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Realized niche width is a phrase relating to ecology defining the actual space that an organism inhabits and the resources it can access as a result of limiting pressures from other species (e.g. superior competitors).
The phenomenon was famously documented by ecologist Joseph Connell's study of species overlap between barnacles on intertidal rocks. He observed that Chthamalus stellatus and Balanus balanoides inhabited the upper and lower strata of intertidal rocks respectively, but only Chthamalus could survive both upper and lower strata without desiccation. If Balanus was removed from the lower strata, Chthamalus was able to occupy its fundamental niche (both upper and lower strata) which is much larger than its realized niche of upper strata.
Niche width vs. realized niche width
The niche width of an organism refers to a theoretical range of conditions that a species could inhabit and successfully survive and reproduce with no competition. The niche width is defined as the parameters of this range which are determined by biotic and abiotic factors such as appropriate food sources and suitable climate respectively.
The niche width often differs from the area that a species actually inhabits, with the area a species actually persists in referred to as its realized niche width. This is due to interspecific competition with other species within their ecosystem and other biotic and abiotic limiting factors. A species realized niche is usually much narrower than its theoretical niche width as it is forced to adapt its niche around superior competing species.
The physical area where a species lives, is its "habitat." The abstract hypercube that defines the limits of environmental features essential to that species' survival, is its "niche." (ECOLOGY, Begon,Harper,Townsend)