Balanced Functional Herbivore Composition Stabilizes Tree-Grass Coexistence and Productivity in a Simulated Savanna Rangeland Ecosystem

Abstract

Savanna ecosystems provide resources supporting the livelihoods of millions of people globally. Ongoing exploitation through high livestock densities, among other factors, however, threatens these ecosystems. Shrub encroachment, a consequence of this exploitation, causes severe declines in fodder biomass for cattle. For example, in Namibia, this leads to losses of land productivity of up to 100%. Consequently, we must rethink land-use options to ensure sustainable utilization of savanna resources. In southern Africa, many landowners and custodians have switched to or included wildlife in their management systems, making it an excellent model region to study how changing herbivory affects ecosystems. The diversity of indigenous wildlife species is expected to promote efficient herbaceous layer utilization with browsing herbivores counteracting nondesirable woody plant encroachment. Nevertheless, surprisingly little research has been conducted on the ecological consequences of altering grazer-to-browser ratios.We developed a large-scale spatially explicit dynamic vegetation model including key hydrological and ecological processes, fire, and an herbivore algorithm that accounts for aggregated animal herds in a heterogeneous savanna landscape. We find that different ratios of wild herbivore feeding types greatly influence vegetation composition and dynamics. By dynamically computing plant biomass as a resource, our model estimates the occurrence of fodder biomass shortages, providing a valuable tool for land users. The model also reports the number of patches with low to medium tree functional cover (10–25%), indicating structural diversity with its implications for species richness, and is thus essential for nature conservation. We find that a herbivore species composition of approximately 40% browsers and 60% grazers is beneficial for plant structural and species diversity. Theoretically, therefore, the browse-graze ratio achieved through wildlife-based land-use can lead to more sustainability and resilience. The interplay between diverse herbivory, vegetation, rain, and fire complicates but also stabilizes the savanna rangeland ecosystem and may contribute to its long-term persistence.