In ecology, a niche is a term describing the relational position of a species or population in an ecosystem. More formally, the niche includes how a population responds to the abundance of its resources and enemies (e. g., by growing when resources are abundant, and predators, parasites and pathogens are scarce) and how it affects those same factors (e. g., by reducing the abundance of resources through consumption and contributing to the population growth of enemies by falling prey to them). The abiotic or physical environment is also part of the niche because it influences how populations affect, and are affected by, resources and enemies.
The description of a niche may include descriptions of the organism's life history, habitat, position in the food chain. According to the competitive exclusion principle, no two species can occupy the same niche in the same environment for a long time.
A niche would be the position occupied by an organism (or group of organisms) within an ecosystem or the conditions making possible a habitat. The conditions needed to survive for an organism is a niche and other competing organisms affect this. This niche defines the organism's role in the ecological community. A niche here implies a species’ ability to survive. Adaptability of a species is important to what niche it can hold.
Different species can hold similar niches in different locations and same species may occupy different niche in different locations. The Australian grasslands grass species, though different from those of the Great Plains grasslands, occupy the same niche. Once a niche is left vacant other organism can fill into that position. The niche that was left vacant by the extinction of the tarpan has been filled by other animals (inparticular a small horse breed, the konik). Plants and animals, though perfectly suited to their environmental niche in their home country, can sometimes become a serious pest when taken to a different location.
The full range of environmental conditions (biological and physical) under which an organism can exist describes its fundamental niche. As a result of pressure from, and interactions with, other organisms (e.g. superior competitors) species are usually forced to occupy a niche that is narrower than this and to which they are mostly highly adapted. This is termed the realized niche. G.E. Hutchinson also defined the ecological niche as a "Hypervolume". This term defines the multiple degrees of resources (i.e., light, nutrients, structure, etc.) available to organisms.
See also: Introduced species, Information ecology