Organisms and Populations - Notes | Class 12 | Part 5: Population Interactions


Population Interactions
Organisms interact in various ways to form a biological community.
Interaction between two species is called Interspecific interactions. They include

Name of interaction

Species A

Species B

Mutualism: Both species are benefitted (+)



Competition: Both species are harmed (-)



Predation: One (predator) is benefitted. Other (prey) is harmed



Parasitism: One (parasite) is benefitted. Other (host) is harmed



Commensalism: One is benefitted. Other is unaffected (0)



Amensalism: One is harmed. Other is unaffected



In predation, parasitism & commensalisms, the interacting species live closely together.

a. Predation 

In a broad ecological context, all carnivores, herbivores etc. are predators. About 25 % insects are phytophagous.

If a predator overexploits its prey, then the prey might become extinct. It results in the extinction of predator. Therefore, predators in nature are ‘prudent’.

Importance of predators
  • Predators control prey populations.
When certain exotic species are introduced into a geographical area, they spread fast due to the absence its natural predators. E.g. Prickly pear cactus introduced into Australia (1920’s) caused havoc by spreading. Later, it was controlled by introducing a cactus-feeding predator moth.
  • Predators are used in Biological control methods.
  • Predators maintain species diversity in a community by reducing competition among prey species.
E.g. the predator starfish Pisaster in the rocky intertidal communities of American Pacific Coast. In an experiment, all these starfishes were removed from an enclosed intertidal area. It caused extinction of over 10 invertebrate species within a year, due to interspecific competition.

Defenses of prey species to lessen impact of predation
  • Camouflage (cryptic colouration) of some insects & frogs.
  • Some are poisonous and so avoided by the predators.
  • Monarch butterfly is highly distasteful to its predator bird. It is due to a special chemical in its body. It is acquired during its caterpillar stage by feeding on a poisonous weed.
  • Thorns (Acacia, Cactus etc.) are the most common morphological means of defense of plants.
  • Many plants produce chemicals that make the herbivore sick, inhibit feeding or digestion, disrupt its reproduction or kill it. E.g. Calotropis produce highly poisonous cardiac glycosides. Therefore cattle or goats do not eat it. Nicotine, caffeine, quinine, strychnine, opium, etc. are defenses against grazers and browsers.
b. Competition 

It is a process in which fitness of one species (‘r’ value) is significantly lower in presence of another species.

Interspecific competition is a potent force in organic evolution.

Competition occurs when closely related species compete for the same limited resources.

Unrelated species can also compete for the resource. E.g. Flamingoes & fishes in some shallow South American lakes compete for zooplankton.

Competition occurs in abundant resources also. E.g. In interference competition, the feeding efficiency of one species is reduced due to the interfering and inhibitory presence of other species, even if resources are abundant.

Evidences for competition:
  • The Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the island, due to greater browsing efficiency of the goats.
  • Competitive release: It is the expansion of distributional range of a species when the competing species is removed.
  • Connell’s field experiments: On the rocky sea coasts of Scotland, there are 2 barnacle species: Balanus (larger & competitively superior) & Chthamalus (smaller). Balanus dominates intertidal area and excludes Chthamalus.
When Connell experimentally removed Balanus, Chthamalus colonized the intertidal zone.

Gause’s ‘Competitive Exclusion Principle’:

It 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 in limited resources, but not otherwise.

Species facing competition may evolve mechanisms for co-existence rather than exclusion. E.g. resource partitioning.

Resource partitioning: 

It is the division of limited resources by species to avoid competition. For this, they choose different feeding times or different foraging patterns. E.g. MacArthur showed that five closely related species of warblers living on a tree could avoid competition and co-exist due to behavioural differences in their foraging activities.

c. Parasitism 

Many parasites are host-specific (they can parasitize only a single host species). They tend to co-evolve. i.e., if the host evolves special mechanisms against the parasite, the parasite also evolves mechanisms to counteract them to remain with the same host species.

Adaptations of parasites: Loss of sense organs, presence of adhesive organs or suckers to cling on to the host, loss of digestive system, high reproductive capacity etc.

Life cycles of parasites are often complex. E.g.
  • Human liver fluke depends on 2 intermediate hosts (a snail & a fish) to complete its life cycle.
  • Malarial parasite needs mosquito to spread to other hosts.
Parasites harm the host. They may reduce the survival, population density, growth and reproduction of the host. They may make the host physically weak and more vulnerable to predation.

Types of parasites:

1. Ectoparasites

Parasites that feed on the external surface of host. E.g.
  • Lice on humans.
  • Ticks on dogs.
  • Ectoparasitic Copepods on many marine fishes.
  • Cuscuta plant on hedge plants.
Cuscuta has no chlorophyll and leaves. It derives its nutrition from the host plant.
Female mosquito is not considered a parasite, because it needs our blood only for reproduction, not as food.

2. Endoparasites

Parasites that live inside the host body at different sites (liver, kidney, lungs, RBC etc).

The life cycles of endoparasites are more complex.

They have simple morphological & anatomical features and high reproductive potential.

Brood parasitism in birds:

Here, the parasitic birds lay eggs in the nest of its host and lets the host incubate them.

During evolution, eggs of the parasitic bird have evolved to resemble the host’s egg in size and colour. So the host bird cannot detect and eject the foreign eggs easily.

E.g. Brood parasitism between cuckoo and crow.

d. Commensalism 

  • Orchid (+) growing as epiphyte on a mango branch (0).
  • Barnacles (+) growing on the back of a whale (0).
  • Cattle egret (+) & grazing cattle (0). The egrets forage close to where the cattle are grazing. As the cattle move, the vegetation insects come out. Otherwise it is difficult for the egrets to find and catch the insects.
  • Sea anemone (0) & clown fish (+). Stinging tentacles of sea anemone gives protection to fish from predators.
e. Mutualism 

  • Lichen: It is a mutualistic relationship between a fungus & photosynthesizing algae or cyanobacteria.
  • Mycorrhizae: Associations between fungi & the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with carbohydrates.
  • Mutualism b/w plant & animal through pollination and seed dispersion: 
  1. Fig trees & wasps. The fig species is pollinated only by its ‘partner’ wasp species. Female wasp pollinates the fig inflorescence while searching for suitable egg-laying sites in fruits. The fig offers the wasp some developing seeds, as food for the wasp larvae.
  2. Orchids show diversity of floral patterns. They can attract the right pollinator insect (bees & bumblebees) to ensure pollination. Not all orchids offer rewards.
  3. ‘Sexual deceit’ of Ophrys (Mediterranean orchid). One petal of its flower resembles female bee in size, colour & markings. So male bee ‘pseudocopulates’ with the flower and is dusted with pollen. When this bee ‘pseudocopulates’ with another flower, it transfers pollen to it.
If the female bee’s colour patterns change slightly during evolution, pollination success will be reduced unless the orchid flower co-evolves to maintain the resemblance of its petal to the female bee.
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