Spatial refuges buffer landscapes against homogenisation and degradation by large herbivore populations and facilitate vegetation heterogeneity

Abstract

Environmental heterogeneity across savanna landscapes, including different seasonal resources at different distances to water, may play a critical role in maintaining the size and diversity of wildlife populations and the sustainability of their resource base. We investigated whether extensive landscapes with functionally diverse seasonal resources and large waterless regions can mediate the effect of herbivory on plant composition, structure and diversity. Vegetation composition, structure and richness in two different vegetation types (mopane and sandveld woodland) at three distance zones (0 km – 5 km, 10 km – 15 km and > 20 km) from the permanent water of the Okavango Delta and Linyanti Swamps were surveyed. We modelled vegetation response of the most abundant species to herbivory in relation to distance from permanent water, and included fire frequency as a covariate. Trees favoured by elephants during the dry season occurred typically as immature, pollarded populations within 5 km of permanent water sources while mature tall populations of these species were found far from water (> 10 km – 15 km). Similarly, short high-quality grazing grasses were higher in abundance within 5 km of permanent water, whereas taller high-quality perennial grasses peaked in abundance beyond 20 km from permanent water. Trends in herbaceous richness with distance from water were contingent upon vegetation type, while tree richness did not change with distance from water. Spatial refuges in waterless regions of landscapes facilitate the creation of heterogeneity of vegetation structure, composition and richness by large herds of mammalian herbivores. Therefore, the extension of herbivore dry season foraging range, for example, by the creation of artificial water points (AWP) in backcountry woodlands, could seriously undermine the resilience of landscapes to herbivory by reducing the availability of spatial refuges. Consequently, it reduces the resilience of herbivore and predator populations that depend on these spatial refuges. We strongly advise that future scientific work, and management and policy actions should be focused on the identification and sustaining of these spatial refuges in wildlife areas.Conservation implications: Management and policy actions should be focused on the identification and sustainability of spatial refuges in wildlife areas. Too many AWP in backcountry woodlands could undermine the resilience of landscapes to herbivory by reducing the proportion of landscapes beyond 15 km from permanent water.

Movement Patterns of African Elephants (Loxodonta africana) in a Semi-arid Savanna Suggest That They Have Information on the Location of Dispersed Water Sources

“…Although water is limiting for both sexes in the dry season, female elephants rarely moved further than 10 km from the nearest water source to forage in the dry season while male elephants accessed forage sites beyond 15 km (Sianga et al, 2017). Furthermore, in the dry season, the directionality of movement path for male elephants was much stronger than the female elephants.…”

“…For instance, in drier environments, elephants take an average interval of 3 days to drink water and the duration of re-visiting water points differ between sexes (Stokke and Du Toit, 2002), with bull elephants drinking every 3-5 days while breeding herds every 2-4 days (Viljoen, 1989;Leggett, 2006). Furthermore, the breeding herds have been reported to forage close to water sources in the dry season compared to the male elephants (Harris et al, 2008;Sianga et al, 2017).…”

“…Based on the elephant's water requirements and the scarcity of water during the dry season, we expect that the movement pattern of the elephant will reflect these seasonal contrasts in water distribution. Although it is not in doubt that the distance to water is a primary environmental factor influencing habitat use by elephants (Verlinden and Gavor, 1998;Chamaillé-Jammes et al, 2007;Smit et al, 2007;Hilbers et al, 2015;Wato et al, 2016;Sianga et al, 2017), it remains unclear how the behavioral responses of elephants change as a result of water scarcity (Polansky et al, 2015). Here, we analyse the movement paths of four male and four female elephants to address the hypothesis that elephant movement patterns show a stronger directional orientation toward water sources in the dry season compared to the wet season.…”

Water and cattle shape habitat selection by wild herbivores at the edge of a protected area

“…Their movements are limited by the location of their kraal. They travel on their own further away from the boundary and may be seeking higher quality grasses (Sianga et al ., ). This suggests that intraspecific competition (Young, Palmer & Gadd, ; Odadi et al ., ) is stronger than the risk of encountering predators (Kuiper et al ., ) or than competition with wild herbivores.…”

“…The difference between elephant and buffalo could result from competitive exclusion of buffalo by cattle (both species being grazers), whereas elephant bulls have access to a broader niche being mixed feeders (Kartzinel et al ., ). Indeed, buffalo are bulk feeders and prefer tall grass species that are often found far away from water (Sinclair, ; Sianga et al ., ). Buffalo may avoid areas used by cattle due to the short term effect of reduced grass height or the long term disappearance of soft‐leaved and nutritious grass species that are preferred by buffalo due to heavy grazing by livestock.…”