Organisms and Populations: Class 12 Biology · Chapter 11

A peacock fans out its tail and shivers it until the feathers rattle. A biologist watching him can ask two quite different questions. How does he do it? The answer lies in muscles, nerves and the way the long train feathers are anchored to the skin. Why does he do it? Now the answer is about peahens: across many generations, males with the showiest displays left more chicks. The first kind of question looks for a mechanism; the second asks what the behaviour is for and how it came about.

Both kinds of question can be asked at every level at which life is organised, from molecules and cells up to whole landscapes. This last unit of the book climbs to the top of that ladder. Ecology (पारिस्थितिकी) studies how organisms interact with one another and with their non-living (abiotic) surroundings: light, water, temperature and soil. Ecologists work at four levels: single organisms, populations of one species, communities of many species, and biomes, the large climatic zones such as deserts, grasslands and tropical forests Fig. 11.1.

11.1 Populations

We begin in the middle of the ladder, with the population (समष्टि). A population is the level at which births, deaths and evolution can be counted, so it is the natural place to start asking how many organisms live somewhere and why that number changes.

11.1.1 Population Attributes

Think of the blackbuck grazing in Velavadar National Park in Gujarat, the water hyacinth choking a city lake, or the yeast cells multiplying in a bowl of idli batter. Each group is a population: members of one species living in the same place at the same time, drawing on the same food, water and space, and able, at least in principle, to breed with one another. Organisms living entirely on their own are the exception in nature. Yeast mostly multiplies by budding rather than mating, yet ecologists still count such asexually produced groups as populations.

Why study populations rather than individuals? Because evolution happens to populations. Natural selection sorts individuals, but what changes from one generation to the next is the mix of alleles in a gene pool, and a gene pool belongs to a population (Chapter 6). That puts population ecology at a crossroads: its questions are shared with population genetics and with evolutionary biology.

Some quantities make sense only for a group. Asking how often one banyan tree is born is meaningless, but asking how many new banyans appear for every existing banyan in a decade gives a birth rate; the matching figure for deaths is the death rate. Ecologists express both per capita, that is, per individual already present, so that large and small populations can be compared fairly.

Count the males and females in the same herd and you have its sex ratio, another number that belongs to the group rather than to any one deer. India's census gives it as females per 1000 males: 943 in 2011. Adult females outnumber males in many deer and antelope herds, because young males leave or die more often.

Ages differ too. Ecologists usually sort members into three classes: pre-reproductive (too young to breed), reproductive and post-reproductive. Draw the percentage in each age group as a horizontal bar, males to the left and females to the right, stack the bars with the youngest at the bottom, and the result is an age pyramid. Its outline predicts the population's future Fig. 11.2.

India's own pyramid is changing. In 1971 it had the broad base of a fast-growing population. Since families became smaller, the bars for the youngest children have stopped widening, and the pyramid is slowly turning into a bell. Japan and much of Europe already show an urn shape.

Above all, ecologists track the number of individuals, written $N$. It is the common currency of the subject: to learn whether a drought, a new disease or a dam has harmed a species, they compare $N$ before and after. Real values span a huge range. Only about 150 Great Indian Bustards were estimated to survive in the wild in 2018, while a single litre of pond water can hold millions of algal cells.

The size is usually reported as a population density, the amount per unit area or volume. Counting heads is the natural way to measure it, but it can mislead when individuals differ hugely in size, and it is ambiguous for grasses that spread by runners, where one plant merges into the next. A plot with one old neem tree and forty tufts of grass shows the problem Fig. 11.3: the grass wins the count by forty to one, yet the neem shades far more ground and holds far more living matter. For such plants per cent cover (the share of ground a species covers) or biomass (its mass of living tissue) is the better yardstick.

How would you report a dense film of bacteria on a petri dish? Counting cells is hopeless, but you can measure the fraction of the dish the colonies cover, how cloudy a liquid culture is, or the dry mass of cells. For very large populations a relative density is often enough. Malaria workers, for example, dip a ladle into ponds and count mosquito larvae per dip; the number rises and falls with the real population, so it reveals a trend without a full count.

Many populations are estimated indirectly, without seeing every animal. Tiger surveys once relied on pugmarks and scats. Today the All India Tiger Estimation also places camera traps along forest trails and recognises each tiger by its unique stripes; the 2022 round put India's tiger population at about 3700. Plant ecologists estimate per cent cover just as indirectly, by dropping points at random over a plot and noting what lies under each one. The simulation below lets you try.

11.1.2 Population Growth

Count the same population year after year and the number almost never repeats itself. Its changes are what tell an ecologist whether the population is thriving or in trouble. However complicated the causes, every rise or fall is the net result of just four kinds of event: individuals are born, die, arrive or leave.

• Natality ($B$): how many individuals are born in the period; births add to $N$.
• Mortality ($D$): how many members die in the period; deaths subtract from $N$.
• Immigration ($I$): members of the species that move into the habitat from outside during the period.
• Emigration ($E$): members that move out of the habitat to settle somewhere else during the period.

Two of these add to the population and two take away. Put them together and you have a simple balance sheet Fig. 11.5.

When a reservoir fills or a lava island cools, nobody is there yet to give birth. The first members must fly, swim or drift in, so immigration drives the early growth of a population in a new habitat. Once the population is established the balance tips the other way: births and deaths account for most of the change, and migration matters mainly in special cases. Either way the rule is simple: $N$ rises when $B+I$ exceeds $D+E$ and falls when it is smaller.

Growth models: exponential growth

Can we predict how $N$ will change over time? Ecologists answer with simple mathematical models. Two classic models, one without limits on resources and one with them, show what difference those limits make, and they help us think about our own species too.

Suppose nothing is ever in short supply: food, water and space are unlimited. Then a population's growth depends only on how often its members reproduce and how often they die. Let $b$ be the per capita birth rate and $d$ the per capita death rate. In a short time each individual adds $b$ births and $d$ deaths, so the whole population changes at the rate

Here $r$ is the intrinsic rate of natural increase. Because it condenses births and deaths into one number, it is the yardstick ecologists reach for when they ask whether a pesticide, a drought or a new parasite is hurting a species: anything that lowers $b$ or raises $d$ lowers $r$. Small, fast-breeding species such as insects and bacteria have large values of $r$; large, slow breeders such as elephants and humans have small ones.

Because $dN\mathrm{/}dt$ is proportional to $N$, the bigger the population gets, the faster it grows. A graph of $N$ against time therefore bends ever more steeply upwards into a J-shaped curve Fig. 11.6. This is exponential growth, also called geometric growth. With a little calculus we can write $N$ itself as a function of time.

Logistic growth

Real habitats run out of room. Picture yeast in a flask of sugar solution or deer on a fenced island: sooner or later every newcomer means less food for the rest. Crowding sharpens competition among members of the same species, so births fall and deaths rise. The population creeps towards a ceiling set by the habitat's resources, its carrying capacity, $K$.

Plot such a population against time and an S-shaped (sigmoid) curve appears, called logistic growth, or Verhulst–Pearl logistic growth after the mathematician and the biologist who described it. Its stages have names. In the lag phase a handful of founders are settling in, so numbers creep up. In the acceleration phase there are more breeders and still plenty of food, so growth speeds up. Past the halfway mark, shortages bite and growth decelerates. Finally $N$ levels off along an asymptote close to $K$.

Read the bracket as a brake. When $N$ is tiny, $(K-N)\mathrm{/}K$ is almost 1 and growth is nearly exponential. When $N=K\mathrm{/}2$, the brake halves the growth, but because $N$ is now large the number added per unit time, $rK\mathrm{/}4$, is the highest it ever gets. As $N$ approaches $K$ the bracket falls towards zero and growth stops. If $N$ ever overshoots $K$, the bracket turns negative and the population shrinks back.

Of the two models, the logistic one fits nature better, because every real population meets a limit sooner or later. It is still a simplification. A herd may overshoot $K$ and crash, numbers may cycle around $K$, and $K$ itself rises in the wet season and falls in the dry one.

11.1.3 Life History Variation

Every organism has a limited budget of energy, which it must split between staying alive, growing and reproducing. Natural selection favours the split that leaves the most surviving descendants, that is, the highest reproductive or Darwinian fitness (for a population, a high $r$). The pattern of growth, breeding and death that results is the species' life history, and it varies remarkably from one species to another.

One difference is how often a species breeds. Neelakurinji shrubs on the hills of Kerala and Tamil Nadu carpet the slopes in blue once in twelve years and then die; many bamboos flower once after decades and die; Pacific salmon swim upriver once, spawn and perish. Such species stake everything on a single breeding event. Others, including mango trees and most birds and mammals, breed season after season.

A second choice is how to split the energy available for offspring. An oyster releases millions of tiny eggs and leaves them to the sea; orchids scatter seeds as fine as dust. At the other extreme an elephant cow carries one calf for about 22 months and then nurses it for years, and a coconut packs each seed with a large food store. Many cheap offspring with poor survival, or few costly ones with good survival Fig. 11.8?

No strategy wins everywhere. Many cheap offspring pay off where conditions are unpredictable and most young die whatever their parents do, as for oysters spawning into the open sea. Investing heavily in a few young pays off in crowded, stable habitats, where a well-fed, well-protected youngster can out-compete others. A species' life history therefore reflects both the physical conditions of its habitat and the other species living there, and explaining particular cases remains a lively research field.

11.1.4 Population Interactions

Look closely at one mango tree in an orchard. Caterpillars chew its leaves, bees and flies visit its flowers, birds and bats carry off its fruit, and fungi and bacteria break down its fallen leaves, releasing minerals that its roots take up again. No species can live alone; even a green plant, though it makes its own food, relies on decomposers to recycle nutrients and often on animals to move its pollen. The populations sharing a place form a community, held together by many links, some obvious and some hidden.

Ecologists sort these links with a simple code. For each of the two species involved, ask: does it gain (+), lose (−) or feel no effect (0)? Each interspecific interaction, one between populations of different species, then gets a pair of signs Fig. 11.9.

The sign pairs make the definitions easy to remember: mutualism (+, +), competition (−, −), predation and parasitism (+, −), commensalism (+, 0) and amensalism (−, 0). Amensalism is the least familiar. The blue-green mould Penicillium releases penicillin, which kills bacteria around it, although the mould gains nothing obvious. Notice also how close the partners are: predators, parasites and commensals usually live in direct contact with the species they exploit or follow, whereas competitors may never meet.

Predation

A predator feeds on another living organism, its prey. The definition is broader than it sounds: a goat browsing a hedge and a field mouse eating grain also feed on living things, so ecologists treat herbivores as predators of plants. Predation (परभक्षण) matters to a community in three ways. First, it carries energy up the food chain: the energy that plants capture from sunlight would stay locked in plant tissue if nothing ate them.

Second, predators regulate prey numbers. Freed from its enemies, a prey species can multiply until it destabilises the whole ecosystem. That is why a species carried to a new land so often turns invasive. Australia learned this the hard way: prickly pear cactus (Opuntia), brought in during the colonial era, had overrun roughly 24 million hectares of farmland by the mid-1920s. In 1926 scientists released a South American cactus moth whose caterpillars tunnel through the cactus pads, and within a few years most of the infested land was clear.

This is the basis of biological control: using a pest's natural predator or parasite to regulate it. India released the Mexican beetle Zygogramma bicolorata against the weed Parthenium (congress grass), and weevils of the genus Neochetina against water hyacinth. Ladybird beetles that devour aphids do the same job in fields and gardens.

Third, predators can protect the variety of species around them. On the rocky shores of Washington state in the 1960s, Robert Paine kept removing the starfish Pisaster from a stretch of shore. Freed from their main predator, mussels spread over the rock and crowded out the barnacles, limpets and other animals that had shared it Fig. 11.10. By eating the strongest competitor, the predator had been making room for the rest.

Prey are not passive. Over many generations they evolve ways to avoid being found, caught or eaten. The Indian dead-leaf butterfly folds its wings into a convincing dry leaf, and stick insects and leaf-litter frogs vanish against their backgrounds: they are camouflaged (cryptically coloured). Other prey are poisonous or foul-tasting, and a bird that tries one seldom tries another. India's plain tiger butterfly is an example: its caterpillars feed on Calotropis and keep the plant's toxins, as the famous Monarch butterfly of the Americas does with milkweed. Predators are held in check too, since one so efficient that it wiped out its prey would soon starve; successful predators in nature are 'prudent'.

Herbivores are the plant world's predators, and insects make up the largest group of them. Rooted to one spot, a plant cannot flee, so it must make itself hard to eat. The commonest physical defences are thorns and spines, as on babul (Acacia), ber and cacti; stinging hairs and tough, silica-rich leaves help too. Many defences are invisible: chemicals that make an animal ill, stop it feeding or digesting, interfere with its breeding or kill it see the comparison.

Walk past an abandoned field and you may see Calotropis (aak) standing untouched among grazed stubble: its cardiac glycosides are poisonous enough that cattle and goats leave it alone. Several plant products that people harvest, such as the caffeine of tea and coffee, the nicotine of tobacco and the quinine of cinchona bark, began as deterrents of this kind.

Competition

In the deserts of the south-western United States, seed-eating ants and seed-eating rodents gather the same seeds. When ecologists fenced rodents out of some plots, ant colonies there increased; when they removed the ants, rodents increased. Ants and rodents are about as unrelated as two animals can be, yet each was holding the other back. This is competition (प्रतिस्पर्धा), and it is wider than the everyday picture of two close relatives fighting over a scarce resource.

The resource need not even be scarce. In interference competition one species directly hinders another, by chasing it off, fouling its surroundings or simply getting in the way, so the victim feeds less efficiently even when food and space are plentiful. Because competition can work in all these ways, ecologists define it by its result, the loss of fitness it causes.

Darwin saw competition between species as a powerful force in evolution, and the Russian ecologist G. F. Gause put the idea to an experimental test in the 1930s. He grew two species of Paramecium in tubes of the same bacterial food. Each thrived alone; together, one always drove the other to extinction. Proving the same thing in the wild is much harder, but some cases are telling. Giant tortoises on Pinta (Abingdon) Island in the Galápagos, already reduced by whalers, faced goats released in 1959 that stripped the vegetation they fed on. Searchers in 1971 found a single survivor, 'Lonesome George', and the species died out with him in 2012.

Joseph Connell's experiments on the rocky coast of Scotland gave cleaner evidence Fig. 11.11. The small barnacle Chthamalus lives high on the shore, above the larger Balanus, although its larvae settle lower down as well. Where Connell scraped Balanus off the rock, young Chthamalus survived and grew in the lower zone too. A species that spreads once a stronger competitor is removed shows competitive release. Field studies of this kind also suggest that plants and herbivores are hit harder by competition than carnivores are.

If the principle always held, how could five very similar warbler species share the same spruce trees in North American forests? Robert MacArthur watched them closely in the 1950s and found that each species hunted insects mostly in its own part of the tree Fig. 11.12, so their diets overlapped far less than it first seemed. Dividing a resource like this, by place, time or feeding method, is called resource partitioning; hawks and owls share rodent prey by hunting at different times of day. Exclusion is expected only when resources are limiting and nothing else changes, and newer studies show that competitors often evolve differences that let them coexist instead.

Parasitism

Living on or inside another organism guarantees a parasite food and shelter, and parasitism (परजीविता) has arisen independently in plants, fungi, protists, worms, insects and even birds. Many parasites can use only one host species. Such pairs are locked in an evolutionary arms race, or co-evolution: each new defence the host evolves, such as an immune trick for recognising the parasite, selects for parasites that can dodge it, and so on.

A tapeworm shows what this lifestyle does to a body. It clings to the gut wall with hooks and suckers, has no eyes or other sense organs, and has no gut at all, since it soaks up food the host has already digested. Most of its body is a chain of segments packed with eggs. Huge output is common among parasites: a female roundworm (Ascaris) can lay about 200 000 eggs a day, because only a tiny fraction will ever reach a new host.

Reaching a new host is the hardest part of a parasite's life, and many solve it with complicated life cycles. Some pass through one or two intermediate hosts before reaching the main host. The Chinese liver fluke, for instance, needs a snail and then a fish, and infects people who eat that fish raw or undercooked Fig. 11.13. Others depend on a vector that carries them between hosts, as the malaria parasite depends on mosquitoes (Chapter 7).

A parasite's gain is its host's loss. Infected hosts may grow more slowly, breed less and die younger, and a host weakened by parasites is easier for a predator to catch. Heavy parasite loads can therefore pull down the density of a host population.

Parasites can be grouped by where they live. Ectoparasites stay on the outside of the host: head lice in human hair, ticks on dogs and cattle, and the copepods that cling to the skin and gills of many marine fish. Plants can be ectoparasites too. Dodder (Cuscuta, अमरबेल), the leafless yellow threads that smother hedges, has lost its chlorophyll during evolution and taps its host for all its food through suckers called haustoria.

Endoparasites such as tapeworms, liver flukes and the malaria parasite live inside the host, in organs from the gut and liver to the lungs and red blood cells. They are the most specialised parasites of all, with the most stripped-down bodies, the greatest egg output and the most elaborate life cycles.

Every spring and summer in Indian towns, the female Asian koel slips into house crows' nests while the owners are away and lays her eggs there. The crows then incubate the eggs and feed the koel chicks as their own. This is brood parasitism: the parasite exploits its host's parental care rather than its body. Over many generations koel eggs have come to look much like crow eggs, so the crows seldom recognise and throw out the intruders.

Commensalism

Commensalism (सहभोजिता) is a one-sided partnership: one species gains, and its partner is left no better and no worse off. The table collects some familiar cases.

In each case the host neither helps nor suffers: a cow is not bothered by the egret at its feet, and the mango tree loses nothing to the orchid, which takes no food from it. The clownfish is a borderline case. Your textbook classes it as commensalism, because the anemone gains nothing obvious; some recent studies suggest that the fish's wastes and its chasing off of anemone-eating fish may help the anemone too, which would make it mutualism.

Mutualism

In mutualism (सहोपकारिता) each partner provides something the other lacks. Look at a grey-green crust on an old wall or tree trunk: it is a lichen, a fungus and a photosynthetic alga or cyanobacterium living as one. The photosynthetic partner makes sugars; the fungus holds water, absorbs minerals and shelters it. Below ground, most plants trade with fungi in mycorrhizae. Fungal threads spread far beyond the roots and pass minerals, especially phosphorus, to the plant, which pays the fungus in sugars.

Flowering plants strike some of the most elaborate bargains, with the animals that move their pollen and seeds. Nectar and spare pollen are the wages of pollinators; a sweet, fleshy fruit is the fee for a bird or bat that swallows the seeds and drops them far away. Any such deal attracts cheaters: a carpenter bee may bite through the base of a flower and drink its nectar without touching the pollen. Plants counter with flower shapes that admit only the right visitors, and pollinators evolve to match, so plant and animal co-evolve.

A fig is not quite a fruit: it is a hollow, almost closed ball lined on the inside with hundreds of tiny flowers. Only a tiny wasp can get in through the narrow opening at its tip, and in many fig species each fig has its own partner wasp species, which can breed nowhere else Fig. 11.14. A pollen-dusted female crawls inside, pollinates flowers as she searches for places to lay eggs, and lays them in some of the flowers. Her larvae eat the developing seeds in those flowers, while the remaining flowers set seed. The fig pays with a few seeds; the wasp pays with pollination.

Orchid flowers come in an extraordinary variety of shapes, many tailored to a single kind of bee. Some orchids pay their pollinators nothing at all. In the Mediterranean orchid Ophrys, one petal mimics a female bee in shape, colour, texture and even scent. Male bees emerge earlier in spring than females, find these 'females' first and try to mate with them, a behaviour called pseudocopulation. Each attempt sticks pollen onto the bee, which delivers it to the next flower that fools him. The trick works only while the petal matches the real female, so any evolutionary change in the bee must be tracked by the orchid: co-evolution again.