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Population Interaction: Predation, Role played by Predators, Defence Against Predation, Practice Problems and FAQs

Have you ever enjoyed watching a television documentary showing how a lion hunts its prey? Or you might have watched how skillful a desert lizard is in protecting itself from potential hunters? We all have seen insects being eaten by lizards, frogs being hunted by snakes and rats being killed by cats, around us.

Do you know what is the name given to this interaction between two animals in which one is hunted and killed by the other? It is known as predation. 

Do you think we, the humans, can be considered as predators as we kill plants and animals to feed on them?? What about a cow who feeds on a plant?? Is it a predator? What would happen if the predators hunt down and kill all the prey in their habitat? If you want the answers to these questions then keep reading.

Table of Contents:


The biological interaction where one organism, the predator, eats another organism, its prey is called predation.


Predation is a natural way of transferring energy. The energy flows through different organisms i.e., food eaten by the first organism (prey) is transferred to the next (predator).


When we think about the predator- prey interaction, most probably it is the tiger and the deer that readily come to our mind. But a bird eating any seed or any herbivores that eats plants are also predators when seen from the broad perspective of an ecosystem.


Role Played by Predators in the Ecosystem

Control of Prey Population

Predators play an important role in controlling the population of their prey in a particular ecosystem and keep the prey population in check. Often alien invasive species start flourishing uncontrollably in a new habitat due to the lack of natural predators. But the introduction of their natural predators can check their population.
In the early 1920’s the prickly pear cactus was introduced into Australia. It caused widespread destruction by spreading rapidly into millions of hectares of rangeland. This was controlled by introducing a predator moth that feeds on prickly pear cactus in its natural habitat.


Based on this principle, microbes are also used to control the invasive pests and the process involved is called biological pest control. For example, spores of the bacterium Bacillus thuringiensis is used to control lepidopteran insect pests, Trichoderma is used against many plant pathogens, Baculoviruses are widely used against many arthropod pests.


Species Diversity Control

Predators help to maintain the species diversity of a habitat by keeping the prey populations in check so as to eliminate competition between them. Let's make it clear through an example of a population interaction.


Here species A is in competition with B and species B is in competition with A. Species C is the predator of species A. 
So what happens when the predator species C is removed? The number of species A will increase dramatically. This would increase the competition between species A and B and eventually species B will be eliminated due to a comparatively smaller population. Hence in this situation Species C keeps A under control and therefore letting A,B and C exist in harmony.


In the rocky intertidal communities of the American Pacific coast, the starfish Pisaster was an important predator. When it was removed, around 10 invertebrates vanished due to competition between the species and the inferior species were gradually eliminated by the superior ones.

Defence Against Predation

What would happen if the predator becomes extremely efficient and ruthless? Yes, you are right. The prey population will rapidly decline and might soon become extinct. Will it affect the predator population in any way?? Yes, the predator would eventually run out of food and would also become extinct. Thus, nature has its way of maintaining a balance by making the predators prudent enough to not exploit the prey population and also providing the prey with mechanisms to evade the predators.

Defence Against Predation in animals

Some preys can merge with their surroundings to avoid predators. This ability is known as camouflage. Some species of insects and frogs are cryptically-coloured or camouflaged to avoid being detected easily by the predator.


Some organisms are poisonous and distasteful and therefore are avoided by the predators. Caterpillars or larvae of monarch butterflies feed on the leaves of the milkweed (poisonous weed) and store the glycosides in their own bodies, which makes the caterpillar toxic. This makes the Monarch butterflies distasteful to the predator (Blue jay) which avoids eating it.


Defence Against Predation in Plants 

It has found that nearly 25 percent of all insects are phytophagous, that is, they feed on plant sap and other parts of plants.

But the plants cannot run away from their predators and have developed morphological and chemical adaptations of defence by the plants towards its predator.

Morphological Defence

The most common morphological means of defence are thorns.

morphological defence

Chemical Defence

Some plants produce and store chemicals which can make the herbivores sick. It can also inhibit feeding and digestion. It may disrupt reproduction and might even be fatal.
Example: Calotropis, a weed grown in fields, contains highly poisonous cardiac glycosides. Hence, no herbivores feed on it.
Some commercial plant products like opium, caffeine, nicotine, quinine, strychnine etc are actually produced by the plants as a defence mechanism.

Chemical defence

Practice Problems of Population Interaction

Question 1. In interspecific interactions, if the organism getting benefitted is represented by ‘+’ sign, the organism getting harmed is represented by ‘-’ sign and the organism which neither gets benefited or harmed is represented by ‘0’ sign, then (+, -) represents which type of population interaction.

a. Mutualism
b. Commensalism
c. Amensalism
d. Predation

Solution: Predation is a type of interaction between two different species in which the predator eats another organism called prey. In predation, the predator is benefited (+) and the prey is harmed (-).

In commensalism one species is benefitted while the other remains unaffected (+, 0).
In amensalism one species is harmed while the other remains unaffected (-, 0)
In mutualism both the species are benefitted (+,+)
Hence the correct option is d.

Question 2. Why do cattle avoid browsing on the plant Calotropis?

a. It gives off foul aroma
b. It produces poisonous cardiac glycosides
c. It bears thorns
d. Both a and b

Solution: Calotropis commonly called milkweed produces very toxic chemicals called cardiac glycosides that affect the heart. Hence, cattle avoid browsing on this plant.

Hence the correct option is b.

Question 3. When Pisaster was removed from an enclosed intertidal area of the American Pacific Coast, more than 10 species of ____(i)_______became extinct because of ____(ii)_______.

a. (i) - vertebrates, (ii) - intraspecific competition
b. (i) - invertebrates, (ii) - interspecific competition
c. (i) - vertebrates, (ii) - interspecific competition
d. (i) - invertebrates, (ii) - intraspecific competition

Solution: The starfish Pisaster is an important predator in the rocky intertidal communities of the American Pacific Coast. In a field experiment, when Pisaster was removed, the population of mussels increased rapidly. It is due to the absence of its predator. These mussels covered the entire intertidal shore. Also there was severe interspecific competition for space. Hence other invertebrate species could not establish themselves in that habitat. The extinction of more than 10 invertebrate species occurred by this interspecific competition for space.
Hence the correct option is b.

Question 4. A habitat possesses three species A, B and C. A and B are competitors and C is the predator of species B. What would happen if we remove C from the habitat?

a. The population of A will increase dramatically
b. The population of B will increase dramatically
c. The population of B will decrease dramatically
d. Both A and B will remain unaffected

Solution: A and B are competitors in a population and removal of C, a predator of B, will lead to a dramatic increase in the population size of B. This would make B a far superior competitor and eventually species A will be eliminated from the habitat.
Hence the correct option is b.

FAQs of Population Interaction

Question 1. How energy flow is connected with predation?

Solution: Predation is a biological interaction where one organism, the predator, eats another organism, its prey. A natural way of transferring energy is through predation. The energy flows from the prey (at a lower trophic level) to the predator (at a higher trophic level).

Question 2. What is camouflage?

Solution: Some preys can merge with their surroundings to avoid predators. And this ability is called camouflage. For example some species of insects and frogs are cryptically-coloured (camouflaged) to avoid being detected easily by the predator.

Question 3. What is the role of a predator in an ecosystem?

Solution: The predators not only help in keeping the prey population in control but also maintain the species diversity of a habitat by eliminating interspecific competition between its prey and its competitors. By keeping the prey population in control it also prevents excessive invasiveness of the prey species over other species.

Question 4. What are the defences taken by the plants against their predators?

Solution: There are morphological and chemical adaptations of defence by the plants towards its predator. The most common morphological means of defence are thorns. They produce and store chemicals which can threaten the survival of predators.

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