Home range and resource use of sable antelope in the Okavango Delta
Hensman, Michael C.
Habitat selection occurs across a range of different spatial scales and is influenced by a variety of factors, ultimately determining how animals distribute themselves across the landscape. Studying the decisions that an individual animal makes across different levels of selection, from its choice in dietary item to predator avoidance strategies, is a fundamental link in understanding the response of groups of animals and ultimately entire populations that may provide insight into population performance. The study formed part of a broader study focused on the decline of rare antelope species. Specifically this study was aimed at establishing the home range and resource use of sable antelope in a region where they were initially expected to be thriving. The levels of selection covered in this study are: the location of home ranges of an individual or social group within the landscape; the use of various habitat components within the home range; and the procurement of food items within those habitats. At the highest level, the broad objectives were (1) to determine where sable occupied home ranges within the landscape, indicating the suitability of various landscape units to sustain sable populations and (2) to determine the relative use of habitat types within those home ranges that may enable sable to avoid predation and acquire resources required to survive and reproduce. At the lowest level of selection the characteristics of forage selection and how the grass quality in the different vegetation types during different seasons affects the success of sable herds was explored. The broad objectives were (1) to determine the effect of seasonal flooding and rainfall on grass greenness in the floodplains and upland vegetation types and the consequent use of those vegetation types by sable antelope and (2) to determine how exploitation of resources in the floodplains and in the uplands contributed to the nutritional status of sable. I additionally quantified the time spent browsing and determined the composition of the browse component of the diet of sable. Adult female sable from each of three adjacent sable herds were fitted with GPS collars providing hourly GPS co-ordinates. Adaptive LoCoH was used to determine home range location and annual, seasonal and core home range extents. A vegetation map was created and the number of GPS locations within each vegetation type was counted to determine their relative use in relation to availability within the home ranges. GPS collars were used to locate herds daily so that foraging observations of browsing and characteristics of the grasses grazed could be attained. Acceptability and dietary contributions of grass species and browse were determined for each sable herd during different seasons. The availability of grass species on the floodplain grasslands and in the upland grasslands and woodlands was estimated. Water and the availability of key resources posed a constraint on where sable home ranges were established. Sable simply did not occupy the region in the north of the study area further than 7 km from permanent water and floodplain grasslands. Herds generally avoided open savanna, mopane woodlands and Kalahari apple-leaf woodlands characterised by sparse grass cover, particularly during the dry season. Home ranges were relatively small compared 4 to the range estimates from herds in Kruger National Park. There was no obvious seasonal difference in home range extent nor were there large areas of overlap between home ranges of adjacent herds. Observations during the study indicated that competitor species, including zebra and wildebeest, concentrated on the floodplain grasslands. Throughout the year H. dissoluta was the most strongly favoured grass species and contributed most to the diet of the sable herds in both the wet and dry season. During the dry season sable herds expanded their diet to include Aristida stipitata and Aristida meridionalis which are generally considered poor forage value species for cattle but that retained some greenness. Additionally, the contribution of browse, especially the leaves of Croton megalobotrys, Philenoptera nelsii and Combretum mossambicense and the flowers of Kigelia africana, constituted an important bridging resource during the extended dry season. Crude faecal protein levels remained above the suggested maintenance levels throughout the annual cycle. Crude faecal protein levels were elevated prior to calving when sable spent more time foraging on the floodplain grasslands where high value forage species such as Paspalum scrobiculatum, Panicum repens and Urochloa mossambicense and sedges were eaten. Indications are that the constraint posed by the distribution of water within the landscape, rather than resource limitations within occupied home ranges, are the primary limitation to population performance in the Kwedi concession.
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters in Science Centre for African Ecology, School of Plant Animal and Environmental Sciences, 2011.