Niche

In geophysical and ecological terms, a niche designates the relationship between a species and its area of inhabitation. The term is specifically used to describe a species' unique position both in terms of physical area, and as a set of characteristics that relate the species' biological and ecological functions to its geophysical environment.

Although not the subject of this article, the term niche is also used to describe a type of glacier (e.g., niche glacier) that forms inside an irregular recess on or within a mountainside.

Four distinct stages of niche theory development in biological ecology can be identified: (1) Joseph Grinnell's original formulation of niche (in 1917 and 1928) as a geophysical spatial unit; (2) Charles Elton's formulation (in 1927) of niche as a functional unit; (3) Gause's (1934) competitive exclusion principle; and (4) E. Evelyn Hutchinson's concept of multidimensional niche in the 1950s.

Although Darwin understood the idea of niche and a few other biologists used the term earlier, Grinnell is credited with its formal development. To Grinnell, niche was a spatial unit that stood for the "concept of the ultimate distributional unit, within which each species is held by its structural and instinctive limitations." His conception of niche was "preinteractive"—that is, it referred to the entire area within which an organism could survive in the absence of other organisms. This is in contrast to the "post-interactive" niche, the actual place occupied by the organism in an environment after it has interacted with other organisms.

At about the same time, Charles Elton was developing the niche concept along somewhat different lines. Elton conceived of niche as a functional unit to describe the organism's "place in the biotic environment, relations to food and enemies." Although Elton presented niche as an organism's ecological position in a larger framework like a community or ecosystem, he then restricted its use to the food habits of an organism. Accordingly, Elton's niche is considered to be postinteractive.

Gause is credited with being the first investigator to perceive the connection between natural selection, competition, and niche and to see the interacting aspects of these concepts. Gause stated that "it is admitted that, as a result of competition, two similar species scarcely ever occupy similar niches, but displace each other in such a manner that each takes possession of certain peculiar kinds of food and modes of life in which it has an advantage over its competitor. Gause experimentally tested the general conclusions drawn from the Lotka-Volterra competitive equations, confirming and amplifying them. These conclusions are summarized in the "competitive exclusion principle," which states that two species cannot coexist at the same locality if they have identical ecological requirements. Gause based the principle on an Eltonian definition of niche.

The Eltonian niche dominated ecological theory during the period 1930–1950 and began to be referred to as an organism's "occupation" or "profession." Hutchinson responded to this rather limited idea of niche by incorporating selected features from both Grinnell's and Elton's niche definitions and redefining niche as an "n-dimensional hypervolume," an abstract multidimensional space defining the environmental limits within which an organism is able to survive and reproduce. Hutchinson's "fundamental niche" is preinteractive, composed of "close to innumerable" dimensions, each corresponding to some requisite for a species. By setting the number of defining dimensions at "close to innumerable," Hutchinson attempted to illustrate the complexity of the systems within which organisms exist and interact. He depicted it by plotting each identifiably important environmental variable along an axis to show the points below which and above which the given organism could not survive.

Hutchinson's "realized niche" usually corresponds to a smaller hypervolume because competition and other interactions serve to restrict organisms from some parts of their fundamental or potential niche. Although most current works in niche theory use some variation of Hutchinson's multidimensional niche, both the Eltonian and the Hutchinson niches are still found in contemporary ecology and are still useful. Any application of niche, however, is only an approximation of reality, because niche dimensions are too numerous to be counted.

See also Archeological mapping; Physical geography; Topography and topographic maps