Water movements, tadpole competition and limits to the distribution of the frogs Ranidella riparia and R. signifera

Ranidella signifera and R. riparia are leptodactylid frogs with largely allopatric distributions which contact and narrowly overlap in the Flinders Ranges of South Australia. One hypothesisto explain the distributional limits is that each species cannot persist in the creeks beyond its range. Survival and growth of tadpoles were measured in enclosures in three creeks, one on either side of the overlap zone and one in it. When each species was kept alone there were no differences between them either in the slow water typical of R. signifera creeks or in fast moving water typical of R. riparia creeks. When the two species were kept together there were no consistent differences in survival or growth in any creek where water flow was shielded in the experimental enclosures. Where water flow was not shielded through the enclosures R. riparia tadpoles had significantly higher survival than R. signifera. A form of interaction is proposed where R. signifera is more frequently exposed to the water current when R. riparia is present. This may explain why R. signifera does not extend its distribution into the adjacent fast rocky creeks occupied by R. riparia, but the experiments did not explain the limitation of the range of R. riparia.