Potential ecological factors contributing to springbok (Antidorcas marsupialis) population declines in the southern Kalahari, Botswana

Abstract

The springbok (Antidorcas marsupialis) population in the southern Kalahari, Botswana has declined precipitously in recent years, but the primary causes of this decline are not known. Springbok play an important role in the ecosystem, so understanding the possible causes of their decline is essential for conservation of the species and ecosystem management. This study identifies possible factors that could contribute to the declining springbok population in the southern Kalahari. Most of the remaining springbok inhabit the Schwelle area of the southern Kalahari, with a small proportion of that population in Mabuasehube. The study was conducted in both locations. Herd composition counts and demographic ratios were used to quantify seasonal changes in recruitment rates by comparing the ratios of each demographic category to adult females. The effects of season and location on herd size, herd composition and demographic ratios were assessed. Larger herds were recorded during the wet than the hot and cold dry seasons. The effects of season and location were significant for herd composition and demographic ratios. The reduction in the juvenile: adult female ratio recorded in this study from 0.37 during the wet season months to 0.10 during the cold dry season suggests a low recruitment rate that could be contributing to low springbok numbers. A high ratio of juvenile: adult female was recorded in the Schwelle, so the results highlight the importance of protecting this region as a wet season calving range. We sampled vegetation characteristics in sites used by and available to springbok to determine forage selection criteria. Springbok used sites with diverse, short grass species in all seasons. Short, diverse shrubs characterised springbok browsing sites. Highly selective species like springbok may experience nutritional stress when forage is scarce and this could compromise reproductive success and survival. 4 Seasonal changes in resource availability determine the location of home ranges, daily movements and habitat selection; these were studied using data from GPS collars that were deployed on five herds of springbok. Home range sizes were larger during the wet than the hot and cold dry seasons. Springbok covered longer distances during the wet than the hot dry season. Movement may have been restricted by high temperatures during the hot dry season, among other factors. High quality resources and reduced predation risk associated with pans were probably important determinants of springbok habitat use. These results highlighted the importance of protecting habitats like pans, which offer critical resources, for conservation of springbok and the ecosystem. The decline in springbok numbers could be caused by low recruitment rate. A lack of rain could lead to low forage quality especially during the late months of pregnancy and during lactation when the nutritional demands for females are high. This could lead to low recruitment of juveniles. More studies on recruitment rates over a longer time period using marked individuals should be conducted. Movement between key habitats should be maintained to facilitate migration between wet and dry season ranges. A declining springbok population could have cascading impacts on the resident large predators, with possible ecosystem wide consequences. Predicted impacts of climate change in Botswana, including higher temperatures and lower rainfall, could exacerbate these problems and cause further declines in arid-adapted antelopes such as springbok.