(i) A ‘Population’ the term used in the science of ecology represents a group of individual organism of the same species in a given area.
(ii) A Community is a group of populations of various species in a particular unit area. It may include are plants, all animals, and microorganisms. A community is formed by the interactions among populations of living animals and plants. It is a convenient biotic sub-division within an ecosystem.
Two concepts are of considerable importance within a community: habitat and niche. Habitat is defined as the specific physical location of an organism, characterized by its biotic and physical characteristics to which the organism must become adapted to survive.
Habitats can be examined at different scales: the macroscale, the mesoscale and the micro scale. Most species have specific habitat requirements with definite limits and sustaining nutrients.
Niche (French nicher, ‘to nest’) means the optimum habitat in an environment for a specific organism. According to Odum, the term ecological niche means the role that the organism plays in the ecosystem.
It also means a set of conditions within which each kind of organism can operate or does operate. It is the way an organism obtains and sustains the physical, chemical and biological factors it needs to survive.
There are three aspects of ecological niche: the habitat niche, the trophic niche, and the reproductive niche.
According to the competitive exclusion principle, no two species can occupy the same niche (food or space) successfully in a stable community. Some species have symbiotic relationship, which is mutually beneficial and sustains each organism.
On the contrary, some species have parasitic relationship i.e. one species kills the other species, thus destroying the parasite’s own niche and habitat.
(iii) An ecosystem:
An ecosystem is the interaction of many communities with the abiotic components of its physical environment. Most often organisms live in groups which is called population. The populations occupying a given area form a community.
The populations and their physical habitat function together as an ecosystem. Such a system may be defined in terms of flora, fauna or both. An ecosystem may be very large like the tropical rain forest of the equatorial region or very small like the small community of bacteria in a dead animal.
A very clear definition of ecosystem is given by F.R. Fosbery (1963). According to him, “an ecosystem is a functioning and interacting system composed of one or more living organisms and their effective environment in a biological, chemical and physical sense, and is a concept applicable at any scale ranging from the planet earth as an ecosystem down to the smallest patch of moss and lichen on a rock surface.”
The term ‘ecosystem’ was coined by A.G. Tansley in 1935. The term ecosystem is self- defining: eco indicates environmental and system indicates a group of coordinated parts.