Population interaction

Population Interaction

  • Interspecific interactions are interactions of populations of two different species.
  • The interactions may be
  • Beneficial/positive effect indicated by +.
  • Harmful/detrimental/negative effect indicated by -.
  • Neutral interaction/no effect on the species indicated by 0.

Table 13.1 Population Interactions

Species A

Species B

Name of Interaction



















(i) Predation

  • It is an interspecific interaction, where an animal, called predator, kills and consumes the other weaker animal called prey.
  • Predation is nature’s way of transferring energy to higher trophic levels, e.g., a tiger (predator) eating a deer (prey), a sparrow (predator) eating fruit or seed (prey), etc.
  • The role of predators:

(a) Predators keep prey population under control. This is called biological control.

(b) Predators also help in maintaining species diversity in a community, by reducing he intensity of competition among prey species.

(c) Besides acting as ‘conduits’ for energy transfer across trophic levels.

  • 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.
  • If a predator is too efficient and over – exploits its prey, then the prey might become extinct and following it, the predator will also become extinct due to the lack of food.
  • The prey defence mechanisms

(a) To avoid being detected easily by the predators, some species of insects and frogs are cryptically coloured (camouflaged).

(b) The Monarch butterfly is highly distasteful to its predator (birds) because of a special chemical present in its body which is acquired by the butterfly feeding on a poisonous weed in its caterpillar stage.

(c) Some plants have thorns or spines for defence mechanism, e.g., Acacia, cactus.

(d) Some plants produce highly poisonous chemicals like cardiac glycosides, nicotine, caffeine, quinine, strychnine, opium, etc., are produced by plants actually as defence against grazers and browsers.

(ii) Competition

  • Competition is a type of interaction where both the species suffer. It may exist between some species (interspecific competition) or between individuals of different species (intraspecific competition).
  • The competition occurs due to limited resources between closely related species.
  • In interspecific competition, the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species, although the resources are abundant.
  • For example, after the introduction of goats in Galapagos Islands, the Abingdon tortoise became extinct within a decade due to greater browsing efficiency of he goats.
  • Gause’s competitive exclusion principle states that two closely related species competing for the same resource cannot coexist indefinitely and the competitively inferior one will be eliminated eventually by the superior one.
  • Resource partitioning: It refers to the phenomenon in which species facing competition might evolve mechanisms that promote coexistence rather than exclusion. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and coexist due to behavioural differences in their foraging activities.

(iii) Parasitism

  • It is the mode of interaction between two species in which one specie (parasite) depends on the other species (host) for food and shelter, and in this process damages the host.
  • Adaptation of parasite:

(a) The parasite have evolved to be host – specific in such a manner that both host and parasite tend to co – evolve.

(b) Loss of unnecessary sense organs.

(c) Presence of adhesive organs or suckers.

(d) Loss of digestive system.

(e) High reproductive capacity.

  • The life cycle of some parasites are complex where one or more intermediate host or vectors to facilitate parasitisation are present.

(a) The human liver fluke depends on two intermediate hosts, a snail and a fish, to complete its life cycle.

(b) Malarial parasite (Plasmodium) needs a vector (mosquito) to complete its life cycle.

  • Parasites may be of two types: ectoparasites and endoparasites.

S. No.








These are the parasites which live inside the host’s body at different sites like liver, kidney, lungs, etc., for food and shelter.

Example, tapeworm, liver fluke, Plasmodium.

These are the parasites which feed on the external surface of the host organism for food and shelter.


Example, live on humans, ticks on dogs, copepods, Cuscuta.

  • The phenomenon in which one organism (parasite) lays its eggs in the nest of another organism is called brood parasitism. E.g. Cuckoo.  

(iv) Commensalism

  • Commensalism is referred to as the interaction between two species where one species is benefited and the other is neither harmed nor benefited.
  • Few examples of commensalism:

(a) An orchid growing as an epiphyte on a mange tree. The orchid gets shelter and nutrition from mango tree while the mange tree is neither benefited nor harmed.

(b) Barnacles growing on the back of whale. Barnacles are benefited to move to location for food as well as shelter while the whales are neither benefited nor harmed.

(c) The egrets are in close association of grazing cattle. The cattle egrets are benefited by the cattle to detect insects because cattle stir up the bushes and insects are flushed out from the vegetation, to be detected by cattle egrets.

(d) The commensalism is also found between sea anemones and the clown fish. The fish is protected from predators and sea anemones are neither benefited nor harmed.

(v) Amensalism

  • Amensalism is referred to as the interaction between two different species, in which one species is harmed and the other is neither benefited nor harmed.
  • For example, the mould Penicillium secretes penicillin which kills bacteria but the mould is unaffected.

(vi) Mutualism

  • Mutualism is referred to as the interspecific interaction in which both the interacting species are benefited.
  • Some examples of mutualism

(a) Lichens represent close association between fungus and photosynthetic algae or cyanobacteria, where the fungus help in the absorption of nutrients and provides protection while algae or cyanobacterium prepares the food.

(b) Mycorrhizae are close mutual association between fungi and the roots of higher plants, where fungi helps the plant for absorption of nutrients while the plant provides food for the fungus.

(c) The male bee pseudocopulates with it and during this process of pseudocopulation, the pollen grains are dusted on the body of male bees.

(d) With such pollen dusts, male bee pseudocopulates to another flower of the same species and pollination takes place.

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12    PMT    Biology    Organisms and Populations    Population interaction