Organisms and Populations

NCERT opens by reminding you of the Class 11 definition: ecology is the study of the interactions among organisms and between the organism and its physical (abiotic) environment. Ecology works at four levels of biological organisation: organisms, populations, communities and biomes. Ecology at the level of the organism is physiological ecology, which asks how an individual copes with its environment. This chapter moves one level up and studies ecology at the level of the population. The next chapter (Ecosystem) works at the community and ecosystem level, and the chapter after it (Biodiversity) at the level of biomes and the whole biosphere.

One line from the introduction is worth memorising because it explains why the whole chapter is built around populations rather than individuals: although an individual organism is the one that has to cope with a changed environment, it is at the population level that natural selection operates to evolve the desired traits. Population ecology therefore links ecology to population genetics and evolution. That sentence is the standard answer to any question that asks why population ecology is important.

A note on the syllabus, because many question banks still carry old questions. The rationalised NCERT chapter (2023 onwards) removed the earlier material on major abiotic factors (temperature, water, light, soil), on how organisms respond to them (regulate, conform, migrate, suspend) and on adaptations such as the kangaroo rat, altitude sickness and desert plants. The current chapter is only about populations: their attributes, their growth, life history variation and the interactions between populations. CBSE sets questions from the rationalised text, so prepare this chapter from the sections below and do not spend time on eurythermal and stenothermal organisms, hibernation or diapause, however often you see them in older papers.

In nature, individuals of a species are rarely alone; they live in groups that share or compete for the same resources, and they can interbreed. NCERT defines a population as a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding. The five NCERT examples are worth reproducing exactly, because 1-mark questions ask for two of them: cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, and lotus plants in a pond. Each of these is a population. Note that the definition includes plants and microbes as much as animals.

A community is the next level up: an assemblage of populations of different species living together and interacting in a given area. The NCERT exercise asks you to define both terms in one question, so learn them as a pair. A population is one species; a community is many populations, and the interactions between them are the subject of the last section of this chapter.

Why study populations at all? Because a population has properties that no individual has. An individual is born once and dies once; a population has a birth rate and a death rate. An individual is male or female; a population has a sex ratio. An individual has an age; a population has an age distribution. And a population has a size or density. These four are the population attributes of the next section. Whenever the board asks 'list the attributes that a population possesses but an individual does not', these are the four to write, with one line of explanation each.

A second reason is practical. The size of a population is not fixed; it changes with food availability, predation pressure and weather. Knowing how populations grow and how they interact is what lets us manage fisheries, control pests, protect endangered species and plan for a human population. NCERT makes the point that governments introduce restraints on human population growth precisely because no population can grow without limit.

Birth rate and death rate. These are per-capita rates, not total counts. NCERT's worked example: a pond had 20 lotus plants last year; through reproduction 8 new plants were added, taking the current population to 28. The birth rate is 8/20 = 0.4 offspring per lotus per year. Notice that you divide by the initial population (20), not the final one (28). The death-rate example: if 4 individuals in a laboratory population of 40 fruit flies died in a week, the death rate is 4/40 = 0.1 individuals per fruit fly per week. Every numerical in this chapter is one of these two divisions or the net-change equation of the next section; get the denominator right and state the unit.

Sex ratio. An individual is either male or female, but a population has a sex ratio, for example 60 per cent females and 40 per cent males.

Age distribution and the age pyramid. A population at any time is made of individuals of different ages. If the per cent of individuals in each age group is plotted, the resulting structure is called an age pyramid. For human populations the pyramid shows males and females side by side. NCERT uses three age groups: pre-reproductive, reproductive and post-reproductive. The shape of the pyramid tells you the growth status of the population: a broad base with many pre-reproductive individuals is a growing (expanding) population, triangular in shape; a pyramid with roughly equal pre-reproductive and reproductive groups and a narrower top is a stable population, bell-shaped; a pyramid with a narrow base and a wide reproductive and post-reproductive band is a declining population, urn-shaped. Age pyramids of human populations are used by governments for planning: an expanding pyramid means more schools and jobs will be needed; a declining one means more old-age care.

Population size or density (N). Population size, technically called population density and designated N, is the total number (or biomass) of individuals in the population. In nature it may be as low as fewer than 10 (Siberian cranes at Bharatpur wetlands in a year) or run into millions (Chlamydomonas in a pond). NCERT is careful to say that density need not be measured in numbers only, and gives four situations that are examined regularly. (1) If a field has 200 Parthenium plants but a single huge banyan tree with a large canopy, calling the banyan's density 'low' underestimates its enormous role in the community; here per cent cover or biomass is the more meaningful measure. (2) When the population is huge and counting is impossible (a dense microbial culture in a petri dish), total number is not a usable measure. (3) For many investigations, relative density is enough: the number of fish caught per trap is a good enough measure of the fish density in a lake. (4) We are often obliged to estimate population size indirectly, without counting or even seeing the animals: the tiger census in our national parks and tiger reserves is based on pug marks and fecal pellets. The 2-mark question 'name two methods to estimate the tiger population' expects exactly these two.

The size of a population is not a static parameter. It changes with time depending on food availability, predation pressure and weather, and the change can be traced to four basic processes. Two of them add individuals and two remove them.

• Natality (B): the number of births during a given period in the population that are added to the initial density.
• Mortality (D): the number of deaths in the population during the given period.
• Immigration (I): the number of individuals of the same species that have come into the habitat from elsewhere during the period under consideration.
• Emigration (E): the number of individuals of the population who left the habitat and went elsewhere during the period.

Natality and immigration increase the density; mortality and emigration decrease it. If N is the population density at time t, then the density at time t + 1 is

N_t+1 = N_t + [(B + I) − (D + E)]

Read the equation as a sentence: the population density will increase if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E), and otherwise it will decrease. A recent board MCQ gave a rat population in a granary with natality 280, mortality 200, immigration 40 and emigration 50 and asked for the net increase: (280 + 40) − (200 + 50) = 320 − 250 = 70. Always show the two brackets, then the subtraction.

NCERT adds a qualification that turns up as a 1-mark or Assertion–Reason item. Under normal conditions births and deaths are the most important factors influencing population density; immigration and emigration assume importance only under special conditions. The example given is a habitat that is just being colonised: there, immigration may contribute more significantly to population growth than birth rates. So if a question asks when immigration matters more than natality, the answer is 'when a new habitat is being colonised'.

The board likes to ask this section as a flow chart: draw a box labelled 'population density (N)', two arrows entering it labelled natality and immigration, two arrows leaving it labelled mortality and emigration, and write the equation beneath. That figure plus the four definitions is a complete 3-mark answer.

This is the most examined part of the chapter, and it appears as a 5-mark question, a 3-mark 'draw and compare' and a 4-mark case with a graph. Learn both models with their equations, curves and conditions.

Exponential (geometric) growth. Resource (food and space) availability is essential for unimpeded growth. When resources in the habitat are unlimited, each species can realise fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection, and the population grows in an exponential or geometric fashion. If a population of size N has a per-capita birth rate b and a per-capita death rate d, the change in N during a unit time period t is dN/dt = (b − d) × N. Writing (b − d) = r, this becomes dN/dt = rN. The r is called the intrinsic rate of natural increase, and NCERT calls it a very important parameter for assessing the impact of any biotic or abiotic factor on population growth. The values to remember: the r for the Norway rat is 0.015, for the flour beetle 0.12, and for the human population in India in 1981 it was 0.0205. Plotting N against time gives a J-shaped curve. The integral form of the equation is N_t = N₀ e^rt, where N_t is the population density after time t, N₀ the density at time zero, r the intrinsic rate of natural increase and e the base of natural logarithms (2.71828). Any species growing exponentially under unlimited resources can reach enormous densities in a short time; Darwin showed how even a slow-growing animal like the elephant could reach enormous numbers in the absence of checks. The NCERT exercise uses the integral form: if a population doubles in 3 years, 2 = e^3r, so r = ln 2/3 = 0.693/3 ≈ 0.231 per year.

Logistic growth. No population in nature has unlimited resources, so exponential growth cannot continue. Limited resources lead to competition between individuals, and eventually the 'fittest' individuals survive and reproduce. A given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible; this limit is called 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 density reaches the carrying capacity. Plotting N against t gives a sigmoid (S-shaped) curve. This is Verhulst-Pearl Logistic Growth, described by dN/dt = rN((K − N)/K), where N is the population density at time t, r the intrinsic rate of natural increase and K the carrying capacity. Notice what the bracket does: when N is small, (K − N)/K is close to 1 and growth is nearly exponential; as N approaches K the bracket approaches 0 and growth stops.

Which is realistic? NCERT's sentence, which should be quoted: since resources for growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered a more realistic one. On the NCERT graph, curve (a) is exponential growth when resources are not limiting and curve (b) is logistic growth when resources are limiting, with K drawn as a horizontal dotted line that the logistic curve approaches.

This short section carries a definition that has appeared as a 3-mark question: Darwinian fitness. Populations evolve to maximise their reproductive fitness, also called Darwinian fitness, which NCERT equates with a high r value, in the habitat in which they live. Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy. The strategies differ in two ways, and each has NCERT examples that are asked by name.

• How often they breed. Some organisms breed only once in their lifetime: the Pacific salmon fish and bamboo. Others breed many times during their lifetime: most birds and mammals. Bamboo flowering is a classic 1-mark question ('mention the unique feature of flowering and fruiting in bamboo'): it flowers once in its life, often after decades, and then dies.
• How many offspring and how large. Some produce a large number of small-sized offspring: oysters and pelagic fishes. Others produce a small number of large-sized offspring: birds and mammals.

NCERT then asks which strategy is desirable for maximising fitness and answers that there is no single answer: ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live. That sentence is the reason a question on this section is usually worded 'why do different organisms have different reproductive strategies'.

An Assertion–Reason on this section usually pairs 'organisms evolve towards the most efficient reproductive strategy' with 'the strategy is decided by the selection pressures of the habitat'; both are true and the reason explains the assertion.

NCERT begins with a question: can you think of any natural habitat on earth inhabited by just a single species? There is none. Animals, plants and microbes do not and cannot live in isolation; they interact in various ways to form a biological community. For any species the minimal requirement is one more species on which it can feed. Even a plant, which makes its own food, cannot survive alone: it needs soil microbes to break down organic matter and return inorganic nutrients for absorption, and it needs an animal agent for pollination. So a community with at least two species is the minimum.

Interspecific interactions arise from the interaction of populations of two different species. They may be beneficial, detrimental or neutral (neither harmful nor beneficial) to one of the species or to both. Assigning + for a beneficial effect, − for a detrimental effect and 0 for a neutral one, the six possible outcomes are the table the board keeps asking for:

• Mutualism: + / +. Both species benefit.
• Competition: − / −. Both species lose.
• Predation: + / −. The predator benefits, the prey is harmed.
• Parasitism: + / −. The parasite benefits, the host is harmed.
• Commensalism: + / 0. One species benefits, the other is neither benefited nor harmed.
• Amensalism: − / 0. One species is harmed, the other is unaffected.

NCERT adds one connecting sentence that examiners like: predation, parasitism and commensalism share a common characteristic, that the interacting species live closely together. A typical 2-mark question gives six pairs of organisms and asks you to name the interaction for each; a 3-mark version gives a table with letters A to D in place of the signs or the names and asks you to fill them in. The rest of the chapter takes the interactions one at a time, in NCERT order: predation, competition, parasitism, commensalism, mutualism. Amensalism has no separate section in NCERT; if you need an example, the inhibition of bacteria by the antibiotic secreted by Penicillium is the standard one.

NCERT starts with a point that has been asked as a 2-mark question: although animals eating plants are categorised separately as herbivores, in a broad ecological context they are not very different from predators. For a plant, the herbivore is the predator.

Roles of predators. The obvious role is that predators act as 'conduits' for energy transfer across trophic levels: the energy fixed by plants moves up the food chain through them. NCERT lists three further roles, and 'mention two roles of predators other than energy transfer' is a regular question.

1. They keep prey populations under control. Without predators, prey species could reach very high densities and cause ecosystem instability. When an exotic species is introduced into a new area it becomes invasive and spreads fast because the invaded land has no natural predators of it. The prickly pear cactus introduced into Australia in the early 1920s spread into millions of hectares of rangeland and was brought under control only when a cactus-feeding predator (a moth) from its natural habitat was introduced. This is the ecological principle behind biological control of agricultural pests: the ability of a predator to regulate its prey population (NCERT exercise question 5).
2. They help maintain species diversity in a community by reducing the intensity of competition among competing prey species. In the rocky intertidal communities of the American Pacific Coast the starfish Pisaster is an important predator; when all the starfish were removed from an enclosed intertidal area in a field experiment, more than 10 species of invertebrates became extinct within a year because of interspecific competition.
3. Predators are 'prudent'. If a predator is too efficient and overexploits its prey, the prey may become extinct and the predator will follow for lack of food. That is why predators in nature are prudent.

Defences of prey animals. Some species of insects and frogs are cryptically coloured (camouflaged) to avoid detection. Some are poisonous and are avoided. The Monarch butterfly is highly distasteful to its predator (a bird) because of a special chemical in its body, which the butterfly acquires during its caterpillar stage by feeding on a poisonous weed. The source of the chemical is the detail the examiner checks.

Defences of plants. Nearly 25 per cent of all insects are phytophagous (they feed on plant sap and other plant parts). Plants cannot run away, so they have evolved morphological and chemical defences. Thorns (Acacia, Cactus) are the commonest morphological defence. Many plants make and store chemicals that make the herbivore sick, inhibit its feeding or digestion, disrupt its reproduction or kill it. The weed Calotropis produces highly poisonous cardiac glycosides, which is why cattle and goats never browse on it. The substances we extract from plants commercially, nicotine, caffeine, quinine, strychnine and opium, are produced by plants as defences against grazers and browsers.

When Darwin spoke of the struggle for existence and survival of the fittest, he was convinced that interspecific competition is a potent force in organic evolution. It is generally believed that competition occurs when closely related species compete for the same limiting resources, but NCERT makes two corrections. First, totally unrelated species can compete: in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton. Second, resources need not be limiting: in interference competition the feeding efficiency of one species is reduced by the interfering and inhibitory presence of the other, even when food and space are abundant. So NCERT defines competition as a process in which the fitness of one species (measured as its r) is significantly lower in the presence of another species.

In the laboratory, as Gause and others showed, when resources are limited the competitively superior species eventually eliminates the other. Evidence for such competitive exclusion in nature is not always conclusive, but there are circumstantial evidences. (1) The Abingdon tortoise in the Galapagos Islands became extinct within a decade after goats were introduced, apparently because of the greater browsing efficiency of the goats. (2) Competitive release: a species restricted to a small area by a competitively superior species expands its range dramatically when the competitor is experimentally removed. Connell's field experiments on the rocky sea coasts of Scotland showed that the larger and competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone. In general, herbivores and plants are more adversely affected by competition than carnivores.

Gause's Competitive Exclusion Principle states that two closely related species competing for the same resources cannot coexist indefinitely, and the competitively inferior one will be eliminated eventually. NCERT qualifies it at once: this may be true if resources are limiting, but not otherwise. More recent studies do not support such gross generalisations. They do not rule out interspecific competition in nature, but point out that species facing competition may evolve mechanisms that promote coexistence rather than exclusion. One such mechanism is resource partitioning: two species that compete for the same resource avoid competition by choosing, for instance, different times for feeding or different foraging patterns. MacArthur showed that five closely related species of warblers living on the same tree coexist because of behavioural differences in their foraging activities. The 2-mark question 'state Gause's principle and how recent studies have modified it' wants the principle, the qualification, and resource partitioning with the warbler example.

The parasitic mode of life ensures free lodging and meals, so it is not surprising that parasitism has evolved in so many groups, from plants to higher vertebrates. Many parasites are host-specific (they can parasitise only a single host species), and host and parasite tend to co-evolve: if the host evolves special mechanisms for rejecting or resisting the parasite, the parasite has to evolve mechanisms to counteract and neutralise them to remain successful with the same host. This is the standard answer to 'explain co-evolution with reference to parasites'.

Adaptations of parasites, four of them, asked almost every year: (1) loss of unnecessary sense organs, (2) presence of adhesive organs or suckers to cling on to the host, (3) loss of the digestive system, and (4) high reproductive capacity.

Life cycles. Parasite life cycles are often complex, involving one or two intermediate hosts or vectors to facilitate parasitisation of the primary host. The human liver fluke (a trematode) depends on two intermediate hosts, a snail and a fish, to complete its life cycle. The malarial parasite needs a vector (mosquito) to spread to other hosts.

Harm to the host. Most parasites reduce the survival, growth and reproduction of the host and reduce its population density, and they may make the host physically weak and so more vulnerable to predation. NCERT asks why natural selection did not produce a totally harmless parasite that thrives without harming its host; the expected reasoning is that a parasite necessarily takes nutrients from the host, so some harm is unavoidable.

Ectoparasites feed on the external surface of the host: lice on humans, ticks on dogs, and the copepods that infest many marine fish. Cuscuta, a parasitic plant found growing on hedge plants, has lost its chlorophyll and leaves in the course of evolution and draws its nutrition from the host. The female mosquito is not considered a parasite, although it needs our blood for reproduction: it does not live on or in the host, it only takes a brief blood meal and leaves. Endoparasites live inside the host body at sites such as the liver, kidney, lungs and red blood cells. Their life cycles are more complex because of their extreme specialisation, and their morphological and anatomical features are greatly simplified while their reproductive potential is emphasised.

Brood parasitism in birds: the parasitic bird lays its eggs in the nest of its host and lets the host incubate them. Over evolution the eggs of the parasitic bird have come to resemble the host's eggs in size and colour, which reduces the chance of the host detecting the foreign eggs and ejecting them. The NCERT example is the cuckoo (koel) and the crow, which you can watch in a park during the breeding season, spring to summer.

Commensalism is the interaction in which one species benefits and the other is neither harmed nor benefited, the + / 0 row of the table. NCERT gives four examples and each has been asked on its own as a 1- or 2-mark question, so learn all four with the reason the second species is unaffected.

• An orchid growing as an epiphyte on a mango branch. The orchid gets a place to grow and access to light; the mango tree derives no apparent benefit and is not harmed, because the orchid does not draw nutrients from it. This is NCERT exercise question 4 and appeared as a 1-mark board question ('an orchid growing on a forest tree: name the interaction').
• Barnacles growing on the back of a whale. The barnacles get a substrate and are carried through plankton-rich water; the whale is unaffected.
• Cattle egret and grazing cattle, a sight common in farmed rural areas. The egrets forage close to the cattle because the cattle, as they move, stir up and flush out insects from the vegetation that the egrets would otherwise find hard to catch. The cattle gain nothing and lose nothing.
• Clown fish and sea anemone. The sea anemone has stinging tentacles and the clown fish 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 fish.

Two comparisons are asked. Commensalism versus mutualism: in mutualism both benefit (lichen, mycorrhiza), in commensalism only one does (epiphytic orchid, egret). Commensalism versus parasitism: in both, the interacting species live closely together, but in parasitism the second species is harmed (Cuscuta on a hedge plant) whereas in commensalism it is unaffected (orchid on a mango branch). When a question asks you to 'differentiate with plant examples', use the orchid for commensalism, Cuscuta for parasitism and mycorrhiza or lichen for mutualism.

One trap: students sometimes call the clown fish and sea anemone relationship mutualism because popular sources say the fish cleans the anemone or lures prey to it. The NCERT position, and therefore the marking scheme's, is commensalism: the anemone does not appear to derive any benefit. Write what NCERT says.

Mutualism confers benefits on both interacting species. Lichens are an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. Mycorrhizae are associations between fungi and the roots of higher plants: the fungi help the plant absorb essential nutrients from the soil, and the plant in turn provides the fungi with energy-yielding carbohydrates. These two are the 'commercially exploited' or 'plant' examples of mutualism that 2-mark questions ask for.

Plant-animal mutualisms. The most spectacular examples are plant-animal relationships. Plants need animals for pollination and seed dispersal, and animals have to be paid 'fees' for these services. Plants offer rewards: pollen and nectar for pollinators, and juicy, nutritious fruits for seed dispersers. The system also has to be safeguarded against 'cheaters', for example animals that steal nectar without pollinating. This is why plant-animal interactions often involve co-evolution of the mutualists: the evolution of the flower and of its pollinator species are tightly linked.

Fig and wasp. In many species of fig trees there is a tight one-to-one relationship with the pollinator species of wasp: a given fig species can be pollinated only by its 'partner' wasp species and by no other. The female wasp uses the fruit as an oviposition (egg-laying) site and uses the developing seeds within the fruit to nourish its larvae. The wasp pollinates the fig inflorescence while searching for suitable egg-laying sites. In return for pollination, the fig offers the wasp some of its developing seeds as food for the wasp larvae. This has been a 2- or 3-mark question in each of the last two board papers; write the four points above in order.

Orchids and Ophrys. Orchids show a bewildering diversity of floral patterns, many evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination. Not all orchids offer rewards. The Mediterranean orchid Ophrys employs 'sexual deceit' to get pollinated by a species of bee: one petal of its flower bears an uncanny resemblance to the female of the bee in size, colour and markings. The male bee is attracted to what it perceives as a female, 'pseudocopulates' with the flower and in the process is dusted with pollen. When the same bee pseudocopulates with another flower it transfers the pollen and so pollinates it. NCERT uses this to show how co-evolution operates: if the female bee's colour patterns change even slightly during evolution, pollination success will fall unless the orchid flower co-evolves to maintain the resemblance of its petal to the female bee. A 3-mark question asks what would happen to the orchid if the bee went extinct: the orchid would fail to be pollinated and would itself decline, because it depends on that one pollinator.