions.
To survive and flourish in any environment, organisms must cope with the changes around them, and there are four possibilities open to them. These are summarised in FC 13.4.
Flowchart
FC 13.4 – Options before an organism facing a stressful environment: Regulate (keep the internal environment constant) OR Conform (let the internal environment change with the external) OR Migrate (move away temporarily and return) OR Suspend (reduce metabolic activity and wait it out)
i. Regulate: Some organisms are able to maintain homeostasis by physiological and behavioural changes which ensure a constant body temperature, a constant osmotic concentration, and so on. All birds and mammals are capable of such regulation (thermoregulation and osmoregulation). Regulation is effective, but it is expensive — a great deal of energy goes into holding the internal environment steady.
ii. Conform: Most animals and plants cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature. In aquatic animals, the osmotic concentration of the body fluids changes with that of the ambient water. These animals and plants are simply conformers. Conforming costs nothing, but it leaves the organism at the mercy of the outside world — which is why conformers are usually confined to habitats that do not fluctuate too violently.
Some species have evolved the ability to regulate, but only within a limited range of environmental conditions, beyond which they simply conform. If the stressful environment is localized, or lasts only for a short period of time, the organism has two further options — it may migrate, or suspend its activities.
iii. Migrate: The organism can move away temporarily from the stressful habitat to a more hospitable area, and return when the stressful period is over. Many animals, particularly birds, undertake long-distance migrations to more hospitable areas during winter.
iv. Suspend: Here the organism stays, but shuts down. In plants, seeds serve as the means to tide over periods of stress; they germinate to form new plants under favourable moisture and temperature conditions. They do so by reducing their metabolic activity and going into a state of dormancy. In animals, an organism that is unable to migrate may go into hibernation during winter, as the polar bear does. Some snails and fish go into aestivation to avoid the summer heat.
13.3 Adaptation
To cope with the extreme variations in their environment, some organisms respond through physiological adjustments, while others do so behaviourally — migration being one example we have already met. All such responses are their adaptations.
We can therefore say that adaptation is an attribute of the organism — morphological, physiological or behavioural — that enables the organism to survive and reproduce in its habitat. Notice both halves of that statement. A feature that keeps an organism alive but leaves it childless is not, in the evolutionary sense, an adaptation at all.
The three kinds of adaptation, with the sort of examples that belong under each, are set out in FC 13.5.
Flowchart
FC 13.5 – Types of adaptation: Adaptation → Morphological (change in form or structure, e.g. spines of Opuntia, blubber of seals) → Physiological (change in functioning, e.g. CAM pathway in desert plants) → Behavioural (change in action, e.g. basking and burrowing in desert lizards, migration in birds)
Adaptations of Plants to Water Scarcity
Many desert plants have a thick cuticle on their leaf surfaces, and have their stomata sunk in deep pits, to minimize the loss of water through transpiration. The waxy cuticle blocks water escaping through the leaf surface, and a stoma lying at the bottom of a pit sits in a pocket of still, humid air, so far less water vapour is drawn away from it.
Desert plants also have a special photosynthetic pathway, CAM (Crassulacean acid metabolism), that enables their stomata to remain closed during the daytime. This is an elegant solution to a real conflict: a plant must open its stomata to take in \(CO_2\), but the daytime desert air would strip it of water the moment it did so. The CAM plant instead opens its stomata at night, when the air is cool and humid, stores the carbon dioxide as an acid, and uses it for photosynthesis the next day behind closed stomata.
Some desert plants, like Opuntia, have their leaves reduced (modified) to spines, and the photosynthetic function is taken over by the flattened stems, as seen in Fig. 13.6. The logic is direct — a broad leaf is mostly surface, and surface is exactly what loses water. Losing the leaf removes the loss; the spine that remains also deters herbivores, and the flat green stem takes over the job of feeding the plant.
Diagram
Fig. 13.6 – Adaptations to harsh habitats: (a) Opuntia with flattened green stem and leaves reduced to spines; (b) T.S. of a desert leaf showing thick cuticle and stomata sunk in deep pits; (c) seal with a thick layer of blubber below the skin; (d) desert lizard basking in the sun and burrowing into sand. Label cuticle, sunken stoma, spine, flattened stem, blubber.
Adaptations of Animals to Cold
Mammals from colder climates generally have a shorter snout, ears, tail and limbs, so as to minimize the loss of body heat. This observation is known as Allen’s Rule.
The reason is a matter of geometry. Heat is lost from the body surface, but it is produced in the body’s volume. Long, thin projections like ears and limbs have a great deal of surface for very little volume, so they bleed heat rapidly. A compact, short-limbed body has less surface for the same bulk, and holds its heat far better.
In the polar seas, aquatic mammals like seals have a thick layer of fat (blubber) below their skin, acting as an insulator to reduce the loss of body heat. Water carries heat away from a body far faster than air does, so a seal cannot rely on shape alone; it needs a physical barrier between its warm core and the freezing sea, and blubber provides exactly that.
Behavioural Adaptations
Some organisms show behavioural responses to cope with variations in their environment. Here the body itself does not change; the animal simply changes what it does.
Desert lizards manage to keep their body temperature fairly constant by such behavioural adaptations. They bask in the sun and absorb heat when their body temperature drops below the comfort zone, but move into the shade when the ambient temperature starts increasing. Some species burrow into the sand to hide and escape from the heat.
This is worth pausing on. A lizard is a conformer — it cannot generate a constant body temperature the way a bird or a mammal can. Yet by moving between sun, shade and burrow, it achieves nearly the same result at almost no energy cost. Behaviour is doing the work that physiology does in a regulator.
13.4 Population
In nature we rarely find isolated, single individuals. Living things occur in groups within a well-defined geographical area, where they share or compete for similar resources, can potentially interbreed, and thus form a population.
Although it is an individual organism that has to cope with changes in the environment, it is at the population level that natural selection operates to evolve the desired traits. A single deer that runs fast may escape a tiger, but “fast-running deer” only becomes a feature of the species when, generation after generation, the faster ones leave more offspring than the slower ones — and that is a statement about a population, not an individual.
Population ecology is therefore an important area of ecology, because it links ecology to population dynamics, genetics and evolution.
Attributes of a Population
A population has certain attributes of its own, which are different from those of an individual. The basic physical characteristics of a population are its size and density. Size speaks for the number of individuals in a population, while density tells us the number of individuals present per unit space at a given time.
Besides size and density, the other characteristics of a population include natality, mortality, immigration, emigration, age pyramids, expanding population, population growth forms and biotic potential.
The distinction that matters most is this: an individual has a birth and a death, but a population has a birth rate and a death rate. You cannot ask what the birth rate of a single sparrow is. Rates, ratios and distributions are properties that appear only when many individuals are taken together.
Natality
Natality is the birth rate of a population. It has the greatest influence on a population’s growth, because births are the one process that adds individuals from within.
Natality may be expressed as a crude birth rate or a specific birth rate. Crude birth rate is used when calculating population size, and is the number of births per 1000 population per year. Specific birth rate is used relative to a specific criterion such as age — for example, the number of births per 1000 women aged 20–24.
Natality is further described in two forms:
- Absolute Natality: the number of births under ideal conditions — with no competition, and an abundance of resources such as food and water.
- Realized Natality: the number of births when environmental pressures come into play.
It must be remembered that absolute natality will always be more than realized natality. Ideal conditions never exist in nature, so the births a population actually achieves are always fewer than the births it is capable of.
Mortality
Mortality is the death rate of a population. The mortality rate, or death rate, is a measure of the number of deaths — in general, or due to a specific cause — in a particular population, in proportion to the size of that population, per unit of time.
Mortality rate is typically expressed in deaths per 1000 individuals per year.
Mortality, like natality, has two forms:
- Absolute Mortality: the number of deaths under ideal conditions — with no competition, and an abundance of resources such as food and water.
- Realized Mortality: the number of deaths when environmental pressures come into play.
It must be remembered that absolute mortality will always be less than realized mortality. In the real world, predators, disease and shortage of resources kill individuals who would have survived under ideal conditions.
Sex Ratio
The sex ratio of a population affects, and is reciprocally affected by, the birth, death, immigration and emigration rates of that population. It is measured as the ratio of the number of individuals of one sex to that of the other sex.
Males and females in a ratio of 1:1 is generally the most common evolutionary stable strategy (ESS). The reasoning behind this is worth grasping: if one sex ever became rare in a population, an individual of that rarer sex would have far more mating opportunities, so parents producing offspring of the rarer sex would leave more grandchildren. This automatically pushes the ratio back towards 1:1. The population settles at the point where no alternative strategy can do better — which is precisely what “evolutionarily stable” means.
Age Distribution and Age Pyramid
A population is composed of individuals of different ages. If the age distribution — the per cent of individuals of a given age or age group — is plotted for the population, the resulting structure is called an age pyramid.
For the purpose of simplicity, the entire population is divided into three age groups: Pre-Reproductive (age 0–14 years), Reproductive (age 15–44 years) and Post-reproductive (45–85+ years).
The shape of the pyramid is not a decoration — it is a forecast. A pyramid with a very broad base is telling you that a huge crowd of children is about to enter the reproductive age group, so the population will keep growing for years to come, whatever the present birth rate is. The four characteristic shapes are shown in Fig. 13.3.
Diagram
Fig. 13.3 – Age pyramids for countries with rapid, slow, zero and negative population growth rates: (a) Rapid growth, broad base, e.g. Kenya, Nigeria; (b) Slow growth, e.g. United States, Australia; (c) Zero growth, straight-sided, e.g. Denmark, Italy; (d) Negative growth, narrow base, e.g. Germany, Hungary. Each pyramid split into male (left) and female (right), with bands for ages 0–14, 15–44 and 45–85+ years.
Measuring Population Size and Density
The size of a population that a habitat can support tells us a great deal about its status in that habitat. Population size in a natural habitat could be as low as less than 10 — Siberian cranes in a bird sanctuary — or run into millions, as with Chlamydomonas in a pond.
Population size, more technically called population density (designated as N), need not necessarily be measured in numbers alone. Although the total number is generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine.
Consider an area with millions of termites or ants, but only a few animals that feed on them. To state that the population density of these predators is “low” would be misleading — a handful of large predators may represent far more living matter, and far more ecological importance, than the raw count suggests.
In such cases, the biomass — the total mass of living material — is a more meaningful measure of population size. The total number is again not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming.
Sometimes, for certain ecological investigations, there is no need to know the absolute population densities; relative densities serve the purpose equally well. For instance, the number of birds or insects caught per trap is a good enough measure of their total population density, because if the trapping effort stays the same, a bigger catch honestly means a bigger population.
We are mostly obliged to estimate population sizes indirectly, without actually counting the animals or even seeing them. The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.
Population Growth
The size of a population of any species is a dynamic parameter. It keeps changing with time, depending on various factors including food, predation pressure and adverse weather. In fact, it is these changes in population density that give us some idea of whether a population is flourishing or declining.
The density of a population in a habitat during a given period fluctuates due to changes in four basic processes. New births (B) and Immigration (I) contribute to an increase in population density, while Deaths (D) and Emigration (E) lead to a decrease.
Immigration (I) is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. Emigration (E) is the number of individuals of the population who left the habitat during the time period. These four processes are summarised in FC 13.6.
Flowchart
FC 13.6 – Four processes changing population density: Natality (B) → increases density; Immigration (I) → increases density; Mortality (D) → decreases density; Emigration (E) → decreases density. Net result: if (B + I) > (D + E) → population grows; if (B + I) < (D + E) → population declines; if equal → density stays constant
So, if \(N\) is the population density at time \(t\), then its density at time \(t+1\) can be calculated as:
\[N_{t+1} = N_t + [(B + I) - (D + E)]\]
Here \(N_{t+1}\) is the population density at the next time interval, \(N_t\) is the present population density, \(B\) is 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 time period. Note that immigration and emigration are the only two of the four that involve movement across the boundary of the habitat; births and deaths happen within it.
Growth Models
Does the growth of a population with time show any specific and predictable pattern? We have been concerned about unbridled human population growth and the problems created by it in our country, and it is therefore natural for us to be curious whether other animal populations in nature behave the same way, or show some restraint on growth.
Two models describe what happens.
i. Exponential growth
Resources like food and space are essential for any growth of a population. Ideally, when resources in the habitat are unlimited, each species has the ability to fully realize its innate potential to grow in numbers. The population then grows in an exponential or geometric proportion, producing the J-shaped curve of Fig. 13.4.
Diagram
Fig. 13.4 – Exponential growth curve of population: population size on the Y-axis against time on the X-axis, showing a J-shaped curve that starts almost flat and then rises ever more steeply with no upper limit.
The steepness is the whole point. In exponential growth, the number added in each interval keeps getting bigger, because every new individual becomes a parent in turn. In symbols, the rate of growth is written as
\[\frac{dN}{dt} = rN\]
where \(N\) is the population density, \(t\) is time, and \(r\) is the intrinsic rate of natural increase — a single number that expresses how fast a species can multiply under ideal conditions. The larger the value of \(r\), the more explosive the growth.
Every species is capable of growing exponentially under unlimited resource conditions, and of reaching enormous population densities in a short time.
But resources like food and space are not always unlimited. They may be unlimited in the beginning; but as population density increases, so does competition for those resources, resulting in a slowdown in the rate at which the population was originally growing. This results in the logistic, or sigmoid, growth curve.
ii. Logistic growth
In logistic growth, competition between individuals for limited resources will weed out the ‘weaker’ ones. Only the ‘fittest’ individuals will survive and reproduce.
In nature, a given habitat has enough resources to support a maximum possible number of individuals, beyond which no further growth is possible. Let us call this limit nature’s carrying capacity (K) for that species in that habitat.
A population growing in a habitat with limited resources shows initially a lag phase, followed by phases of acceleration and deceleration, and finally an asymptote, when the population density reaches the carrying capacity. A plot of population density (N) in relation to time (t) results in a sigmoid (S-shaped) curve. This type of population growth is called Verhulst–Pearl Logistic Growth. The curve, with its named phases, is shown in Fig. 13.5, and the sequence of phases in FC 13.7.
Diagram
Fig. 13.5 – Logistic growth curve of population: growth (size or weight of the organism) on the Y-axis against time on the X-axis, showing an S-shaped curve with the lag phase at the bottom, the log or exponential phase in the steep middle, the diminishing growth phase as it bends over, and the stationary phase as a flat asymptote at the carrying capacity K.
Flowchart
FC 13.7 – Phases of the logistic (sigmoid) growth curve: Lag phase (few individuals, slow start) → Log or exponential phase (resources still plentiful, rapid acceleration) → Diminishing growth phase (competition rises, growth decelerates) → Stationary phase (density reaches carrying capacity K; growth becomes zero)
This slowing is written mathematically as
\[\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)\]
where \(K\) is the carrying capacity. Look at what the bracket does. When \(N\) is very small compared with \(K\), the bracket is close to 1, and growth is almost exponential. As \(N\) climbs towards \(K\), the bracket shrinks towards zero, and growth grinds to a halt. When \(N = K\), the bracket is exactly zero and the population stops growing. The formula, in other words, contains its own brake.
Since resources for the growth of most animal populations are finite and become limiting sooner or later, the logistic growth model is considered the more realistic one.
Reproductive Strategies
Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy. There is no single “best” way to reproduce — only the way that works in a given habitat.
Some species produce a large number of small-sized offspring, as oysters and pelagic fishes do. Others produce a small number of large-sized offspring, as birds and mammals do. The first strategy is a gamble on numbers: almost all the young will die, but a few will survive by sheer chance. The second is a gamble on care: few young are produced, but each is large, protected and given a much better chance of reaching adulthood.
13.5 Population Interactions
There is no natural habitat which has only one species of animals or plants. For any species, the minimal requirement is another species as food. Even a plant species, which has photosynthetic abilities and can make its own food, cannot survive alone; it needs soil microbes to break down the organic matter in the soil and release the inorganic nutrients that its roots can absorb.
It is obvious, then, that in nature animals, plants and microbes do not — and cannot — live in isolation. They interact in myriad ways to form a biological community. Having studied the organism and the population, we now step up to the level where populations meet.
Intraspecific and Interspecific Interactions
Interactions may be intraspecific, that is, existing between organisms of the same population, or interspecific, that is, between members of different species.
The interspecific interactions occur between a minimum of two organisms — plant and plant, animal and animal, or plant and animal. Such interactions may be classified into four types: neutralism, negative (harmful) interactions, positive (beneficial) interactions, and interactions that are both positive and negative.
Even in the simplest communities many interactions exist, and not all of them may be easily seen. Interspecific interactions arise from the interaction of populations of two different species, and they could be beneficial, detrimental, or neutral — neither harming nor benefiting — to one of the species or to both.
The Sign Convention
The neatest way to classify these interactions is to ask a simple question about each of the two species involved: does it gain, does it lose, or is it unaffected? This is exactly what Table 13.6 does, using three symbols — a plus for benefited, a minus for inhibited, and O for not affected.
Key: + = benefited, – = inhibited, O = not affected.
The classification follows directly from the nature of the interaction for one or both species. Both species are benefited in mutualism, and one or both may be harmed in competition.
In parasitism and predation, only one species benefits — the parasite and the predator, respectively — and the interaction is detrimental to the other species, the host and the prey respectively.
The interaction where one species is benefited and the other is neither benefited nor harmed is called commensalism. In amensalism, on the other hand, one species is harmed whereas the other is unaffected.
Protocooperation also benefits both species, but it differs from mutualism in one important way: it is not obligatory. The partners gain from being together, yet each can survive perfectly well on its own if the other disappears. Mutualism, as we shall see next, is not so forgiving.
Flowchart
FC 13.8 – Classification of interspecific interactions by outcome: Interaction between two species → Both benefited (+ +) → Mutualism (obligatory) or Protocooperation (not obligatory); One benefited, other unaffected (+ O) → Commensalism; One harmed, other unaffected (– O) → Amensalism; Both harmed (– –) → Competition; One benefited, other harmed (+ –) → Parasitism or Predation; Neither affected (O O) → Neutralism
Mutualism
This interaction is obligatory and interdependent, and it benefits both the species. Neither partner is doing the other a favour — each is simply unable to manage without the other.
Lichens represent an intimate, mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria, as shown in the section in Fig. 13.7. The division of labour is exact: the photosynthetic partner makes food from sunlight, which the fungus cannot do, while the fungus supplies anchorage, water and minerals, and shields its partner from drying out. Together they colonise bare rock, where neither could have lived alone.
Diagram
Fig. 13.7 – V.T.S. of a lichen thallus showing algal and fungal components: upper fungal layer, a distinct band of algal cells below it, loose fungal hyphae beneath; label alga, fungus, lichen.
The most spectacular and evolutionarily fascinating examples of mutualism are found in plant–animal relationships. Plants need the help of animals for pollinating their flowers and for dispersing their seeds. Animals obviously have to be rewarded for this service — with pollen and nectar in the case of pollinators, and with juicy, nutritious fruits in the case of seed dispersers.
But the mutually beneficial system must also be safeguarded against ‘cheaters’ — for example, animals that try to steal nectar without aiding in pollination. A flower that gives away its reward to a thief has paid for a service it never received, so selection favours flowers whose shape makes theft difficult.
This is why plant–animal interactions often involve co-evolution of the mutualists: the evolution of the flower and that of its pollinator species are tightly linked with one another. As Fig. 13.8 shows, the long, narrow, tubular flower and the long, slender beak of the humming bird have been shaped by each other over evolutionary time — the flower excludes the wrong visitors, and the bird gains a nectar source no rival can reach.
Diagram
Fig. 13.8 – Co-evolution of humming birds and plants: a humming bird hovering with its long slender beak inserted into a long tubular red flower; label long tubular corolla, long beak, anthers touching the bird’s head.
Competition
Competition is the type of interaction where both the species are at a loss.
Two ideas here often surprise students. The first is that totally unrelated species may compete for the same resource. In the shallow creeks on the west coast of Mumbai, visiting flamingos and resident fish compete for their common food, the zooplankton. Competition is decided by what an organism eats, not by what it is related to.
The second is that resources need not always be limiting for competition to occur. In competition, the feeding efficiency of one species may be reduced simply due to the interference or the inhibitory presence of the other species, even if resources such as food and space are abundant. Leopards, for instance, do not hunt in close proximity to a lion pride — not because there is no prey, but because there is a lion. This is captured in Fig. 13.9.
Diagram
Fig. 13.9 – A lion standing over a dead leopard, illustrating the direct removal of a competitor from the habitat.
Competition is therefore best defined as a process in which the fitness of one species is significantly lower in the presence of another species.
Gause’s Competitive Exclusion Principle
Gause’s ‘Competitive Exclusion Principle’ states that two closely related species competing for the same resources cannot co-exist indefinitely, and the competitively inferior one will be eliminated eventually.
Note carefully the condition attached to the principle. It is true if resources are limiting, but not otherwise. In interspecific competition with sufficient resources, species facing competition will evolve mechanisms that promote co-existence rather than exclusion.
One such mechanism is resource partitioning. If two species compete for the same resource, they could avoid competition by choosing different times for feeding — or, equally, different parts of the habitat, or different sizes of the same food. In effect, the two species stop doing the identical job, and so stop being in each other’s way. You will recognise this as the niche idea from Section 13.1 in action: by narrowing their realized niches, both species survive.
Evidence for competition also comes from what happens when it is lifted. A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species is found to expand its range when the competing species is removed — showing that the boundary was never set by climate or soil, but by a rival.
Parasitism
Parasitism has evolved in so many taxonomic groups, from plants right up to higher vertebrates, that it must be regarded as one of the most successful ways of making a living.
Many parasites have evolved to be host-specific — they can parasitize only a single species of host — in such a way that both host and parasite tend to co-evolve against each other. Every improvement in the host’s defence puts pressure on the parasite to find a way past it, and every improvement in the parasite puts pressure back on the host.
In accordance with their lifestyles, endoparasites — those living inside the host’s body — have evolved special adaptations: the loss of unnecessary sense organs, the presence of adhesive organs or suckers to cling on to the host, the loss of the digestive system, and a high reproductive capacity.
Each of these makes sense once you picture the habitat. Inside a host there is nothing to see, so eyes are useless baggage. The host’s gut is a current that would sweep the parasite away, so suckers are essential. The food arriving there is already digested, so a digestive system is an unnecessary expense. And the chance that any single offspring will ever reach another host is minute, so enormous numbers must be produced.
The life cycles of parasites are often complex, involving intermediate hosts or vectors to facilitate transfer to the host. The malarial parasite Plasmodium vivax needs a vector — the mosquito — to spread to other hosts.
The majority of parasites harm the host. They may reduce the survival, growth and reproduction of the host, and may even lead to the death of the host, thus reducing its population density. They might also render the host more vulnerable to predation by making it physically weak.
Parasites that feed on the external surface of the host organism are called ectoparasites. The most familiar examples of this group are the lice on humans and the ticks on dogs. Many marine fish are infested with ectoparasitic copepods.
Cuscuta, a parasitic plant commonly found growing on hedge plants, has lost its chlorophyll and its leaves in the course of evolution. It derives its nutrition from the host plant which it parasitizes. Once a plant can draw ready-made food from a host, the machinery of photosynthesis becomes a cost with no return — and selection strips it away, exactly as it strips the gut from an endoparasite.
Brood Parasitism
Brood parasitism in birds is a fascinating example, in which the parasitic bird lays its eggs in the nest of its host bird and lets the host bird incubate them. The parasite outsources the most expensive part of parenting — incubation and feeding — to another species.
During the course of evolution, the eggs of the parasitic bird have evolved to resemble the host’s eggs in size and colour, so as to reduce the chances of the host bird detecting the foreign eggs and ejecting them from the nest. This resemblance is not a coincidence; it is the visible record of a long arms race, in which every host that learned to spot a strange egg selected for parasites whose eggs looked less strange.
The eggs of the parasitic bird, the Asian koel, hatch before those of its host, the common Indian crow — a head start that gives the koel chick the first claim on the food the crow brings, as Fig. 13.10 shows.
Diagram
Fig. 13.10 – A crow feeding an Asian koel hatchling in its own nest; label host (crow), brood parasite (koel chick).
Flowchart
FC 13.9 – Brood parasitism in the Asian koel: koel lays egg in crow’s nest → egg resembles the host’s egg in size and colour → crow fails to detect it and incubates it → koel egg hatches earlier than the crow’s own eggs → koel chick is fed by the host crow → koel fledges without its own parents having incubated or fed it
Predation
When we think of predator and prey, most probably it is the tiger and the deer that come readily to mind — but a sparrow eating a seed is no less a predator. The seed is a living embryo, and it is being eaten.
Although grazers are animals that eat plants and are classified as herbivores, they are not very different from predators in ecological terms. In each case, one organism is transferring energy from another living organism into itself.
Predators play many important roles in a community.
First, they keep prey populations under control. Without predators, prey species would reach very high population densities and cause ecosystem instability. In the absence of frogs, for example, locusts increase in density and destroy large tracts of agricultural land.
Second, the methods of biological control adopted in agricultural pest control are based on this very ability of a predator to regulate prey populations. Instead of spraying a chemical, the farmer introduces or protects the pest’s natural enemy.
Third, predators help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. A predator that keeps the strongest competitor in check prevents that competitor from monopolising the resource and driving the others out — so predation, paradoxically, allows more species to survive together.
Flowchart
FC 13.10 – What happens when a predator is removed from a community: predator removed → prey population is no longer checked → prey density rises steeply → the strongest prey competitor monopolises resources → weaker prey species are competitively excluded → species diversity falls and the ecosystem becomes unstable (e.g. loss of frogs → locust outbreak → destruction of crops)
Invasive Species and Prudent Predators
When certain exotic species are introduced accidentally or intentionally into a new geographical area, they become invasive and start spreading rapidly, due to the absence of a natural predator in the new area. The zebra mussels in the intertidal zone of North America are an example. Freed from the enemies that held them in check at home, their population growth for a while resembles the exponential curve of the previous section — a J-shaped explosion with no brake on it.
If a predator over-exploits its prey, then the prey might become extinct, and following it the predator will also become extinct for lack of food. This is the reason why predators in nature are ‘prudent’ — a predator that wiped out its prey would be destroying its own food supply, so selection favours restraint.
Prey Defences
Prey species have evolved various defences to reduce the impact of predation. It is a kind of ‘evolutionary arms race’. Better camouflage for concealment and faster speed for escape are the prey’s weapons, and the predator must counter these by means of its own — sharper vision, greater speed, better stealth. Neither side ever wins permanently; both simply keep improving.
Some species of insects and frogs are cryptically coloured (camouflaged) to avoid being detected easily.
The Monarch butterfly takes a different route. It is highly distasteful to its predator, the bird, because of a special chemical present in its body. Interestingly, the butterfly does not manufacture this chemical at all — it acquires it during its caterpillar stage by feeding on a poisonous weed. Its bright warning colouration, seen in Fig. 13.11, then advertises that unpleasantness, so that a bird which has tasted one Monarch avoids every Monarch thereafter.
Diagram
Fig. 13.11 – An adult Monarch butterfly displaying its warning colour pattern: bright orange wings with black veins and white-spotted black borders.
Plant Defences Against Herbivores
For plants, herbivores are the predators. A plant cannot run away, so its defences must be built into its body or its chemistry. Plants have therefore evolved a variety of morphological and chemical defences against herbivores.
Thorns, as in Acacia and Cactus, are the most common morphological means of defence.
Many plants produce and store chemicals that make the herbivore sick. When these chemicals or the produce containing them are eaten, they inhibit the feeding or digestion of the herbivore, disrupt its reproduction, or even kill it. Calotropis, growing in abandoned fields, produces highly poisonous cardiac glycosides — and that is why you never see any cattle or goats browsing on this plant.
A wide variety of chemical substances that we extract from plants on a commercial scale — nicotine, caffeine, quinine, strychnine, opium and others — are secondary metabolites, produced by plants actually as defences against grazers and browsers. It is a striking thought: substances that human beings cultivate, trade and consume were evolved by plants to poison animals that tried to eat them.
Commensalism
This is the interaction in which one species benefits and the other is neither harmed nor benefited — the (+ O) relationship of Table 13.6.
An orchid growing as an epiphyte on a branch of a mango tree gets a benefit: it is lifted up into the sunlight, away from the shaded forest floor, without having to build a trunk of its own. The mango tree derives no benefit from this, but neither is it harmed — the orchid takes nothing from it and merely uses the branch as a perch.
The cattle egret and grazing cattle in close association is a classic example of commensalism. Cattle egrets always forage close to cattle: as the cattle move, they flush out insects from the grass that might otherwise be difficult for the egrets to find and catch. The egret gains a meal; the cattle neither gain nor lose.
Another example of commensalism is the interaction between the sea anemone, which has stinging tentacles, and the clown fish that lives among them. The fish gets protection from predators, which stay away from the stinging tentacles. The anemone does not appear to derive any benefit from hosting the clown fish. Both examples are shown in Fig. 13.12.
Diagram
Fig. 13.12 – Commensalism: (a) cattle egret foraging beside a grazing buffalo; (b) clown fish sheltering among the tentacles of a sea anemone.
13.6 Summary
The living world is organised in levels: macromolecules, cells, tissues, organs, individual organism, population, communities, ecosystems and biomes. Ecology studies the interactions among organisms, and between organisms and their physical (abiotic) environment. The term was first used by Reiter, but E. Haeckel gave substance to it.
The ecological hierarchy has four sequential levels of increasing complexity: organism → population → community → biome. The individual organism is its basic unit. There are six major groups of terrestrial biomes, and their distribution is decided mainly by mean annual temperature and mean annual precipitation.
Habitat is the place, or the set of environmental conditions, to which an organism must adapt in order to survive and prosper. Niche (J. Grinnell, 1917) is the functional role played by an organism in its environment. Habitat is the postal address; niche is the profession. A habitat contains many niches and supports many species; a niche is species-specific and no two species can share the same niche.
Niches are of three types: spatial (or habitat) niche, trophic niche, and multidimensional (hypervolume) niche. The fundamental niche is the niche occupied in the absence of all competitors; the realized niche is the smaller niche actually occupied when competitors are present.
The key abiotic factors that regulate a habitat are temperature, water, light and soil.
Temperature is the most ecologically relevant factor. It ranges from subzero in polar regions and high altitudes to 50 °C in tropical deserts in summer, 80–100 °C in hot springs, and about 400 °C at deep-sea hydrothermal vents. It affects enzyme kinetics, and therefore the whole metabolism and physiology of the organism. Eurythermal organisms tolerate a wide range of temperature; stenothermal organisms tolerate only a narrow range.
Water: salinity is less than 5 ppt in fresh waters, 30–35 ppt in the seas and oceans, and up to 100 ppt in hypersaline lagoons. Euryhaline organisms tolerate a wide range of salinity; stenohaline organisms tolerate only a narrow range, and cannot be moved between fresh and sea water because of osmotic problems.
Light drives photosynthesis, the only source of energy for the entire ecosystem. Forest-floor herbs and shrubs are adapted to photosynthesise under very low light. For animals, the photoperiod is a clue for timing foraging, reproduction and migration.
Soil: its composition, grain size, pH, mineral content and topography decide percolation and water-holding capacity, which decide the vegetation of an area, which in turn dictates the animals found there.
Organisms maintain a constant internal environment despite external variation — this is homeostasis. Four responses are open to them: regulate (maintain constancy physiologically and behaviourally, as all birds and mammals do), conform (allow the internal environment to change with the external), migrate (move away temporarily), and suspend (reduce metabolic activity — seed dormancy in plants, hibernation in the polar bear, aestivation in some snails and fish).
Adaptation is any attribute of an organism — morphological, physiological or behavioural — that enables it to survive and reproduce in its habitat. Desert plants show a thick cuticle, stomata sunk in deep pits, and the CAM pathway that keeps stomata closed by day; Opuntia has leaves reduced to spines with flattened photosynthetic stems. Cold-climate mammals have shorter snout, ears, tail and limbs (Allen’s Rule); seals have insulating blubber. Desert lizards regulate body temperature behaviourally by basking, shifting to shade, and burrowing.
A population is a group of individuals of the same species in a well-defined geographical area, sharing resources and potentially interbreeding. Natural selection operates at the population level.
Population attributes include size, density, natality, mortality, immigration, emigration, sex ratio, age pyramids, growth forms and biotic potential. An individual has a birth and a death; a population has a birth rate and a death rate.
Natality is the birth rate (crude or specific). Mortality is the death rate, usually expressed per 1000 individuals per year. Absolute natality is always more than realized natality; absolute mortality is always less than realized mortality.
The sex ratio is the ratio of individuals of one sex to the other; 1:1 is generally the most common evolutionary stable strategy (ESS).
An age pyramid plots the population as Pre-Reproductive (0–14 years), Reproductive (15–44 years) and Post-reproductive (45–85+ years) groups. Its shape predicts rapid, slow, zero or negative growth.
Population density (N) may be measured as total number, as biomass, as relative density (catch per trap), or indirectly — for example, the tiger census from pug marks and fecal pellets.
Density changes through four processes: natality (B) and immigration (I) increase it; mortality (D) and emigration (E) decrease it. Hence
\[N_{t+1} = N_t + [(B + I) - (D + E)]\]
Exponential growth occurs when resources are unlimited, giving a J-shaped curve; Darwin showed that even the elephant could reach enormous numbers under such conditions. Logistic growth occurs when resources are limited: the population passes through a lag phase, a log (exponential) phase, a diminishing growth phase and a stationary phase, levelling off at the carrying capacity (K) and giving a sigmoid curve. This is called Verhulst–Pearl Logistic Growth, and it is the more realistic model.
Reproductive strategies differ under different selection pressures: many small offspring (oysters, pelagic fishes) or few large offspring (birds, mammals).
Interactions may be intraspecific (within a population) or interspecific (between species). Interspecific interactions are classified by their effect on each species: mutualism (+ +), protocooperation (+ +, but not obligatory), commensalism (+ O), amensalism (– O), competition (– –), parasitism and predation (+ –), and neutralism (O O).
Mutualism is obligatory and interdependent — lichens (fungus with alga or cyanobacteria), and plant–pollinator relationships, which often show co-evolution and must be safeguarded against ‘cheaters’.
Competition lowers the fitness of one species in the presence of another, and may occur between unrelated species (flamingos and fish competing for zooplankton) and even when resources are abundant (leopards avoiding a lion pride). Gause’s Competitive Exclusion Principle states that two closely related species competing for the same resources cannot co-exist indefinitely and the inferior one is eliminated — true when resources are limiting. Where resources are sufficient, resource partitioning allows co-existence.
Parasitism: parasites are often host-specific and co-evolve with the host. Endoparasites lose sense organs and the digestive system, gain adhesive organs or suckers, and have a high reproductive capacity. Life cycles may need vectors — Plasmodium vivax needs the mosquito. Ectoparasites include lice, ticks and copepods; Cuscuta is a parasitic plant that has lost chlorophyll and leaves. In brood parasitism, the Asian koel lays eggs resembling those of its host, the common Indian crow, and they hatch earlier.
Predation keeps prey populations in check (frogs and locusts), underlies biological control of pests, and maintains species diversity by reducing competition among prey. Exotic species without natural predators become invasive (zebra mussels). Predators are ‘prudent’. Prey defences include camouflage and warning colouration (the Monarch butterfly, whose distasteful chemical is acquired as a caterpillar from a poisonous weed). Plants defend themselves with thorns (Acacia, Cactus) and with secondary metabolites — Calotropis cardiac glycosides, nicotine, caffeine, quinine, strychnine, opium.
Commensalism benefits one species and leaves the other unaffected — an orchid epiphyte on a mango branch, cattle egrets beside grazing cattle, and the clown fish among the tentacles of a sea anemone.
Flowchart
FC 13.11 – The chapter in one chain: Abiotic factors (temperature, water, light, soil) shape the habitat → the organism responds by regulating, conforming, migrating or suspending → over evolutionary time these responses become adaptations (morphological, physiological, behavioural) → individuals of a species form a population with its own attributes (natality, mortality, sex ratio, age pyramid, density) → the population grows exponentially under unlimited resources but logistically towards carrying capacity K under limited resources → populations of different species interact (mutualism, competition, parasitism, predation, commensalism, amensalism, neutralism) → together they form the biological community
13.7 Points to Ponder
An organism’s habitat can be described by its address, and its niche by its profession. If a species were somehow able to keep its habitat but lose its niche, would it still be able to survive there? What does your answer tell you about which of the two is more essential to an organism?
A regulator spends a great deal of energy to hold its internal environment steady, while a conformer spends none at all and simply lets its body track the surroundings. If regulation is so obviously advantageous, why has natural selection allowed the vast majority of animals and plants to remain conformers?
Hibernation and aestivation are both ways of “waiting out” a hostile season by shutting down. What, then, decides whether a species evolves the ability to migrate instead of the ability to suspend? Think about body size, energy reserves, and the distance to a more hospitable habitat.
A desert plant closes its stomata in the daytime and takes in carbon dioxide at night. What is the cost of this arrangement, and why do plants in wet habitats not adopt it, even though it saves water?
Should an ideal parasite be able to thrive within its host without harming it at all? If a harmless parasite would be so successful, why has natural selection not produced one?
Absolute natality is always greater than realized natality, and absolute mortality is always less than realized mortality. What does the size of the gap between the absolute and the realized figure tell you about the environment a population is living in?
The age pyramid of a country with a very broad base predicts continued growth even if birth rates were to fall today. Why does a population continue to grow for years after its birth rate has been brought down? What does this mean for planning food, water and schools in such a country?
Every species is capable of exponential growth, yet almost no species in nature actually shows it for long. What is holding them back — and what does it suggest about a human population that appears, at present, not to be held back in the same way?
What will happen when the carrying capacity of a habitat is exceeded? Can a population overshoot K, and if so, what must follow?
Gause’s Competitive Exclusion Principle predicts that one of two closely related competitors must eventually disappear. Yet forests, coral reefs and grasslands are full of closely related species living side by side. How do you reconcile these two facts?
Predators are described as ‘prudent’. Is prudence something a predator possesses, or something that the process of natural selection produces? What is the difference, and why does it matter to how we explain behaviour in biology?
Nicotine, caffeine, quinine, strychnine and opium were all evolved by plants as chemical defences against animals that tried to eat them. What does it say about our relationship with plants that we now cultivate them on a commercial scale for those very chemicals?
Monocot plants like cereals and pulses, and dicot plants like mango, adopt very different reproductive strategies. What could be the reason behind this difference, and what selection pressures might have favoured each?
What could be the causes of a deviation from the 1:1 sex ratio in a natural habitat? If 1:1 is an evolutionarily stable strategy, what force must be at work whenever a population departs from it?
13.8 Objective Questions
- Which factor of an ecosystem includes plants, animals and microorganisms?
- Biotic factor
- Abiotic factor
- Direct factor
- Indirect factor
- An assemblage of individuals of different species living in the same habitat and having functional interactions is …………….
- Biotic community
- Ecological niche
- Population
- Ecosystem
- Association between sea anemone and clown fish is that of …………….
- Mutualism
- Commensalism
- Parasitism
- Amensalism
- Select the statement which explains parasitism best.
- One species is benefited.
- Both the species are benefited.
- One species is benefited, other is not affected.
- One species is benefited, other is harmed.
- Growth of bacteria in a newly inoculated agar plate shows …………….
- exponential growth
- logistic growth
- Verhulst-Pearl logistic growth
- zero growth
13.9 Check Your Concepts
Two bird species live in the same forest, feed at the same height, and eat exactly the same insects. Predict what will happen to them if the insect supply becomes limiting, and name the principle on which your prediction rests.
Assertion (A): A habitat can support many species at a time, but a niche supports only one. Reason (R): A single habitat contains numerous niches. Examine whether A and R are both correct, and whether R correctly explains A.
Why is the realized niche of a species always smaller than its fundamental niche? Under what single condition would the two become identical?
Arrange the following in the correct sequence of increasing complexity in the ecological hierarchy, and justify the order: Community, Biome, Organism, Population.
Temperature is called the most ecologically relevant environmental factor. Explain, at the level of the cell, why a change of a few degrees can decide whether an organism survives in a habitat.
A student claims that “heat and cold are equally dangerous to an enzyme.” Study the shape of the enzyme activity–temperature curve and explain why this claim is incorrect.
What happens to a freshwater fish, at the level of its cells, if it is released into the sea? Explain in terms of osmosis, and state why a euryhaline fish would fare better.
Herbs and shrubs growing on a forest floor photosynthesise optimally under very low light. Why is this an adaptation and not merely a disadvantage?
Birds begin their migration before the winter cold actually sets in. What environmental cue are they using, and why is it a more reliable signal than temperature itself?
Complete the causal chain and explain each arrow: soil grain size → water-holding capacity → ? → ?
Regulation keeps the internal environment constant but is energetically expensive; conformity is free but leaves the organism exposed. Why, then, have most animals and plants remained conformers?
A polar bear hibernates and a desert snail aestivates. Both are described as using the same strategy. Name the strategy and explain what makes the two cases the same despite the opposite seasons.
Explain, using the idea of surface area and volume, why Allen’s Rule holds. Would you expect Allen’s Rule to apply to a mammal living in a hot desert in the opposite direction? Give your reasoning.
What would happen to a CAM plant if it were forced to keep its stomata open during the day in a desert? Explain the trade-off that the CAM pathway resolves.
A desert lizard is a conformer, yet it keeps its body temperature remarkably constant through the day. How is this possible? What does this tell you about the power of behavioural adaptation?
Assertion (A): Absolute natality is always greater than realized natality. Reason (R): Environmental pressures such as competition and shortage of resources reduce the number of births actually achieved. Examine whether A and R are correct, and whether R explains A.
A population records a very high birth rate in a given year, and yet its density falls. Using the four basic processes that change population density, explain how this is possible.
Why is a 1:1 sex ratio described as an evolutionary stable strategy? Explain what would happen in a population in which one sex became rare.
Two countries have identical present birth rates, but one has a broad-based age pyramid and the other a narrow-based one. Predict the population of each 15 years from now, and justify your prediction from the shape of the pyramid.
Why is the total number of individuals a poor measure of population density in an area with millions of termites and only a few anteaters? Name a better measure and justify it.
The tiger census is based on pug marks and fecal pellets rather than on direct counting. Is this a weakness of the method or a strength? Defend your answer.
In the logistic growth curve, the population is largest at the stationary phase but is growing fastest in the log phase. Explain why these two are not the same thing, and identify the common error a student makes here.
Look at the logistic growth equation \(\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)\). Explain in words what happens to the growth rate as \(N\) approaches \(K\), and what happens when \(N = K\).
Why did Darwin use the elephant — the slowest breeder he could think of — rather than a fast-breeding animal, to make his point about the potential for exponential growth? What conclusion was he actually driving at?
Order the phases of the sigmoid growth curve and describe what is happening to the population in each: diminishing growth phase, lag phase, stationary phase, log phase.
Match the following interactions with their sign pairs, and give one example of each.
| Interaction | Sign pair |
|---|---|
| a. Commensalism | i. (+ +) |
| b. Amensalism | ii. (+ O) |
| c. Mutualism | iii. (– O) |
| d. Competition | iv. (– –) |
| e. Predation | v. (+ –) |
Both mutualism and protocooperation are (+ +) interactions. What single feature separates them, and why does a lichen belong to the first rather than the second?
Why must a mutualistic system such as a flower and its pollinator be safeguarded against ‘cheaters’? Describe how the shape of a flower can act as such a safeguard.
Leopards avoid hunting close to a lion pride even when prey is abundant. Which feature of competition does this illustrate, and why does it show that competition does not require resources to be limiting?
Gause’s principle predicts that one of two closely related competitors must be eliminated, yet a single forest may contain many closely related bird species. Resolve this contradiction using the idea of resource partitioning.
An endoparasite has lost its digestive system and its sense organs, but has an enormous reproductive capacity. Explain how these three features are connected, and why each makes sense in the habitat the parasite occupies.
The eggs of the Asian koel closely resemble those of the common Indian crow in size and colour. Explain how this resemblance could have arisen, and why koel eggs hatching earlier than the host’s eggs is an advantage to the koel chick.
What would happen to the species diversity of a community if all its predators were removed? Trace the sequence of consequences, and explain why predation can actually increase the number of species that coexist.
Why do exotic species so often become invasive in a new geographical area? Relate your answer to the growth curve such a population would follow at first.
Camouflage hides the prey; warning colouration advertises it. Both are anti-predator defences. Explain why warning colouration only works for a prey species that is genuinely distasteful or poisonous.
The Monarch butterfly is distasteful to birds, but it does not manufacture the chemical responsible. Where does it come from, and at which stage of the life cycle is it acquired?
Cattle and goats never browse on Calotropis. Give the reason, and explain how this is a chemical defence in the same category as the thorns of Acacia is a morphological one.
“Predators in nature are prudent.” Does this mean the predator deliberately spares its prey? Explain what actually produces this apparent restraint.
In a labelled diagram of a lichen thallus, mark the algal and the fungal components. Explain, from their positions, why the arrangement is well suited to the partnership.
You observe two species living together. Species A gains, and species B is completely unaffected. Which two of the interactions studied in this chapter could this be, and what one further observation would let you decide between them?
Define the following terms: (30–40 words)
- Commensalism
- Parasitism
- Camouflage
- Give one example for each: (30–40 words)
- Mutualism
- Interspecific competition
- Name the type of association: (30–40 words)
- Clown fish and sea anemone
- Crow feeding the hatchling of Koel
- Humming birds and host flowering plants
What is the ecological process behind the biological control method of managing pest insects? (30–40 words)
How is the dormancy of seeds different from hibernation in animals? (80–100 words)
If a marine fish is placed in a fresh water aquarium, will it be able to survive? Give reason. (80–100 words)
Name the important defence mechanisms in plants against herbivores. (80–100 words)
An orchid plant is growing on the branch of a mango tree. How do you describe this interaction between the orchid and the mango tree? (80–100 words)
Distinguish between the following: (80–100 words)
- Hibernation and Aestivation
- Ectotherms and Endotherms
- Parasitism and Mutualism
- Write a short note on: (80–100 words)
- Adaptations of desert animals
- Adaptations of plants to water scarcity
- Behavioural adaptations in animals
Define Population and Community. (80–100 words)
With the help of a suitable diagram, describe the logistic population growth curve. (150–200 words)
Enlist and explain the important characteristics of a population. (150–200 words)
13.10 Practice With Numericals
In a pond there were 150 tilapia last year. Through reproduction, 600 new fish were added during the year. Calculate the birth rate in offspring per fish per year.
A population of 2000 individuals records 36 deaths in a year. Express the mortality rate as deaths per 1000 individuals per year, and as a percentage of the total population.
A population of langurs in a sanctuary has a density of 850 at time \(t\). During the next year, 120 infants are born, 25 langurs move in from a neighbouring forest, 60 langurs die, and 45 langurs leave the sanctuary. Using \(N_{t+1} = N_t + [(B + I) - (D + E)]\), calculate the population density at time \(t+1\), and state whether the population is growing or declining.
In a grassland, a population has 400 births, 150 immigrants and 250 deaths in a year, and the population density is found to be unchanged at the end of the year. Calculate the number of individuals that must have emigrated.
A deer population in a reserve is at its carrying capacity of \(K = 1200\). Using the logistic equation \(\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)\), calculate the value of \(\frac{dN}{dt}\) when \(N = 1200\), and state what this means for the population.
For the same reserve, with \(K = 1200\) and an intrinsic rate of natural increase \(r = 0.2\) per year, calculate the growth rate \(\frac{dN}{dt}\) when the population density \(N\) is 300. Compare your answer with the growth rate at \(N = 1100\), and explain the difference in terms of competition for resources.
A census of a village of 5000 people records 105 births and 45 deaths in one year, with no immigration or emigration. Calculate the crude birth rate and the crude death rate per 1000 population per year, and find the net change in population size over the year.