Niche overlap and diffuse competition.

Current theory predicts a distinct upper limit on the permissible degree of niche overlap; moreover, theory suggests that maximal tolerable overlap should be relatively insensitive to environmental variability. Data presented here demonstrate that, within the lizard subset of natural desert communities, niche overlap decreases both with increasing environmental variability and with increasing numbers of lizard species. The latter two factors are themselves positively correlated. A partial correlation analysis is interpreted as indicating that the extent of tolerable niche overlap does not necessarily decrease due to environmental variability, but rather that overlap is probably more closely related to the number of potential interspecific competitors in a community, or what has been termed "diffuse competition." This result lends support to the "niche overlap hypothesis," which asserts that maximal tolerable overlap should vary inversely with the intensity of competition. Moreover, this empirical discovery indicates that niche overlap theory could be profitably expanded to incorporate the number of competing species. Although the average amount of overlap between pairs of species decreases with the intensity of diffuse competition, the overall degree of competitive inhibition tolerated by individuals comprising an average species could nevertheless remain relatively constant, provided that extensive niche overlap with a few competitors is roughly equivalent to lower average overlap with a greater number of competitors.