Realized niche width is a phrase relating to ecology, is defined by 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). An organism's ecological niche is determined by the biotic and abiotic factors that make up that specific ecosystem that allow that specific organism to survive there. The width of an organism's niche is set by the range of conditions a species is able to survive in that specific environment.
Definition
The fundamental niche width of an organism refers to the theoretical range of conditions that an organism could survive and reproduce in without considering interspecific interactions. The fundamental niche exclusively considers limiting biotic and abiotic factors such as appropriate food sources and a suitable climate. The fundamental niche width often differs from the realized niche width (the areas where actually inhabited by a given species). This differentiation is due to interspecific competition with other species within their ecosystem while still considering the biotic and abiotic limiting factors. A species' realized niche is usually much narrower than its fundamental niche width as it is forced to adjust its niche around the superior competing species.
The physical area where a species lives, is its habitat. The set of environmental features essential to that species' survival, is its "niche." (Ecology. Begon, Harper, Townsend)
Importance
The difference between the realized and the fundamental niche is important in understanding how interactions with a variety of different species in one environment affects the fitness of another species. This is not only important in understanding how a species functions in an ecosystem, but it is also important in determining the potential and realized success of invasive species. Invasive species could thrive or be killed off in an environment where they would theoretically be able to exist based on the presence or lack of there of different species. To survive, an invasive species first has to successfully survive the journey to the new area, they then have to be able to survive in that habitat. After this, they then must to be able to successfully compete and reproduce with the other species already in the new, invaded environment. Considering these factors, not all invasive species are devastating to the new environment they inhabit as they must first overcome these other challenges before they can negatively affect their new environment.
In an organism's niche, the abiotic and biotic factors determine the ability of a species to survive; however, both the abiotic and biotic factors of that environment can be changed by that species' existence. A species' impact on its biotic environment in its niche tend to effect not only that species' ability to survive, but the other species it coexists with. Again, these changes are important in understanding the effects of invasive species in a new habitat. The ability of a new species to change an environments abiotic and biotic factors can make a previously habitable environment for a species uninhabitable. The extinction of this species can further change the biotic factors of an environment. Invasive species not only directly affect the biotic environment, but they indirectly effect this environment by affecting the species able to survive in this habitat.
Niche theory states that a species' ranges are limited by their physiological tolerances (fundamental niche) and their biotic limitations (realized niche). The survival rates of organisms facing rapid niche shifts help scientists predict the future effects of climate change and invasive species on current ecological communities. The ability of organisms to shift niches also help scientists understand community formation and speciation. Niche shifts for invasive species in their native environment differ from those in their newly invaded environment. After an invasive species is introduced to their new environment, they have to cope with new biotic factors, environmental constraints, and climate differences. These variables play a role in determining how the organism's niche will evolve. Biophysical models use links between an organism's preferred climate and their functional traits to determine where an organism could survive without taking biotic factors into account.