Water temperature affects life-cycle duration of tadpoles of Natal cascade frog

Abstract

Direct and indirect effects of climate change on amphibians include range shifts and changes in community structure. The Natal cascade frog Hadromophryne natalensis has an altitudinal range of some 2 400 m in KwaZulu-Natal, and presents an opportunity to assess how increased water temperatures may impact on widely distributed amphibians. We sampled populations of tadpoles at three sites near the upper and lower altitudinal extremes of their range, measuring total length, mass and habitat water temperatures. Except for the spring survey, frequency–size class data showed bimodal curves at the two high-altitude sites and unimodal curves at the low-altitude site, suggesting a two-year aquatic life cycle in the cooler, higher rivers and a one-year life cycle at the warmer site. These data were consistent with cumulative degree days, which doubled from the higher- to the lower-altitude sites. Tadpoles at the warmer, lower-altitude site were in worse condition than those at the cooler, higher-altitude sites. We hypothesise that, while thermal changes may not result in rapid, large range shifts for this species, there are a number of unknown variables that may result from potential reductions in life-cycle length, including changes in competitive behaviour and community structure linked to asynchronies between species.