An experimental demonstration that predation influences antelope sex ratios and resource-associated mortality

Abstract

Smaller, enclosed reserves lacking large mammalian predators are an increasingly popular commercial model in southern Africa and elsewhere. The presence or absence of predation is likely to have major effects on the population dynamics of sexually dimorphic ungulates, with contradictory implications for multiple-use reserves, and to provide fundamental insights into predator–prey relationships.Over a two- and four-year period we determined the adult sex ratios and juvenile mortality of two substantial populations of impala Aepyceros melampus in South Africa – one in predator-free Ithala Game Reserve (IGR), the other in neighbouring predator-laden Hluhluwe-iMfolozi Park (HiP). Data were collected monthly, over a five-day period, by repeated road transects covering a representative sample of the reserves’ habitat types. We assessed differences in adult sex ratios by applying Pearson's chi-square test, whilst to explore the relationship between juvenile mortality, the advance of the breeding year, rainfall and the presence or absence of predators, we used a generalized linear model.We found that the impala adult male to adult female ratio was significantly lower in the presence of predation (HiP=0.43, IGR=0.69). The generalized linear model revealed that the overall proportion of juveniles in breeding herds (defined as herds containing at least one juvenile) declined, over the breeding year, at a faster rate in the presence of predators.Impala juvenile mortality over the breeding year was not significantly affected by lower rainfall in the absence of predators, but under predation juvenile mortality declined at a faster rate over a drier year compared to years of near average rainfall – a novel finding amongst African antelope.Such fundamental insights into predator–prey relationships are especially relevant to predator-free reserves, where management and planners should be aware of these influences and, depending on the business model, consider replicating them artificially.