Resource Variability and Survival of Black and White Rhinoceroses at Lapalala Wilderness Nature Reserve

Abstract

Population dynamics and vital rates of savanna large herbivores, such as the black (Diceros bicornis) and white rhinoceros (Ceratotherium simum), are influenced by intrinsic and extrinsic factors, among which environmental fluctuations play an important role as they affect food resources. Given the differences in diets between the two rhino species and the differences in relationships between rainfall and woody versus herbaceous plants, there should also be differences in their survival and other vital rates in response to environmental stochasticity. We used historical rhino-monitoring, rainfall and vegetation greenness data to investigate rhino survival and other vital rates of two rhino populations at Lapalala Wilderness Nature Reserve. Mean population growth rates during the study were 4.8% for black rhinos and 8.9% for white rhinos, and both populations illustrated no evidence of density-dependence. Cox proportional hazard models showed that survival of both species was stage-specific and increased with rainfall. Vegetation greenness models similarly showed that survival increased with integrated annual greenness for all stages and proved to be the better proxy for white rhino survival. Additionally, density effects were somewhat trivial for both black and white rhinos as the populations seem to be rather resource limited. With droughts expected to be more common in an already variable system, species-specific assessment of how environmental stochasticity affects vital rates will be necessary for effective conservation of species in semi-arid regions.