Habitat and forage dependency of sable antelope (Hippotragus niger) in the Pretorius Kop region of the Kruger National Park

Abstract

The distribution of animals across landscapes is driven by processes operating across a multitude of spatial scales. In essence, the spatial and temporal variability in nutrient availability characteristic of savanna ecosystems, superimposed on the spatial pattern of the distribution of predator risky areas, govern the herbivore foraging response. Thus studying the foraging behaviour of individual herds is a fundamental link in ultimately understanding demographic responses of entire populations. This study formed part of a broader research programme managed by the Centre for African Ecology (CAE) specifically focusing on the decline of rare antelope species in the Kruger National Park (KNP). Ultimately the aim was to contribute towards identifying the causal factors of a recent decline in sable antelope (Hippotragus niger) in the KNP. Specifically, this research was designed to span two levels of selection. Firstly to identify the forage resources that sable depend on by investigating the acceptability and dietary contribution of grass species and by examining the way in which the selection of particular species is influenced by changes in grass phenology and structure. In addition, with this study I attempted to describe the characteristics of sable foraging habitat and to identify the landscape features that distinguish areas suitable for feeding from those areas that remain unaccepted for feeding. I predicted at the level of the grass species that factors influencing the distribution and concentrations of nutrients between species and between tufts of the same species should influence the relative acceptance of a species by sable. Similarly, I expected sable’s use of foraging areas and feeding sites to be governed largely by nutrient distributions across the landscape, but to be restricted within safe areas with high visibility where the probability of the timely detection of predators is high. Four herds of sable were fitted with GPS/GSM collars and tracked from the early dry season to the start of the wet season for a total of two years during which characteristics of the foraging area and forage selection were recorded. The dietary contribution and the attributes of the foraging area remained largely descriptive and only involved analysis of seasonal and herd differences. Grass species and phenological and structural features influencing species acceptance were analysed using generalised linear models (GLM). A similar analysis technique was employed to identify the landscape attributes that played an important role in the distinction between feeding and non-feeding sites. The grass species that were consistently highly accepted by all four herds and contributed considerable proportions to the diet of each herd, included Panicum maximum, Heteropogon contortus, Hyperthelia dissoluta and Setaria sphacelata. Sable increased the dietary contribution of P. maximum and H. dissoluta during the dry season by feeding more frequently in areas where it was abundantly available. Regardless of the identity of the grass species, sable were more likely to feed from tufts that were green relative to the greenness available in that season. Sable also adjusted their acceptance of grass species based on the height of the tuft and were more likely to feed from tufts greater than 20 cm in height. The foraging area was mostly located on upper catena positions and a lack of a dry season increase in the use of bottomlands suggested that nutrients were either not accumulating in bottomlands as expected, or that sable were not responding to an accumulation of nutrients. Sable foraged and fed readily in low to high shrub cover and showed no response to the increased predation risk that would be expected to be associated with increased shrub cover. Sable were more likely to feed in areas with a relatively high tree canopy cover and more likely to feed in areas with a relatively green sward. However, sable still fed fairly frequently in open areas or areas with a predominantly brown sward. Overall, sable seemed unexpectedly tolerant of landscapes that would be predicted to range widely in nutrient distributions and forage quality as well as relative predation risk.