Observed variation in species richness on ecological gradients and betweenregions has attracted several different explanations. Climatic factors, such asenergy availability, precipitation and mean annual temperature, are frequentlycited to explain differences in species richness (Wright et al. 1993). The relativeamount of variation in these variables may be of importance as well as absolutevalues (Stevens 1989, 1992). Alternatively, the history of colonization andextirpation have been used to explain regional variation in diversity (Guo et al. 1998; Latham & Ricklefs 1993a, b). Area is an important factor, for exampleon an elevational gradient the tops of mountains are not only cooler than thebottoms, but they are also much smaller (Rahbek 1997).