Competition between the tadpoles of Japanese toads versus frogs.

Competition within and among species can play a key role in structuring the assemblages of anuran tadpoles. Previous studies have reported that tadpoles of the invasive cane toad (Rhinella marina) are more strongly disadvantaged by the presence of native frog tadpoles than by the same number of conspecific toad tadpoles. That effect might arise from a lack of coevolution of the invasive toad with its competitors; and/or from a generalized superiority of frog tadpoles over toad tadpoles. To clarify those possibilities, we conducted experimental trials using the larvae of a native rather than invasive toad (Bufo japonicus formosus in Japan) exposed to larvae of native anurans (the sympatric frogs Rana japonica and Rana ornativentris and the parapatric toad Bufo japonicus japonicus). In intraspecific competition trials, higher densities of B. j. formosus prolonged the larval period and reduced size at metamorphosis, but did not affect survival. In interspecific competition trials, the effects of the other anuran species on B. j. formosus were similar to the effects of the same number of conspecific larvae. This similarity in impact of interspecific versus intraspecific competition argues against any overall competitive superiority of frog larvae over toad larvae. Instead, the vulnerability of larval cane toads to frog tadpoles may result from a lack of coevolutionary history.