5 Main Forms of Population Interactions | Biology

The following points highlight the five main forms of interaction between population. The forms are: 1. Predation 2. Competition 3. Parasitism 4. Commensalism 5. Mutualism.

Form # 1. Predation:

It is an interspecific interaction, where an animal called predator kills and consumes the other weaker animal called prey. This is a biological control method.

It is the nature’s way of transferring energy to the higher trophic levels, which is fixed by plants. For example, tiger and the deer.

Important roles of predators are as follows:

(i) They keep prey population under control.

(ii) They help in maintaining species diversity in a community by reducing the intensity of competition among prey species.

(iii) In absence of predators, prey species could achieve very high population densities and cause instability. So, besides acting as ‘conduits’ for energy transfer across trophic levels, predators play very important role to provide population stability.

(iv) When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast because the invaded land does not have natural predators.

(v) If a predator is too efficient and exploits its prey, then the prey might become extinct. Following it, the predator will also become extinct because of the lack of food.

(vi) Prey species have evolved various defence mechanisms to lessen the impact of predation.

These are as follows:

(a) Some species of insects and frogs are cryptically coloured (camouflage) to avoid being detected easily by the predator. Some are poisonous and therefore, avoided by the predators.

(b) Monarch butterfly is highly distasteful to its predators (birds) because of a special chemical present in its body. The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.

(c) Some plants have thorns or spines for defence from herbivores as predators. Nearly 25% of all insects are known to be phytophagous (feeding on plant sap and other parts of plants).

(d) Thorns of Acacia and cactus are the most common morphological means of defence.

(e) Some plants produce highly poisonous chemicals like cardiac glycosides, nicotine, caffeine, quinine, strychnine, opium, etc. These are actually defence mechanisms against grazers and browsers.

Form # 2. Competition:

Competition occurs when closely related species compete for the same resources that are limited:

a. It can be 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.

b. It is a type of interaction, where both the species suffer.

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

d. Some totally unrelated species could also compete for the same resources. For example, in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, i.e., zooplanktons.

e. 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 plenty.

f. For example, when goats were introduced in Galapagos Islands, the Abingdon to noise became extinct within a decade due to greater browsing efficiency of the goats.

g. Competitive release is a phenomenon, in which a species whose distribution is restricted to a small geographical area is found to expand its distributional range dramatically, when the competing species is experimentally removed.

h. Connel’s elegant field experiments showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Baranus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone.

Gause ‘s competitive exclusion principle states that the two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually.

i. Resource partitioning states that if two species compete for the same resource, they could avoid competition by choosing. For instance, different times of feeding or different foraging patterns. In this relation, McArthur 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.

Form # 3. Parasitism:

It is the mode of interaction between the two species in which one species (parasite) depends on the other species (host) for food and shelter and damages the host.

In this process, one organism is benefitted (parasite), while the other being harmed (host).

(i) Adaptation Methods of a Parasite:

(a) Parasite is host-specific in a way 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.

(ii) The life cycles of parasites are often complex, involving one or two intermediate hosts or vectors to facilitate parasitisation of its primary host.

For example,

(a) Human liver fluke (a trematode parasite) depends on two intermediate hosts (a snail and a fish) to complete its life cycle.

(b) Malarial parasite {Plasmodium) needs a vector (mosquito) to spread disease to other hosts.

(iii) Majority of parasites harm the host. They reduce the survival, growth and reproduction of the host. They reduce its population density by making it physically weak.

Types of Parasites:

Parasites are broadly divided as:

(a) Ectoparasites feed on the external surface of the host organism for food and shelter. Examples are lice on humans, ticks on dogs, copepods in marine fishes and Cuscuta, a parasitic plant that grow on hedge plants.

(b) Endoparasites live inside the host’s body at different sites like liver, kidney, lungs, etc., for food and shelter. Examples are tapeworm, liver fluke, Plasmodium, etc. The life cycles of endoparasites are more complex because of their extreme specialisation.

(c) Brood parasitism is a phenomenon in which one organism (parasite) lays its eggs in the nest of another organism. For example, eggs of cuckoo (koel) and the crow resemble in size and colour, to reduce the chances of the crow (host) detecting the foreign eggs (cuckoo’s) and ejecting them out from the nest.

Form # 4. Commensalism:

It is the interaction between two species, where one species is benefitted and the other is neither harmed nor benefitted.

Some examples of commensalism are:

a. An Orchid growing as an epiphyte on a mango tree gets shelter and nutrition from mango tree, while the mango tree is neither benefitted nor harmed.

b. Barnacles growing on the back of whale gets benefitted to move to different locations for food as well as shelter, while the whales are neither benefitted nor harmed.

c. Egrets always forage close to where the cattle are grazing. Because, the cattle egrets are benefitted by the cattle to detect insects as the cattle stir up the bushes and insects are flushed out from the vegetation to be catched by cattle egrets.

d. Sea anemone has stinging tentacles and the clown fish lives among them. The fish gets protection from predators, which stay away from the stinging tentacles. The anemone does not appear to derive any benefit by hosting the clown fish.

Form # 5. Mutualism:

It is an interaction that confers benefits to both the interacting species.

Some examples of mutualism are:

a. Lichens represent an intimate mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria. Here, the fungus helps in the absorption of nutrients and provides protection, while algae prepare the food.

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

c. Plants need help from animals for pollination and dispersal of seeds. In return, plants provide nectar, pollens and fruits to the pollinators.

For example, the female wasp uses the fruit not only as an oviposition (egg-laying) site but uses the developing seeds within the fruit for nourishing its larvae. The wasp pollinates the fig inflorescence, while searching for suitable egg-laying sites. In return, fig provides the wasp some seeds as food for the developing wasp larvae.

d. Mediterranean Orchid, Ophrys employs ‘sexual deceit’ to get pollinated by a species of bee. One petal of its flower bears an uncanny resemblance to the female of the bee in size, colour and markings.

e. The male bee is attracted to what it perceives as a female and ‘pseudo-copulates’ with the flower. During that process, pollen is dusted from the flower. When the same bee pseudo-copulates with another flower, it transfers pollen to it and pollinates another flower.