© Eric R. Pianka

Intercontinental comparisons of independently-evolved
desert lizard assemblages

Understanding why more species occur at some places than at others has been a focal point of my research for many years. In graduate school during the early 1960's, my Ph.D. research was on species diversity of North American desert lizards. The first chapter of my dissertation was a review of concepts of latitudinal gradients in diversity, published in The American Naturalist in 1966. It has been reprinted in several collections of important papers and was celebrated at a symposium in 2016 entitled "Latitudinal Gradients in Species Diversity: 50 years since Pianka" and recognized in a 2017 historical comment: Latitudinal Gradients in Species Diversity: Reflections on Pianka's 1966 Article and a Look Forward.

After publishing a companion paper on species diversity of lizards in North America, I embarked on a 3-year postdoctoral with Robert H. MacArthur at Princeton University, 18 months of which was spent studying the ecology and diversity of Australian desert lizards. My ex-wife Helen and I found the most diverse lizard assemblages known and discovered half a dozen as yet undescribed species, two of which are named after us. During 1970, with the able assistance of Raymond Huey, I extended these studies to include the Kalahari Desert in southern Africa.

North American deserts have the lowest species richenss with 12 genera and only 14 species. Australian deserts are the richest with 21 lizard genera and 69 species. The Kalahari desert of Africa is intermediate with 14 genera and 22 species.

I have now spent half a century collecting extensive data on ecological relationships of lizard faunas at some 32 desert study sites at similar latitudes on three continents: western North America, the Kalahari desert of southern Africa, and Australia's Great Victoria Desert. These are the most comprehensive surveys of lizard assemblages carried out anywhere in the world.

A major virtue of these data is that identical methods and resource categories were used by the same investigator for each of three continental desert-lizard systems, enabling meaningful intercontinental comparisons. This unique body of data has thus allowed detailed analyses of resource utilization patterns and community structure in these historically independent lizard faunas.

Detailed comparisons of these three indepently-evolved desert-lizard assemblages have shown that lizards from more species-rich systems are not more specialized than those from less rich saurofaunas. Moreover, average niche overlap is lower in more diverse systems, suggesting that diffuse competition from a greater number of competitors structures these natural ecosystems. Food is a major niche dimension separating lizards in North America, but not in the Kalahari where differences in time and place niches are considerable. All three niche dimensions are important in Australia. Differences in species richness between continents stem primarily from differences in the overall diversities of resources utilized or the size of lizard niche space occupied.

My intercontinental comparisons of diversity constitute a classic example featured in many ecology textbooks.