Conservation biology of the giant bullfrog, Pyxicephalus adspersus (Tschudi, 1838)

Abstract

The giant bullfrog, Pyxicephalus adspersus, is a large, explosive-breeding anuran from southern Africa, which spends most of the year buried in a state of torpor. In South Africa this species is considered to be Near-Threatened by habitat loss and other factors, especially in the densely human populated Gauteng Province. The aim of this thesis was to obtain essential outstanding information about the ecology of P. adspersus to contribute towards improved conservation management of this species. A model was used to predict the geographic range of P. adspersus in southern Africa, and recent land cover data were used to determine the amount of suitable habitat remaining for this species in Gauteng. As a step towards identifying P. adspersus conservation management units, genetic structure and gene flow for populations from 23 localities in Gauteng and seven additional localities in the north-eastern interior of South Africa was quantified using 708 base pairs of the mitochondrial gene cytochrome b. To investigate the unpredictable activity and unknown spatial habitat requirements of P. adspersus, a population’s spawning and non-breeding activity was monitored, and the movements of 70 adult frogs were radio- or spool-tracked during five summers at a site in Diepsloot, Gauteng. Using skeletohronology, the age distribution of breeding P. adspersus at this and two other peri-urban sites near Johannesburg, Gauteng, was examined. Bioclimatic conditions were predicted to be suitable for P. adspersus in the temperate to semi-arid interior, but not the low-lying eastern subtropical and arid western sides of southern Africa. Limited genetic data suggested that P. adspersus was common in the north-eastern interior of South Africa, and that populations in the Free State Province represent an evolutionary significant unit of this species. In central Gauteng, where P. adspersus may have declined by > 90%, populations < 20 km apart exhibited significant genetic differentiation, possibly as a result of genetic drift. At Diepsloot, both annual numbers of spawning events and numbers of spawning males were positively correlated with rainfall, although other meteorological variables also affected the activity of P. adspersus. Radio- or spool-tracked frogs showed high fidelity to their breeding site and burrows, which were situated up to 1 km away from the water. Male P. adspersus probably live 20 years in the wild, but at some peri-urban breeding sites adult life expectancy and body size may be declining. The geographic range of P. adspersus was predicted to be slightly smaller than that reported by other authors, and deserves phylogeographic validation. The main conservation priority for P. adspersus in South Africa should be the protection of terrestrial habitat for adult foraging and aestivation around, and for juvenile dispersal and gene flow between, breeding sites. In Gauteng, the conservation of a P. adspersus metapopulation is critical, and could most likely be achieved in the northern region of this province. Populations in the Free State Province deserve improved protection given their reported genetic uniqueness. At local spatial scales specific threats (e.g. pollution) should be ameliorated, and long-term monitoring should be implemented to detect real population trends.