The spatial ecology of cane toads (Bufo marinus) in tropical Australia: why do metamorph toads stay near the water?

Cane toads (Bufo marinus) have invaded large areas of Australia, killing many native predators as they have done so. The metamorph stage of the life cycle - the first terrestrial phase, immediately after transformation from the tadpole - is critical for ecological impact (because these animals are small enough to be prey for many native predators) and for potential control of toad populations (because small body size renders metamorphs vulnerable to desiccation).To quantify the spatial and temporal distribution of metamorph toads, and the biotic and abiotic factors that might affect their distributions, we surveyed toad breeding sites in Australia's wet-dry tropics (Adelaide River floodplain, NT) in both the wet season and the dry season. Metamorphs were concentrated close to the water's edge during the dry season, especially at midday when desiccation rates were highest. During the wet season, metamorphs were widely dispersed through the landscape. Our surveys indicate that abiotic factors (risk of desiccation) are most favourable for metamorph toads close to the pond edge, but biotic factors (food supply, and risk of competition and cannibalism) are most favourable away from the water. Operative temperatures were spatially homogeneous and sublethal, and so are unlikely to influence metamorph distribution. Desiccation risk fluctuated on a diel cycle as well as seasonally.We predict that metamorph toads benefit from dispersing as soon as desiccation risk allows them to do so, and hence the distribution of metamorph toads will shift dynamically in response to weather-mediated changes in rates of evaporative water loss.