Invasive cane toads (Bufo marinus) cause mass mortality of freshwater crocodiles (Crocodylus johnstoni) in tropical Australia

Invasive species are frequently blamed for faunal declines, but there is little direct evidence about the pathways, magnitude and size-selectivity of mortality induced by invaders. Top predators are of particular interest in this context, because their removal can generate substantial cascades of secondary effects on community composition. Cane toads (Bufo marinus) are large South American anurans currently spreading rapidly through tropical Australia. Native predators that attempt to consume these highly toxic toads may die as a result. During surveys of the Victoria River in the semi-arid tropical region of the Northern Territory, we documented massive mortality of freshwater crocodiles (Crocodylus johnstoni) at the toad invasion front. Dead crocodiles spanned a wide size range (0.6-2.1 m long) but with significant biases; intermediate-sized animals (0.6-1.5 m long) were more likely to be found dead. Population densities of crocodiles plummeted by as much as 77% following toad invasion, and population size-structures changed. The negative impacts of toads on crocodiles appear to be greater in these hot semi-arid landscapes than in cooler, higher rainfall areas where crocodiles have access to a wider prey base, and the toads are less prone to desiccation and can rehydrate in small, scattered water bodies rather than in the main river. Hence, the impact of cane toad invasion on this top predator may increase with increasing aridity.