Why are there so many coexisting species of lizards in Australian deserts?

Because Australian skinks of the genus Ctenotus display very high local spe cies richness in arid-zone spinifex grasslands but not in mesic habitats, t hese lizards have been used as "model organisms" to ask why ecologically si milar taxa coexist under some circumstances but not others. Previous work h as involved detailed studies within small areas, and has looked for differe nces in ecological processes between arid versus mesic habitats. We suggest a radically different explanation for the high species-richness of arid-zo ne Ctenotus, by shifting attention to a larger spatial scale: the regional species pool. Analyses of the geographic distributions of Ctenotus species confirm that more species coexist at sites in the arid-zone (mean =9.3 spec ies per site) than in other climatic zones (means 2.4-7.6). However, the to tal number of species occurring within the arid-zone is actually lower, per km(2) of habitat, than is the case in some other climatic zones. That is, arid-zone Ctenotus show a higher local (alpha) species diversity, but a low er regional (gamma) diversity, than their mesic-habitat congeners. This app arent paradox occurs because most arid-zone species occur over vast areas ( mean =1,035,000 km(2)), whereas congeners from other climatic zones have sm aller geographic ranges (200-373,000 km(2)). The broad distributions of ari d-zone taxa reflect the great spatial homogeneity in climatic conditions in this zone. That is, the "climate spaces" occupied are similar for Ctenotus species from all bioclimatic regions. Thus, a given amount of climatic spa ce translates into a larger geographic distribution (and hence, more sympat ry) in the arid-zone than in other areas. In summary, the high number of co existing Ctenotus species in arid-zone habitats may simply reflect the fact s that the arid zone is large (so that many species have evolved therein) a nd climatically homogeneous (so that any species evolving in that habitat t ype can disperse very widely, and thus overlap with many other species). Ou r approach explains much of the variance in local-assemblage species richne ss from regional to site scales; but explanations invoking biological attri butes of the species concerned, the nature of their interactions with other species or with particular resources (such as prey or shelter) may still b e significant at microhabitat scales. For lizard communities in Australia, species richness at a site may be determined more by continental biogeograp hy rather than by ecological interactions.