Predator-prey interactions are dynamic and the ability to predict their impact on prey species has become an important aspect in ecology. One method to predict the impact of a predator species on a prey population is by analysing the predator's functional response. However, predators are not all functionally similar and may differ intraspecifically. Predators are also not limited to prey from other species as they can cannibalise vulnerable individuals within their own population. The African clawed frog (Xenopus laevis) is a predator with a broad diet, known to consume multiple prey species, including its congeners. They are notorious cannibals with populations consisting of different sized conspecifics. They occur in sympatry with several congeners including the endangered X. gilli which are thought to be under threat through competition, hybridisation and predation from X. laevis. In this study, I investigated the role of predator size on the functional response of X. laevis predators using mosquito larvae (Culex pipiens) as a common prey. I also investigated the threat of X. laevis predation on X. gilli using choice and no-choice experiments to evaluate the relative vulnerability of X. laevis and X. gilli larvae to X. laevis predation. For the functional response experiments, predators were classified by size into small (15-30mm snout-vent length), medium (50-60mm) and large (105-120mm) size classes. Predator-prey interactions were filmed in order to compare handling time and attack rate to the functional response model. In the choice and no-choice experiments, both X. laevis and X. gilli larvae species were collectively and separately exposed to treatments with the presence or absence of a predator. Results showed that the functional response of X. laevis predators change with size: small predators were found to have a Type II response, while medium and large predators had a Type III response. Both functional response and behavioural data showed an inversely proportional relationship between predator attack rate and predator size. Small and medium predators had the highest and lowest handling time, respectively. That the functional response was found to change with the size of predator suggests that predators with overlapping cohorts may have a dynamic impact on prey populations. Therefore, predicting a predator's impact from the functional response of a single size-matched predator experiment may be a misrepresentation of the predator's potential impact on a prey population. Results from the choice and no-choice experiments showed that large X. gilli showed a significantly higher vulnerability to X. laevis predation compared to small X. laevis. Large and small X. laevis larvae, and same size X. gilli and X. laevis larvae showed no significant differences in relative vulnerability. Behaviour may be a factor in contributing to large X. gilli larvae's vulnerability to X. leavis predation, and this will likely have negative implications for the population structure of the endangered X. gilli.

To explain the impact of Xenopus laevis of aquatic ecosystems (including its impact on an endangered species)