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Two lichens on a rock, in two different ecological niches

In ecology, a niche (pronounced "nich," "neesh" or "nish")^[1] is a term describing the relational position of a species or population in its ecosystem^[1]. The ecological niche describes how an organism or population responds to the distribution of resources and competitors (e. g., by growing when resources are abundant, and predators, parasites and pathogens are scarce) and how it in turn alters those same factors (e.g., limiting access to resources by other organisms, acting as a food source for predators and a consumer of prey)^[2].

The different dimensions, or plot axes, of a niche represent different biotic and abiotic variables. These factors may include descriptions of the organism's life history, habitat, trophic position (place in the food chain), and geographic range. According to the competitive exclusion principle, no two species can occupy the same niche in the same environment for a long time^{[citation needed]}.

The word "niche" is derived from the Middle French word nicher,meaning to nest. The term was coined by the naturalist Joseph Grinnell in 1917, in his paper "The niche relationships of the California Thrasher."^[3] However, it was not until 1927 that Charles Sutherland Elton, a British ecologist, gave the first working definition of the niche concept. He is credited with saying: "[W]hen an ecologist says 'there goes a badger,' he should include in his thoughts some definite idea of the animal's place in the community to which it belongs, just as if he had said, 'there goes the vicar.'"^[4]

The niche concept was popularized by the zoologist G. Evelyn Hutchinson in 1958^[5]. Hutchinson wanted to know why there are so many different types of organisms in any one habitat.

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. The ecological niche has also been termed by G.E. Hutchinson a "hypervolume." This term defines the multi-dimensional space of resources (i.e., light, nutrients, structure, etc.) available to (and specifically used by) organisms. An adaptive zone is a set of ecological niches that may be occupied by a group of species that exploit the same resources in a similar manner. (After Root, 1967.)^{[citation needed]}

It should be noted that Hutchinson's "niche" (a description of the ecological space occupied by a species) is subtly different from the "niche" as defined by Grinnell (an ecological role, that may or may not be actually filled by a species—see vacant niches).

Different species can hold similar niches in different locations and the same species may occupy different niches in different locations. The Australian grasslands species, though different from those of the Great Plains grasslands, occupy the same niche.^[6] Once a niche is left vacant, other organisms can fill into that position. For example, the niche that was left vacant by the extinction of the tarpan has been filled by other animals (in particular a small horse breed, the konik). When plants and animals are introduced into a new environment, they can occupy the new niches or niches of native organisms, outcompete the indigenous species, and become a serious pest.

• Niche differentiation
• Fitness landscape
• Habitat (ecology)

• Concept of ecological niche
• Environmental Niche - Extinction of the Dinosaurs
• Ontology of the niche