Possible causes of divergent population trends in sympatric African herbivores

Abstract

Sympatric herbivores experience similar environmental conditions but can vary in their population trends. Identifying factors causing these differences could assist conservation efforts aimed at maintaining fully functional ecosystems. From 1996–2013, tsessebe and wildebeest populations in the Okavango Delta, Botswana, declined by 73% and 90%, respectively, whereas zebra populations remained stable. These sympatric, medium sized herbivores are exposed to similar natural and anthropogenic pressures, but apparently differ in their responses to those pressures. To identify factors that could cause these differences, we fitted GPS-enabled collars to six zebra, eight tsessebe and seven wildebeest in the Moremi Game Reserve, Botswana. We calculated utilisation distributions (UDs) from GPS data, and used 95% isopleths to compare seasonal home range size between species. We calculated utilisation intensity (UI) from the UDs and generated spatial layers representing resources and disturbances, and then used model averaging to identify factors affecting UI for each species. We calculated second and third order habitat selection ratios to determine whether species were habitat specialists or generalists. Zebra occupied larger home ranges than tsessebe and wildebeest, showed weaker responses to spatial variables and displayed no third order habitat selection; zebra social systems are also more fluid, allowing for information exchange between stable harems. Herbivore species that are sedentary, occupy small home ranges, are habitat specialists and exist in relatively isolated groups are likely to be less resistant and resilient to the rapid pace of environmental change forecast by climate change scenarios. Resources contained within existing protected areas are unlikely to maintain populations of such species at sufficiently high levels, potentially leading to functional extinction. Special precautions may be needed to ensure that such species can persist in the wild, such as buffer zones around existing protected areas, which would allow greater potential for adaptive movement should current environmental conditions change.

Predicting wildlife corridors for multiple species in an East African ungulate community

“…Conceptually, space use of habitat generalists may best represent movement of a range of other species. Indeed, space use of zebra usually shows little response to spatial variables, which is indicative of a habitat generalist [ 96 ].…”

Genetic patterns in three South African specialist antelope species: Threats, conservation management and their implications

“…However, artificial water points can adversely affect certain species such as the tsessebe, as well as the utilization of the surrounding habitat. The presence of competitive species was correlated with artificial waterpoints in various studies, with waterpoints attracting generalist water-dependent species such as zebra and larger browser species like elephants, which may lead to degradation of essential grass for specialist grazers (Bennit et al, 2019;Muposhi et al, 2017). This is especially true during the dry season when surface water is limited and these waterpoints are the most accessible to all species, with specialist species becoming most susceptible to extirpations during these dry months (Bennit et al, 2019;Chirima et al, 2018).…”

“…All three species are habitat specialists. The oribi and tsessebe thrive in flat grassland, although both tree and bush cover in these habitats may vary (Bennit et al, 2019; Brashares & Arcese, 2013). The blue duiker is an obligate browser that prefers forest habitats with high wood cover and simultaneous high canopy and floor cover and has been shown to avoid habitats that lack one of these (Ehlers‐Smith et al, 2017; Gagnon & Chew, 2000).…”

“…Multiple species are often managed together in confined areas, chiefly due to limited available land and the need to diversify products to maximize profit (Child et al, 2013; Lindsey et al, 2007). Generalist herbivorous species, such as wildebeest ( Connochaetes ), buffalo ( Syncerus caffer ) and zebra ( Equus ), have been shown to have strong competitive effects on the populations of tsessebe, as these generalist species have wider home range preferences and greater adaptability (Bennit et al, 2019; Child et al, 2013; Grant et al, 2002; Pacifici et al, 2015). These generalist species are coincidentally preferred by wildlife farmers and as such are introduced across a majority of private wildlife land (Taylor et al, 2016).…”

Water-Level Fluctuations and Ungulate Community Dynamics in Central Uganda

“…Competition for resources is greatest when resources are low, leading to competitive exclusion [29]. Habitat specialist animals, such as sitatunga, face greater risk of extinction due to climate change compared to habitat generalists such as waterbuck, even when subjected to the same conditions in the same ecosystem [30].…”