Change in Mesoherbivore Browsing Is Mediated by Elephant and Hillslope Position
June
Change in Mesoherbivore Browsing Is Mediated by Elephant and Hillslope Position
D. D. Georgette Lagendijk 0 1 2
Maria Thaker 0 1 2
Willem F. de Boer 0 1 2
Bruce R. Page 0 1 2
Herbert H. T. Prins 0 1 2
Rob Slotow 0 1 2
0 1 School of Life Sciences, University of KwaZulu-Natal , Westville Campus, Private Bag, X54001, Durban, 4000, South Africa , 2 Centre for Ecological Sciences, Indian Institute of Science , Bangalore, 560012 , India , 3 Resource Ecology Group, Wageningen University , Droevendaalsesteeg 3a, 6708 PB, Wageningen , The Netherlands , 4 Department of Genetics , Evolution and Environment , University College , London , United Kingdom
1 Funding: This research was financed by the Amarula Elephant Research Programme, University of KwaZulu-Natal, and the South African National Research Foundation , Grant FA2006032300024 to RS
2 Academic Editor: Jin Chen, Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences , CHINA
Elephant are considered major drivers of ecosystems, but their effects within small-scale landscape features and on other herbivores still remain unclear. Elephant impact on vegetation has been widely studied in areas where elephant have been present for many years. We therefore examined the combined effect of short-term elephant presence (< 4 years) and hillslope position on tree species assemblages, resource availability, browsing intensity and soil properties. Short-term elephant presence did not affect woody species assemblages, but did affect height distribution, with greater sapling densities in elephant access areas. Overall tree and stem densities were also not affected by elephant. By contrast, slope position affected woody species assemblages, but not height distributions and densities. Variation in species assemblages was statistically best explained by levels of total cations, Zinc, sand and clay. Although elephant and mesoherbivore browsing intensities were unaffected by slope position, we found lower mesoherbivore browsing intensity on crests with high elephant browsing intensity. Thus, elephant appear to indirectly facilitate the survival of saplings, via the displacement of mesoherbivores, providing a window of opportunity for saplings to grow into taller trees. In the short-term, effects of elephant can be minor and in the opposite direction of expectation. In addition, such behavioural displacement promotes recruitment of saplings into larger height classes. The interaction between slope position and elephant effect found here is in contrast with other studies, and illustrates the importance of examining ecosystem complexity as a function of variation in species presence and topography. The absence of a direct effect of elephant on vegetation, but the presence of an effect on mesoherbivore browsing, is relevant for conservation areas especially where both herbivore groups are actively managed.
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Competing Interests: Co-author Rob Slotow is a
PLOS ONE Editorial Board member. This does not
The African savanna biome, characterised by a high degree of horizontal and vertical spatial
heterogeneity, harbours one of the most diverse assemblages of large herbivores [1,2]. As
would be expected, species use the landscape differently depending on their specific nutritional
requirements and the spatial heterogeneity of the available resources [3]. Many complex factors
determine herbivore foraging behaviour and the consequent use of the landscape, including
the quality, availability, spatial distribution of resources, and specific nutritional requirements
[3,4], as well as predation risk [5] and competition from other herbivores [6]. Therefore, the
impact of herbivores on vegetation is spatially heterogeneous across the savanna landscape
[7,8].
In savanna ecosystems, much of the spatial heterogeneity of herbivory is understood as a
function of the distance to major water sources, such as rivers and waterholes, at the landscape
level [9,10]. However, ecohydrological conditions can vary at much smaller spatial scales, such
as within hillslopes.
During the wet season, water and sediments are also transported downslope through runoff,
where they are captured by the vegetation in lower areas (run-on) and partially stored in the
soil [11,12]. This runoff also prolongs the growing seasons in low-lying areas, resulting in a
different composition of plant species along slopes [13]. The nutrients that thus accumulate at
lower slope positions result in a high quality soil resource base for plant regeneration and
growth [11,12]. Consequently, plants at lower slope positions tend to be of a higher nutritional
quality and more palatable than plants on crests [14]. The effect of slope position (e.g.,
footslopes vs. crests) on the foraging behaviour of grazers has been well studied [14,15]. For
example, Macandza et al. [15] found that African buffalo Syncerus caffer grazing on granite
landscapes in Kruger National Park, South Africa, (...truncated)