The Diversity and Abundance of Lizards in Arid Australia: A New Hypothesis

We attempt to integrate previous partial explanations for the greater diversity and abundance of lizard species in arid Australia with new ideas concerning spinifex grasslands, the habitat with the most startling lizard diversity, in order to present a more comprehensive and testable theory than has hitherto been provided. Our synthesis focuses on the factors allowing diversification in local assemblages, not on between-habitat diversity. Our argument has four major components. 1. Spinifex grasses grow on unusually infertile soils, and spinifex is nutritionally very poor. Thus, herbivorous animals are relatively uncommon, whereas termites are exceedingly common and diverse. Acacia shrubs grow on firm soils that are only slightly more fertile than sands; and termites are probably even more abundant here than in the grasslands. Chenopod shrubs and tussock grasses, which grow on relatively fertile soils, probably have considerably fewer termites. 2. The diversity of lizards increases from fertile vegetational formations to infertile spinifex grasslands and acacia shrublands. We suggest that termites, being individually small but abundant, provide a food resource particularly suitable for lizards. Consequently, lizards have been able to diversify, especially where sandy soils allow subterranean activity. 3. Four contributory factors appear to have enhanced the abundance and diversity of Australian lizard species. Vegetational production in arid Australia is uncertain because the variability in rainfall is high and droughts may be lengthy; hence, ectothermic lizards are favored over birds and mammals. Nevertheless, aridity is mild and vegetational cover is usually high, thereby providing abundant food for termites. Because many termites forage nocturnally, abundant resources are available for exploitation by nocturnal lizards. Finally, arboreal lizards are prominent because climatic peculiarities allow trees to grow sparsely throughout arid Australia; the added structural complexity allows further diversification of termites and lizards. 4. Our interpretation is broadly consistent with available literature on lizard and termite faunas in arid Australia, North American deserts, and the Kalahari Desert, although important gaps remain. Our hypothesis consists of a web of forces rooted in the edaphic and climatic characteristics of arid Australia. It suggests that lizards are better suited than other vertebrates to infertile and arid environments because of their morphological and physiological characteristics; this phenomenon reaches its climax in Australia.