The macroecology of Australian frogs

Abstract

1. The Australian continent provides an important test of macroecological patterns given its unique biota and long-term geographical isolation. However, macroecological contributions from the Australian continent are rare. We explored the relationship between abundance and geographical range for Australian frogs (Order Anura) across complete geographical ranges, and investigated how adult body size relates to both abundance and the size of geographical ranges. 2. Our analyses followed two complementary paths. First, we employed cross-species correlations at a number of taxonomic levels to analyse present-day ecological associations among the macroecological variables. Secondly, we tested whether there were correlated evolutionary divergences among the variables by use of the phylogenetic regression. In the event, cross-species patterns and correlated divergence patterns proved quite similar for this dataset. 3. For Australian frogs there is a strong, significant positive correlation between abundance and geographical range, that is not mediated through body size, and that is found for both cross-species correlations and as correlated evolutionary divergences. Among biological mechanisms that have been proposed, some can be rejected. Feasible explanations would invoke resource use or intrinsic rates of increase or both. 4. There is also a significant positive correlation between adult body size and geographical range. We were able to discard four of the five proposed mechanisms describing this relationship. The remaining mechanism relates to homeostasis and environmental variability. 5. Previous studies of the relationship between adult body size and abundance have revealed a strong negative correlation when studies are performed over areas that encompass most or all of the geographical ranges of the species in question. Contrary to these studies, a negative relationship was not found for Australian frogs. None of the eight explanations postulated from other studies can account for this discrepancy, leading to the conclusion that Australian frogs provide a contrast to macroecological patterns prevailing elsewhere. 6. Body size could not explain directly the relationship between abundance and geographical range. While there was a significant positive correlation between body size and geographical range, there was no relationship between body size and abundance. If a mechanism incorporating the intrinsic rates of increase of species is to explain the positive relationship between abundance and geographical range, then ecological factors other than body size must be determining differences in intrinsic rates of increase.