First is the realized niche
, which consists of the climate conditions in a species' native range.
In addition, we hasten to point out that these analyses typically model the fundamental niche, whereas in nature, species will typically occupy a more geographically and ecologically restricted realized niche
. Further, our analysis is based on presence only data, rather than including information on absence and abundance, which may bias the results with respect to the "real" distribution of a species (Anderson, 2012, 2015).
The realized niche
, [N.sub.R], is the reduction of the existing fundamental niche by the geographic effects of species interactions, and is extremely complex to characterize (Chase & Leibold 2003).
Following Mac-Arthur (1972), we hypothesized that for both Characidium species realized niche
vary among sites and niche breadth is reduced in sites were the species co-exist.
Hutchinson (1959) further defined the ecological niche concept by distinguishing between the fundamental and realized niches
. The fundamental niche consists of the total potential area that meets all the physical and biological requirements of a species; the actual distribution of a species is determined by a variety of factors such as dispersal, history, and physical barriers--an area described as the realized niche
This portion of the fundamental niche is called a realized niche
. It may be much smaller than the fundamental niche (see Figure 1).
Although this micro orientation is justified by the strong empirical findings to date, it seems to rely on an incomplete conception of the niche, one grounded in the realized niche
. Other broader environmental factors are not ignored entirely, but they are not given much theoretical attention, usually getting relegated to the status of control variables.
The "fundamental niche" represents that portion of the environmental gradient within which existence is possible, while the "realized niche
" reflects the proportion of the fundamental niche that is actually occupied.