Stable isotope evidence for nutritional stress, competition, and loss of functional habitat as factors limiting recovery of rare antelope in southern Africa

A major focus in population ecology is understanding factors that limit rare species. We used stable isotope approaches to diet to determine whether remaining rare antelope populations in Kruger National Park (KNP), South Africa experience i) nutritional stress; ii) competition with sympatric bulk grazers; iii) reduced habitat heterogeneity. Rare species consumed near-pure C4 grass-based diets throughout the seasonal cycle, in contrast to field observations that reported significant levels of C3 consumption (browse) by these taxa. This finding, coupled with low faecal %N at the height of the dry season, may indicate nutritional stress, but recent isotopic studies of the same species elsewhere in Africa suggest that field observations overestimated levels of browse consumption. We find little evidence for diet niche overlap between rare antelope with bulk grazing species. This partitioning of resources (interpreted mainly as tall- versus short-grass grazing, respectively), is consistent with reported differences in observed diet, and comparative oral morphology. Last, we find less seasonal diet variations amongst bulk grazers feeding in rare antelope habitats compared with other landscapes. We propose that loss of functional heterogeneity, apparently brought about by high densities of artificial waterholes, limits recovery of diet- and habitat-selective rare antelope populations in KNP.