Research Article: Possible causes of divergent population trends in sympatric African herbivores

Date Published: March 12, 2019

Publisher: Public Library of Science

Author(s): Emily Bennitt, Tatjana Y. Hubel, Hattie L. A. Bartlam-Brooks, Alan M. Wilson, Bi-Song Yue.


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.

Partial Text

Wildlife population trends fluctuate spatially and temporally with natural environmental variation, such that the same species in different environments can have divergent population growth patterns [1,2], as can sympatric species within one environment [3]. Globally, mammalian herbivore populations are declining in response to a variety of factors [4], mainly linked to anthropogenic impacts such as hunting [5], land-use changes [6], habitat loss and fragmentation [7], barriers to movement [8,9], and climate change [10]. These factors often create sudden environmental changes [11] that can affect behaviour [12], reproductive success [13] and gene flow between populations [14], ultimately impacting individual fitness and population dynamics [15].

Not all animals were collared for complete seasons (Table 1), and animals collared for less than a month within a season were discarded from analyses, so sample sizes varied with analysis methods.

Sympatric herbivores have access to similar resources and are adapted to co-exist within their environment [44]. Zebra, tsessebe and wildebeest have similar body masses, but differ in their life history traits, with different digestive systems, social organisation, mouth morphology, and resource requirements. In the Okavango Delta, zebra home ranges were larger than those of tsessebe and wildebeest, and the first two species had larger home ranges during the rainy season. None of the species showed particular avoidance for indicators of anthropogenic activity, such as proximity to roads or villages, and water availability seemed to be the main driver of herbivore distribution. All species displayed second order habitat selection, but, whereas tsessebe and wildebeest also displayed third order habitat selection, zebra used the habitats within their home ranges in proportion to their availability. Wildebeest showed a stronger preference for floodplain habitats than the other two species. When selection was significant, most woodland habitats were avoided by all species, although zebra showed some preference for riparian woodland, especially during the rainy season.

Larger home ranges sizes, lack of third order selection and a propensity to form larger, more fluid groups suggest that zebra are more adaptable and better able to track changing environmental conditions than tsessebe or wildebeest. The natural, stochastic environmental changes in flood levels in the Okavango Delta could have a detrimental impact on sedentary species such as tsessebe and wildebeest, especially during prolonged flooding periods lasting several years. In this ecosystem, collared tsessebe and wildebeest moved seasonally and changed their patterns of habitat selection, but they may have been constrained by their lower capacity for adaptive movement in response to changing conditions when compared to zebra. Climate change scenarios predict higher levels of environmental fluctuation, particularly in combination with proposed anthropogenic developments upstream of the Delta [73], so populations of sedentary herbivores may continue to decline. The floodplains of the Okavango Delta currently extend far beyond the boundaries of the protected and wildlife management areas, so expanding the protected areas, particularly during extended periods of high flooding, could provide more extensive areas for colonisation by vulnerable herbivore species. Buffer zones extending beyond protected areas could be established with flexible land use strategies, such that during periods of high flood, they could be used as additional eco-tourism areas, and during periods of low flood, they could be developed as cultural tourism destinations.




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