Factors affecting the occurrence and abundance of the Natal Cascade frog Hadromophryne natalensis at Mariepskop South Africa

Abstract

Amphibians have long been recognized as excellent indicators of both biological and ecological health of ecosystems, as they occupy many habitats and environments and have a diverse range of impacts on their immediate environments. It is thus important to investigate the critical habitat requirements, as well as the preferred environmental variables that are associated with different amphibian species in order to provide insight into conservation and management plans, and to predict what effects climate change and disease might have on amphibian populations. The aim of this study was to investigate the abundance and occurrence of the Natal cascade frog, Hadromophryne natalensis on Mariepskop Mountain in Limpopo, South Africa. In this regard, we investigated what environmental factors had an effect on the occurrence and temporal variation of H. natalensis (i.e. pH, water temperature, stream substrate) and, determined when the optimal breeding period of H. natalensis is, and over which seasons they are most active. Taking into account the habitat requirements of these frogs, we also explored in which ways climate change may affect the abundance and distribution of these amphibians. Finally, we investigated if the amphibian disease, chytridiomycosis, is present in the population, and if so what effect this disease is having on the H. natalensis population. Our results revealed that forest cover had a significant positive effect on the number of cascade frog observations, rather than low temperatures. Cascade frog distributions seem to be governed by forest distribution and their association with low temperatures is a coincidental result of the distribution of southern African indigenous forest, mostly in the mountains. With reference to climate change and temperature, we predict that H. natalensis is unlikely to be directly affected by increased temperatures brought on by climate change as they are able to survive at lower altitudes and warmer temperatures. However, indirect effects of climate change on the distribution of their forest habitat may result in range reductions or range shifts for this species. With reference to disease, chytridiomycosis incidence was significantly affected by tadpole size, with larger tadpoles more likely to be infected with the disease. We found that 28% of tadpoles sampled at Mariepskop showed visible signs of the disease, suggesting that chytridiomycosis extends much further northwards than reported to date. Overall, the population of H. natalensis at Mariepskop appears to be healthy with numerous individuals found when sampling. Interestingly, we found that H. natalensis breeds during the winter months, contrary to other studies conducted in South Africa on this species. Breeding appears to be successful as significant numbers of tadpoles, metamorphs and adults were observed during the study period. However, no data are available for previous seasons with regards to population size or disease prevalence, so it is not possible to determine the long term trends for these variables and their effect on this population, thus the importance for long term monitoring.