Organisms and Populations

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 11.

One starfish, ten missing species

On the rocky Pacific coast of America, ecologists once removed a single kind of starfish, Pisaster, from a patch of shore. Within a year more than ten species of invertebrates in that patch had disappeared. Nobody had touched them. This chapter explains how removing one animal can empty a shore. It does so in two steps: how a group of the same species behaves as a population, and how different species help or harm each other.

Ecology and its four levels

Ecology is the study of interactions among organisms and between organisms and their physical (abiotic) surroundings. It is organised into four levels, and this chapter works on the middle two and the links between species.

Populations: a group has traits that no individual has

Individuals of a species rarely live alone. They live in a well-defined geographical area, share or compete for similar resources and can potentially interbreed. That group is a population: cormorants in a wetland, rats in an abandoned house, teak trees in a forest tract, bacteria on a culture plate, lotus in a pond. Even a clone-like group from asexual reproduction counts. Selection acts on populations, which is why this topic links ecology to evolution.

An individual is born, lives and dies. A population has attributes that make sense only for the group.

Age pyramids. For humans, the pyramid shows males and females, split into three bands: pre-reproductive (bottom), reproductive (middle) and post-reproductive (top). Compare the bottom two bands and you can read the future.

Population density and how it is measured

Population size is called population density, written N. It does not have to be a head count. These examples show why:

Why a population changes: four processes

Density can change in only four ways. Natality (B) and immigration (I) raise it; mortality (D) and emigration (E) lower it.

The population in one line: N(t+1) = N(t) + [(B + I) − (D + E)]

Under normal conditions B and D dominate. In a newly colonised habitat, immigration may add more than births do.

Pause and predict: a population of 500 has 60 births, 40 deaths, 10 immigrants and 30 emigrants in a year. Will it grow?

Check your answer: N(t+1) = 500 + [(60 + 10) − (40 + 30)] = 500 + 0 = 500. It stays the same, even though births exceed deaths, because emigration quietly cancelled the gain.

Population growth: two curves, two equations

Exponential growth. If food and space are unlimited, every individual reproduces at its full potential, and the new individuals reproduce too. The growth feeds on itself. If the per capita birth rate is b and the death rate is d, then (b − d) is called r, the intrinsic rate of natural increase, and:

Exponential growth: dN/dt = rN, which integrates to N(t) = N(0) e^(rt)

It plots as a J-shaped curve. Some sample r values: Norway rat 0.015, flour beetle 0.12 and human population of India in 1981, 0.0205. A higher r means a faster rise, and r is the parameter used to judge how any factor affects population growth.

Worked example. With r = 0.2 per individual per year and N = 100, dN/dt = 0.2 × 100 = 20 individuals per year. Notice that r is a rate per individual, not a percentage of anything else.

Logistic growth. No habitat has unlimited resources. Competition begins, and only the fittest individuals survive and reproduce. A habitat can support only a maximum population, the carrying capacity (K). As N approaches K, growth slows and finally stops. This is the Verhulst-Pearl logistic growth:

Logistic growth: dN/dt = rN[(K − N)/K]

Read the bracket as a brake. When N is small, (K − N)/K is close to 1 and growth is nearly exponential. As N nears K, the bracket falls towards zero and growth stops. The curve is sigmoid (S-shaped) with a lag phase, an acceleration phase, a deceleration phase and an asymptote at K.

Worked example. K = 1000, r = 0.5, N = 200. dN/dt = 0.5 × 200 × (1000 − 200)/1000 = 100 × 0.8 = 80. Without the brake you would get 100, which is the trap answer.

Beyond the textbook. For the exam: a population climbs to K and stays there as a smooth S-curve. In reality: K is not a fixed number. It shifts with seasons and with food and space, and real populations often overshoot it or fluctuate around it. The equation is also not just an exam device: its fastest growth occurs at N = K/2, which is why simple fishery models place the maximum sustainable harvest near K/2. Why it matters: it explains why the logistic model is a good description but not a prediction.

Life history: how a species spends its energy

Every species has a limited energy budget, so evolution has to decide whether to spend it on many offspring or on a few well cared for. Populations evolve whatever pattern gives the highest reproductive fitness (Darwinian fitness, which means high r) in their habitat. This is shaped by the abiotic and biotic conditions of that habitat.

Ecologists often label "many small, fast" species r-selected and "few large, well cared for" species K-selected.

Population interactions: six ways two species can meet

No habitat holds a single species. Even the smallest community needs something for each species to feed on, and plants need pollinators and soil microbes. Interactions between two species are shown with signs: + beneficial, − harmful, 0 neutral.

Predation, parasitism and commensalism have one thing in common: the species live closely together.

Predation

Predators move energy fixed by plants to higher trophic levels, keep prey populations in check and maintain species diversity. In a broad sense a herbivore is also a predator of plants. A predator that hunted its prey to extinction would starve, so predators are "prudent".

The Australian cactus. Prickly pear cactus was introduced into Australia in the early 1920s and spread over millions of hectares of rangeland until a cactus-feeding moth was introduced. This is how biological control of pests works, and it shows that invasive species explode when their natural enemies are missing.

The starfish experiment. When Pisaster was removed from an enclosed intertidal area, more than ten invertebrate species disappeared within a year because of interspecific competition. The starfish had been eating the strongest competitor. Remove the predator, and that competitor crowds out the rest.

Pause and predict: removing a predator should help its prey, yet the prey species vanished. Why?

Check your answer: the prey were not dying from the starfish but from each other. The starfish had been eating the strongest competitor, and without it that competitor crowded out the rest.

Beyond the textbook. For the exam: predators reduce competition among prey and so maintain diversity. In reality: ecologists call a species like Pisaster, whose effect on a community is much bigger than its numbers suggest, a keystone species. The idea was developed from this very experiment. Why it matters: it connects this chapter to biodiversity conservation, where protecting a keystone species protects many others.

How prey fight back. Camouflage (cryptic colouration) hides some insects and frogs. The monarch butterfly is distasteful to birds because of a chemical it acquires as a caterpillar by feeding on a poisonous weed. It does not make the poison itself. Plants face herbivores too, since about 25 per cent of insects feed on plants. Plant defences are thorns (Acacia, cactus) and chemicals: the cardiac glycosides of Calotropis are so poisonous that cattle and goats do not browse it. The commercial chemicals nicotine, caffeine, quinine, strychnine and opium are plant defences that humans later put to use.

Competition

Darwin saw competition between species as a powerful force in evolution. It is not limited to closely related species, and it does not even need scarce resources. Flamingoes and resident fishes in some South American lakes compete for zooplankton though they are unrelated. In interference competition one species lowers another's feeding efficiency just by being present, even when food is plentiful. A good definition: competition is a process in which the fitness (r) of one species is significantly lower in the presence of another species.

In the laboratory, Gause's experiments showed that the superior species eliminates the inferior one. This became the competitive exclusion principle: two closely related species competing for the same limiting resources cannot coexist indefinitely. In nature the evidence is "not always conclusive" and herbivores and plants seem more affected by competition than carnivores.

Resource partitioning is the escape route. MacArthur showed that five closely related warbler species on the same tree avoided competing by differing in their foraging behaviour: where they hunt on the tree and when. They share the resource by splitting it.

Parasitism

Parasitism gives free lodging and meals, so it has evolved in many groups from plants to vertebrates. Many parasites are host specific, and host and parasite co-evolve: the host evolves resistance, the parasite counters it. Their adaptations are: loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system and high reproductive capacity. Life cycles are often complex and use intermediate hosts or vectors. The human liver fluke needs two intermediate hosts, a snail and a fish. The malarial parasite needs a vector, the mosquito.

Most parasites harm the host by lowering its survival, growth, reproduction and density, and by making it easier prey. Brood parasitism is a clever case: the parasitic bird's eggs have evolved to look like the host's eggs in size and colour, so the host does not detect and remove them. The female mosquito is not considered a parasite, although it needs blood for reproduction.

Commensalism and mutualism

Commensalism (+/0) has four classic examples.

Mutualism (+/+) is where both gain.

Plant–animal relations are guarded against cheaters and often involve co-evolution. Many fig species have a one-to-one link with a single wasp species. Some orchids offer no reward at all. The Mediterranean orchid Ophrys uses sexual deceit: one petal looks like the female of a bee species in size, colour and markings. The male bee tries to mate with the flower ("pseudocopulation"), picks up pollen and carries it to the next flower. If the female bee's colour pattern evolves, the orchid evolves to match it.

Beyond the textbook. For the exam: the six interactions come with clean signs, and amensalism is simply (−/0). In reality: a standard example of amensalism is allelopathy, where a plant releases chemicals that stop neighbours from growing, as the black walnut tree does. Nature also blurs the boxes: a mutualism can sour into parasitism when one partner cheats, and the same pair may be commensal in one habitat and mutualistic in another. Why it matters: when a question looks ambiguous, answer by the sign pattern of the example.

Exam traps

60-second revision

Where this chapter connects

• Next: Ecosystem takes the same organisms and follows the energy and nutrients passing between them.
• Class XII link: Biodiversity and Conservation shows what happens when keystone species, alien invaders and co-extinctions strip a community.
• Class XII link: Evolution explains how fitness, competition and adaptation drive the traits met here.
• Class XI link: Biological Classification introduces lichens and mycorrhiza, two of the mutualisms in this chapter.