On the relationship between niche and distribution

Applications of Hutchinson's n-dimensional niche concept are often focused on the role of interspecific competition in shaping species distribution pa tterns. In this paper, I discuss a variety of factors, in addition to compe tition, that influence the observed relationship between species distributi on and the availability of suitable habitat. In particular, I show that Hut chinson's niche concept can be modified to incorporate the influences of ni che width, habitat availability and dispersal, as well as interspecific com petition per se. I introduce a simulation model called NICHE that embodies many of Hutchinson's original niche concepts and use this model to predict patterns of species distribution. The model may help to clarify how dispers al, niche size and competition interact, and under what conditions species might be common in unsuitable habitat or absent from suitable habitat. A br ief review of the pertinent literature suggests that species are often abse nt from suitable habitat and present in unsuitable habitat, in ways predict ed by theory. However, most tests of niche theory are hampered by inadequat e consideration of what does and does not constitute suitable habitat. More conclusive evidence for these predictions will require rigorous determinat ion of habitat suitability under field conditions. I suggest that to do thi s, ecologists must measure habitat specific demography and quantify how dem ographic parameters vary in response to temporal and spatial variation in m easurable niche dimensions.