Ecological traits of declining amphibians in upland areas of eastern Australia

Abstract

Catastrophic declines of amphibian populations have been reported from upland areas around the world, yet within regions not all species have been affected. Herein, we examine the relationships between conservation status and ecological traits for over 60 amphibian species from upland areas of eastern Australia. Among the 24 phylogenetic groups examined, eight groups contained only declining species, eight groups contained both declining and non-declining species and eight groups had no declining species, suggesting species declines are influenced by their evolutionary history. Declines have been reported for only one species (Philoria frosti) of 20 species (i.e. from genera Assa, Cophixalus, Austrochaperina and remaining Philoria/Kyaranus) with direct development in the terrestrial environment. Of the remaining 40 species that include an aquatic stage in the life history, species-specific traits associated with declines suggest species with stream-dwelling tadpoles are more likely to be declining than species that breed in ephemeral and isolated ponds. However, low ovarian clutch size is the most important ecological trait significantly associated with the tendency for a species to be declining (independent of phylogeny). Species with restricted geographic ranges are also more likely to be declining than widespread species (independent of phylogeny). While ovarian clutch size and geographic range are correlated they are both good predictors for the likelihood of decline for upland species independent of their phylogenetic history. We propose that low ovarian clutch size is an index of low fecundity, and indicates a reduction in population resilience, making those species more susceptible to population declines and extinction than highly fecund species. This provides the first ecological explanation for why some species are declining while other sympatric species are not. Geographic range provides an easily applied tool that can be used to predict the likelihood of species decline in other regions. These results support the hypothesis that a causal factor may be affecting all species of frogs, yet only those species with naturally low population resilience are susceptible to declines.