Do invasive cane toads (Chaunus marinus) compete with Australian frogs (Cyclorana australis)?

Abstract

Abstract Despite widespread concern about the ecological impacts of invasive species, mechanisms of impact remain poorly understood. Cane toads (Chaunus [Bufo] marinus) were introduced to Queensland in 1935, and have now spread across much of tropical Australia. One plausible impact of toad invasion concerns competition between toads and native frogs, but there has been no previous experimental evaluation of this possibility. We examined interactions between toads and a morphologically similar species of native frog (Cyclorana australis) by manipulating toad and frog densities within large outdoor enclosures beside a floodplain in the wet‐dry tropics of the Northern Territory. Toads differed from frogs significantly in dietary composition and feeding rates, even in comparisons controlling for body‐size differences between these two taxa. Perhaps reflecting the abundant insect biomass, manipulating anuran densities or the presence of the putatively competing species did not influence food intake or dietary composition. However, the presence of toads suppressed activity levels of native frogs. The degree to which the invasion of cane toads influences attributes such as the activity levels, food intake and dietary composition of native frogs warrants further study, but our study suggests that competitive effects are likely to be minor compared with other pathways (such as direct poisoning during ingestion attempts) by which toads can affect frog populations.