This MCQ module is based on: Predation Competition
TOPIC 52 OF 64
Predation Competition
🎓 Class 12
Biology
CBSE
Theory
Ch 11 – Organisms and Populations
⏱ ~14 min
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This assessment will be based on: Predation Competition
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Population Interactions - Predation and Competition
Can you think of any natural habitat on earth that is inhabited just by a single species? There is no such habitat, and such a situation is even inconceivable. For any species the minimal requirement is one more species on which it can feed.
Even a plant species, which makes its own food, cannot survive alone: it needs soil microbes to break down the organic matter in soil and return the inorganic nutrients for absorption. And how would the plant manage pollination without an animal agent? In nature, animals, plants and microbes do not and cannot live in isolation but interact in various ways to form a biological community. Even in minimal communities, many interactive linkages exist, although all may not be readily apparent.
The six kinds of interaction
Interspecific interactions arise from the interaction of populations of two different species. They could be beneficial, detrimental or neutral — neither harm nor benefit — to one of the species or both. Assigning a ‘+’ sign for a beneficial interaction, a ‘−’ sign for a detrimental one and 0 for a neutral one, we can list all the possible outcomes.
| Species A | Species B | Name of interaction |
|---|---|---|
| + | + | Mutualism |
| − | − | Competition |
| + | − | Predation |
| + | − | Parasitism |
| + | 0 | Commensalism |
| − | 0 | Amensalism |
Both species benefit in mutualism, and both lose in competition. In both parasitism and predation only one species benefits — the parasite and the predator respectively — and the interaction is detrimental to the other, the host and the prey. 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 while the other is unaffected.
One thing predation, parasitism and commensalism share. The interacting species live closely together. That is what distinguishes them from competition, in which the two species may never meet at all — they need only consume the same resource.
Predation
What would happen to all the energy fixed by autotrophic organisms if the community had no animals to eat the plants? You can think of predation as nature's way of transferring to higher trophic levels the energy fixed by plants.
When we think of predator and prey, it is most probably the tiger and the deer that come to mind — but a sparrow eating a seed is no less a predator. Although animals eating plants are categorised separately as herbivores, they are, in a broad ecological context, not very different from predators.
Four roles of predators
(i) Conduits for energy transfer. Predators move the energy fixed by plants up across trophic levels.
(ii) They keep prey populations under control. But for predators, prey species could achieve very high population densities and cause ecosystem instability.
(iii) They restrain invasive species. When certain exotic species are introduced into a geographical area they become invasive and start spreading fast, because the invaded land does not have its natural predators. The prickly pear cactus introduced into Australia in the early 1920s caused havoc by spreading rapidly into millions of hectares of rangeland. The invasive cactus was finally brought under control only after a cactus-feeding predator — a moth — from its natural habitat was introduced into the country. Biological control methods adopted in agricultural pest control are based on this same ability of a predator to regulate a prey population.
(iv) They 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. In a field experiment, when all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year, because of interspecific competition.
The Pisaster experiment is worth pausing on. Removing a predator did not increase diversity, as you might expect — it destroyed it. With the starfish gone, the competitively superior prey species were no longer being cropped, so they monopolised the rock surface and excluded the rest. A predator can therefore be the reason a community is diverse, not a threat to its diversity.
Why predators are 'prudent'
If a predator is too efficient and overexploits 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’.
Prey defences
Prey species have evolved various defences to lessen the impact of predation.
- Some species of insects and frogs are cryptically-coloured, that is camouflaged, to avoid being detected easily by the predator.
- Some are poisonous and therefore avoided by predators.
- The Monarch butterfly is highly distasteful to its predator, a bird, because of a special chemical present in its body. Interestingly, the butterfly acquires this chemical during its caterpillar stage, by feeding on a poisonous weed.
Plant defences against herbivores
For plants, herbivores are the predators. Nearly 25 per cent of all insects are known to be phytophagous — feeding on plant sap and other parts of plants. The problem is particularly severe for plants because, unlike animals, they cannot run away from their predators.
Plants have therefore evolved an astonishing variety of morphological and chemical defences against herbivores.
Morphological. Thorns, as in Acacia and cactus, are the most common morphological means of defence.
Chemical. Many plants produce and store chemicals that make the herbivore sick when eaten, inhibit feeding or digestion, disrupt its reproduction, or even kill it. You must have seen the weed Calotropis growing in abandoned fields: the plant produces highly poisonous cardiac glycosides, and that is why you never see cattle or goats browsing on it.
A striking consequence. A wide variety of chemical substances that we extract from plants on a commercial scale — nicotine, caffeine, quinine, strychnine, opium and others — are produced by them actually as defences against grazers and browsers. Your morning cup of coffee is a plant's insecticide.
Competition
When Darwin spoke of the struggle for existence and survival of the fittest in nature, 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 resources that are limiting — but this is not entirely true, on two counts.
First, totally unrelated species could also compete for the same resource. In some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake.
Second, resources need not be limiting for competition to occur. In interference competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if resources — food and space — are abundant.
Competition is therefore best defined as a process in which the fitness of one species — measured in terms of its r, the intrinsic rate of increase — is significantly lower in the presence of another species.
Notice how much this definition improves on the intuitive one. It says nothing about relatedness, nothing about shared food, and nothing about scarcity. It says only that one species does worse when the other is present, and it makes the claim measurable.
Evidence from nature
It is relatively easy to demonstrate in laboratory experiments, as Gause and other experimental ecologists did, that when resources are limited the competitively superior species will eventually eliminate the other species. But evidence for such competitive exclusion occurring in nature is not always conclusive. Strong and persuasive circumstantial evidence does exist in some cases.
The Abingdon tortoise. In the Galapagos Islands it became extinct within a decade after goats were introduced on the island, apparently due to the greater browsing efficiency of the goats.
Competitive release. 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 distributional range dramatically when the competing species is experimentally removed. Connell's elegant field experiments showed that on the rocky sea coasts of Scotland 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 appear to be more adversely affected by competition than carnivores.
Gause's Competitive Exclusion Principle — and its limits
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.
This may be true if resources are limiting, but not otherwise. More recent studies do not support such gross generalisations about competition. While they do not rule out the occurrence of interspecific competition in nature, they point out that species facing competition might 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, for instance, different times for feeding or different foraging patterns. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist, due to behavioural differences in their foraging activities.
📐 Activity: remove the starfish
In a field experiment on the rocky intertidal coast of the American Pacific, every individual of the starfish Pisaster was removed from an enclosed area, and the area was watched for a year. Pisaster is an important predator there.
Predict: with the predator gone, did the number of invertebrate species in the enclosure go up, stay the same, or go down? Give your reasoning before revealing.
It went down, sharply: more than 10 species of invertebrates became extinct within a year.
Why removing a predator destroyed diversity. Because of interspecific competition. The starfish had been cropping all its prey species, including the competitively superior ones, which kept every prey population below the density at which it could monopolise the limited space on the rock. Remove that cropping and the best competitor multiplies, occupies the surface and excludes the others — competitive exclusion on a grand scale, played out in a single year.
The general principle. Predators help in maintaining species diversity in a community by reducing the intensity of competition among competing prey species. A predator's effect on any one prey species is negative; its effect on the number of species can be strongly positive.
The practical lesson. Conservation aimed at protecting one attractive species while removing its predators can dismantle the community that species belongs to. And it explains the Australian cactus: it was not a stronger plant in Australia than at home, only an unpredated one.
🎯 Interactive: Name That Interaction
Six situations from this part. Choose one to see which interaction it is, with the signs for each species.
🎯 Competency-Based Questions
Q1. Name the important defence mechanisms in plants against herbivory, and explain why plants rely on these rather than on the defences animals use.
Why plants need a different kind of defence. For plants, herbivores are the predators, and nearly 25 per cent of all insects are known to be phytophagous, feeding on plant sap and other parts of plants. The problem is particularly severe for plants because, unlike animals, they cannot run away from their predators. Flight, hiding and fighting back are all unavailable, so a plant's defence must work while the plant stands still and is being eaten.
Morphological defences. Thorns are the most common morphological means of defence — as in Acacia and cactus. They work passively, deterring the herbivore before it feeds, and cost the plant nothing to maintain.
Chemical defences. Many plants produce and store chemicals that make the herbivore sick when eaten, inhibit feeding or digestion, disrupt its reproduction, or even kill it. The weed Calotropis, common in abandoned fields, produces highly poisonous cardiac glycosides, which is why cattle and goats are never seen browsing on it.
The remarkable commercial footnote. A wide variety of chemical substances that we extract from plants on a commercial scale — nicotine, caffeine, quinine, strychnine, opium and others — are produced by them actually as defences against grazers and browsers. Nearly every plant-derived drug and stimulant in human use began as an anti-herbivore poison.
The pattern worth noticing. An immobile organism defends itself by making itself unpleasant to eat — mechanically or chemically. A mobile one can simply leave.
Q2. What is the ecological principle behind the biological control method of managing pest insects? Illustrate with the Australian prickly pear.
The principle. Biological control methods adopted in agricultural pest control are based on the ability of the predator to regulate the prey population. Predators keep prey populations under control; but for predators, prey species could achieve very high population densities and cause ecosystem instability.
Why pests reach such densities in the first place. When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast, because the invaded land does not have its natural predators. The pest is not stronger in the new land; it is merely unpredated, so its growth is exponential where it would have been logistic at home.
The prickly pear in Australia. Introduced in the early 1920s, it caused havoc by spreading rapidly into millions of hectares of rangeland. It was finally brought under control only after a cactus-feeding predator — a moth — from its natural habitat was introduced into the country. The remedy was to restore the missing half of the interaction.
Why it works better than a pesticide, in principle. A predator reproduces, spreads and concentrates where its prey is dense, so control is self-sustaining and self-targeting. A pesticide has to be reapplied, acts on non-target species too, and selects for resistance.
And why it must be done with great care. The control agent is itself an introduced species. Choosing a predator specific to the pest — taken from the pest's own natural habitat, as the moth was — is essential, or the cure becomes the next invasion.
Q3. The intuitive definition of competition — closely related species fighting over a scarce resource — is wrong on two counts. Explain both, and state the better definition.
First error: it need not be closely related species. Totally unrelated species can compete for the same resource. In some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton in the lake — a bird and a fish, as distant as two animals can conveniently be. What creates competition is a shared requirement, not a shared ancestry.
Second error: resources need not be limiting. In interference competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, even if food and space are abundant. One species can simply get in the other's way, or harass it, or make the site unusable.
The better definition. Competition is best defined as a process in which the fitness of one species — measured in terms of its r, the intrinsic rate of increase — is significantly lower in the presence of another species.
Why this definition is superior. It drops every assumption that turned out to be false: nothing about relatedness, nothing about which resource, nothing about scarcity, and nothing about mechanism. And it is operational — it tells you what to measure. Determine r with the other species present and absent, and you have the answer, whatever the mechanism turns out to be.
Q4. State Gause's Competitive Exclusion Principle. Given that five closely related warbler species live on one tree, is the principle wrong?
The 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.
Its proper scope. This may be true if resources are limiting, but not otherwise — and that qualification is part of the principle, not an escape from it. It is relatively easy to demonstrate in laboratory experiments, as Gause and other experimental ecologists did, that where resources are limited the competitively superior species eliminates the other. Evidence for competitive exclusion occurring in nature is not always conclusive, although strong circumstantial evidence exists in some cases — the Abingdon tortoise in the Galapagos became extinct within a decade after goats were introduced, apparently due to the greater browsing efficiency of the goats.
So is it wrong? No — it is incomplete. More recent studies do not support such gross generalisations about competition. They do not rule out interspecific competition in nature, but they point out that species facing competition might evolve mechanisms that promote co-existence rather than exclusion.
How the warblers escape it. By resource partitioning. If two species compete for the same resource, they could avoid competition by choosing different times for feeding or different foraging patterns. MacArthur showed that five closely related species of warblers living on the same tree co-existed because of behavioural differences in their foraging activities. They are not competing for the same resource after all: each has a different part of the tree, so the premise of the principle no longer holds.
The deeper point. Competitive exclusion is not only an outcome but a selection pressure. Species that divide the resource leave more descendants than species that fight over it, so what exclusion mostly produces, over evolutionary time, is partitioning.
Q5. 'Predators in nature are prudent.' Explain what this means and why natural selection should produce prudence rather than maximum efficiency.
What the statement means. If a predator is too efficient and overexploits 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’ — they do not, in practice, drive their prey to extinction.
Why maximum efficiency is self-defeating. A predator's food supply is a reproducing population, not a fixed stock. A predator that takes everything destroys the thing that renews its food, so its lineage ends with the prey's. Prudence is not restraint in the moral sense; it is what remains after the imprudent have eliminated themselves.
How prudence is actually achieved. Not by any individual predator choosing to eat less, but by the interaction settling into a balance. Prey species have evolved various defences to lessen the impact of predation — cryptic colouration in some insects and frogs, poisons in others, and the special chemical that makes the Monarch butterfly highly distasteful to birds, which it acquires as a caterpillar by feeding on a poisonous weed. As prey become harder to catch, predator numbers fall; as predators become scarce, prey recover. The result looks like prudence and is in fact an equilibrium.
Why this matters for the rest of the section. It is the same reason predators maintain species diversity. A prudent predator keeps every prey population below the density at which it could exclude the others — which is exactly what the Pisaster removal experiment demonstrated, when more than 10 invertebrate species became extinct within a year of the starfish being taken away.
🧠 Assertion–Reason Questions
For each pair, decide whether both statements are true and whether the reason correctly explains the assertion.
Assertion (A): Removing the starfish Pisaster from an intertidal area caused more than 10 invertebrate species to become extinct within a year.
Reason (R): Predators help maintain species diversity by reducing the intensity of competition among competing prey species.
Both A and R are true, and R is the correct explanation of A.
With the predator gone, the competitively superior prey species were no longer cropped, monopolised the limited space and excluded the rest. The extinctions were caused by interspecific competition, which the predator had previously been holding in check.
Assertion (A): Cattle and goats do not browse on Calotropis.
Reason (R): The plant produces highly poisonous cardiac glycosides.
Both A and R are true, and R is the correct explanation of A.
Chemical defence is one of the two great classes of plant defence, alongside morphological defences such as the thorns of Acacia and cactus. Plants cannot run away from their predators, so they make themselves dangerous to eat instead.
Assertion (A): Competition occurs only between closely related species when resources are limiting.
Reason (R): Competition is a process in which the fitness of one species is significantly lower in the presence of another species.
A is false but R is true.
R is the correct modern definition — and it is precisely what makes A false. Totally unrelated species can compete, as flamingoes and fishes do for zooplankton in shallow South American lakes; and resources need not be limiting, since in interference competition the feeding efficiency of one species is reduced by the inhibitory presence of the other even when food and space are abundant.
Frequently Asked Questions - Predation and Competition
Why can no species live in isolation?
There is no natural habitat on earth inhabited by just a single species, and such a situation is even inconceivable. For any species the minimal requirement is one more species on which it can feed. Even a plant, which makes its own food, needs soil microbes to break down organic matter and return inorganic nutrients for absorption, and needs an animal agent for pollination. In nature animals, plants and microbes interact in various ways to form a biological community.
What are the six types of population interaction?
Mutualism, in which both species benefit; competition, in which both lose; predation and parasitism, in which one species benefits and the other is harmed; commensalism, in which one benefits and the other is neither harmed nor benefited; and amensalism, in which one is harmed and the other is unaffected.
What is predation and what roles do predators play?
Predation is an interaction in which one species benefits and the other is harmed, and it is nature's way of transferring to higher trophic levels the energy fixed by plants. Besides acting as conduits for energy transfer, predators keep prey populations under control, since without them prey species could achieve very high population densities and cause ecosystem instability; they restrain invasive species; and they help maintain species diversity in a community by reducing the intensity of competition among competing prey species.
How did predators control the prickly pear cactus in Australia?
The prickly pear cactus introduced into Australia in the early 1920s caused havoc by spreading rapidly into millions of hectares of rangeland, because the invaded land did not have its natural predators. It was finally brought under control only after a cactus-feeding predator, a moth, from its natural habitat was introduced into the country. Biological control methods in agricultural pest control are based on this same ability of a predator to regulate a prey population.
What happened when the starfish Pisaster was removed from an intertidal area?
In the rocky intertidal communities of the American Pacific Coast the starfish Pisaster is an important predator. In a field experiment, when all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year, because of interspecific competition. This shows that predators help maintain species diversity by reducing the intensity of competition among competing prey species.
Why are predators in nature described as prudent?
Because if a predator is too efficient and overexploits its prey, then the prey might become extinct and, following it, the predator will also become extinct for lack of food.
What defences have prey species evolved against predation?
Some species of insects and frogs are cryptically coloured, that is camouflaged, to avoid being detected easily by the predator. Some are poisonous and therefore avoided by predators. The Monarch butterfly is highly distasteful to its bird predator because of a special chemical in its body, which it acquires during its caterpillar stage by feeding on a poisonous weed.
What defences do plants have against herbivores?
Plants have evolved an astonishing variety of morphological and chemical defences, because unlike animals they cannot run away from their predators. Thorns, as in Acacia and cactus, are the most common morphological means of defence. Chemically, many plants produce and store substances that make the herbivore sick, inhibit feeding or digestion, disrupt reproduction or even kill it; Calotropis produces highly poisonous cardiac glycosides. Nicotine, caffeine, quinine, strychnine and opium are all produced by plants as defences against grazers and browsers.
How is competition best defined, and why not by relatedness or scarcity?
Competition is best defined as a process in which the fitness of one species, measured in terms of its r or intrinsic rate of increase, is significantly lower in the presence of another species. Relatedness is not the criterion because totally unrelated species can compete, as flamingoes and fishes do for zooplankton in shallow South American lakes. Scarcity is not required either, because in interference competition the feeding efficiency of one species may be reduced by the interfering and inhibitory presence of the other even when food and space are abundant.
What is Gause's Competitive Exclusion Principle and what is resource partitioning?
Gause's 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. This may be true if resources are limiting but not otherwise, and species facing competition might instead evolve mechanisms that promote co-existence. One such mechanism is resource partitioning, in which two species avoid competition by choosing different times for feeding or different foraging patterns. MacArthur showed that five closely related species of warblers living on the same tree co-existed because of behavioural differences in their foraging activities.
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