11.3. Ecological NicheThe ecological niche is not a notion of quantitative population ecology despite of several attempts to define it quantitatively. There were numerous definitions of ecological niche. Grinnell (1917) defined it as all the sites where organisms of a species can live (where conditions are suitable for life). Elton (1927) described the niche as the function performed by the species in the community of which it is a member. The first definition emphasized the "address" of the species and the second one emphasized its "profession" (Miller 1967).
Hutchinson (1957) defined a niche as a region (n-dimensional hypervolume) in a multi-dimensional space of environmental factors that affect the welfare of a species. This definition is more close to Grinnell's definition. It became popular because the range of tolerance to ecological factors can be easily measured, whereas species "profession" is hardly measurable. It is believed that the intensity of competition is proportional to the degree of niche overlapping. However, this kind of statements should be accepted with caution because: (1) measurement of niche volume is a subjective procedure, (2) some important dimensions of the niche may be not known, (3) niches change in the life-cycle, (4) niches change from one geographical region to another.
More information about niches can be found in Pielou, chapter 13.