In any habitat, plants, animals and microbes interact in various ways to form a community. When populations of two different species interact, it leads to interspecific interactions. Such interactions may benefit, harm or have a neutral effect on one or both species.
Darwin had mentioned that competition was a powerful force in the organic evolution as species struggled for survival. The feeding efficiency of one species is reduced in the presence of another species even if resources are abundant. Such a competition is known as interference competition. Therefore, competition is best defined as a process in which the fitness of one species, measured in terms of its intrinsic rate of natural increase, is significantly lowered in the presence of another species. Moreover, experiments conducted by Gause and other ecologists show that when resources are limited, the competitively superior species will eventually eliminate other species. In fact, Gause’s ‘Competitive Exclusion Principle’ states that two closely related species competing for the same resources cannot co-exist indefinitely and the species which is competitively inferior will be eventually eliminated. Some recent studies propound that species facing competition might evolve mechanisms to promote co-existence rather than exclusion. One such mechanism is ‘resource partitioning’, that is, if two species compete for the same resource, they could avoid competition by choosing different developing or foraging patterns.
Apart from competition, parasitism is another interspecific interaction observed in nature. In a parasitic mode of life, the parasite gets food from the host. Many parasites have evolved to be host-specific. In such cases, both the host and the parasite tend to co-evolve. That is, if the host develops special mechanisms for resisting its parasite, the parasite too evolves mechanisms to counteract them to continue its association with the same host species. Depending on where parasites live, they are classified as ectoparasites or endoparasites. Parasites that feed on the surface of a host are called ectoparasites while those that live inside a host are called endoparasites. Studies show that a large number of parasites are detrimental to their hosts as they make them more susceptible to predation by making them physically weak, reduce their growth, survival, reproduction as well as reduce the size of their population.
Thus, interspecific interactions such as parasitism and competition guarantee the survival and reproduction of species.