Relationships between body size and geographical range size among Australian mammals: has human impact distorted macroecological patterns?

Extinction and artificial reduction in the size of geographical ranges of many species have occurred extensively across the globe because of human activities. In particular; Australian mammals have suffered heavily in the last two hundred years, with the highest number of reported cases of mammal extinctions anywhere. In the present study, we investigated the extent to which human impact has affected contemporary macroecological patterns in Australian terrestrial mammals. After examining patterns relating to body size and range size among the contemporary mammal fauna, we removed the effects of the last two hundred years of human impact by exploring patterns in the pre-European assemblage. This permitted us to determine whether contemporary macroecological patterns are distortions of pre-European patterns. In contrast to the expected pattern of a significant positive relationship between body size and range size, our results showed no significant association for the complete fauna in both cross-specie and phylogenetic analyses, even when data were corrected for species extinctions and range reductions. Analyses within families and among species with the same dietary strategy revealed three significant positive relationships (Macropodidae, Peramelidae, and herbivores) and one significant negative relationship (insectivores) within the contemporary assemblage that disappeared when the pre-European assemblage was analysed. A positive relationship also emerged in the pre-European assemblage for the Vombatidae that was not apparent in the contemporary fauna. Thus, correcting for human impact revealed important distortions among contemporary macroecological relationships that have been brought about by human-induced range reduction and extinction. These findings nor only provide further evidence that the Australian continent presents a unique and valuable opportunity with which to test the generality of macroecological patterns, but they also have important ramifications for the analysis and interpretation of contemporary macroecological datasets.