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Among rodents and rhinos: Interplay between small mammals and large herbivores in a South African savanna

Among rodents and rhinos: Interplay between small mammals and large herbivores in a South African savanna
Nicole Hagenah

2006

Department of Environmental Sciences, Wageningen University, 6708 PB Wageningen, THE NETHERLANDS.

ABSTRACT

Mankind has caused species extinction of many groups of organisms through the transformation and fragmentation of once continuous natural habitats. In order to protect and restore natural biodiversity hotspots such as the African savannas we need to understand the determinants of their community structure and species diversity. Evidence has accumulated that body size differences that balance species facilitation and competition interactions as well as spatial heterogeneity may be key factors in the functioning of these ecosystems. This study explores the ecological consequence of the interplay between murid rodents and large herbivore species in a South African savanna and investigates the effect of abiotic factors on community interactions.

The herbivores studied showed strong interactions that were influenced by both fire and rainfall. Exclusion of large herbivores resulted in plant species composition changes and grass cover modifications, leading to higher rodent abundances. Middle-sized and large herbivore species responded to short-term postfire succession patterns with large species being the pioneers on recently burnt vegetation patches. Both middle-sized and large species affect murid rodents in the time of recolonisation burnt areas, mostly through increasing and prolonging their predation risk via grass cover changes. Additionally, the grass cover played a major role in the habitat selection and space use of grass-eating rodents, although the availability of high quality food resources was also important, especially with male and females having different priorities. Furthermore, the recruitment of the encroaching woody species Dichrostachys cinerea was not only positively (less competition for water and light with grasses) and negatively (higher risk of being trampled or desiccation) affected by large herbivores and rainfall but also influenced through predation by invertebrates and murid rodents. The findings of this study suggest that herbivore communities from very small (murid rodents) to very large (elephant) potentially interact in a South African savanna and that this interplay is of importance for the maintainance of the biodiversity in these ecosystems. Large herbivores influence murid rodents mainly through top-down effects (via grass cover modifications) rather than through competitive and facilitative interactions for food (bottom-up). Additionally, by changing the vegetation structure large herbivores may have an impact on the woody species recruitment, but to what extent this imposes a feedback on them remains unclear. Furthermore, the study supports the hypothesis that herbivores, in interaction with abiotic factors, strongly modify African savanna ecosystems. Under low rainfall conditions woody species recruitment is mostly limited by seed predators and browsers, while fire plays a bigger role under high rainfall conditions. However, fire seems to be one possible management tool to maintain spatial heterogeneity in savannas in order to sustain coexistence opportunities for different- sized herbivore species.