Top-down effects of an herbivorous primate: Ecosystem processes mediated by geladas and domestic herbivores in the Ethiopian highlands
The gelada monkey (Theropithecus gelada) is the only gramnivorous primate in the world. As this once widespread terrestrial primate represents the majority of the native large herbivore biomass throughout the Ethiopian Highlands, an investigation into their impact on ecosystem processes is needed. Despite growing acceptance of the importance of biodiversity in regulating ecological, biogeochemical, and hydrological processes, developing strategies to protect and enhance ecosystem function remain hindered due to a lack of understanding of the mechanisms driving these processes. This challenge has historically been exacerbated due to the different scales (i.e. microhabitat, landscape, regional) at which these processes operate, and the difficulty of studying them in an integrated manner. In addition, despite the potential ecological value of regenerating degraded Afroalpine patches, few studies have measured differences in animal and plant communities across disturbance gradients in Ethiopia. Here, I conducted the first broad-scale census of T. geladathroughout their central range to evaluate their potential role as keystone species in the Afroalpine ecosystem. Direct count surveys of Geladas and domestic herbivores were conducted in 2014 and 2015 from Guassa Community Conservation Area in the North Shoa highlands, north across the Wollo highlands, to the Simien Mountains. I sampled random stratified 2 x 2 m vegetation plots chosen across 6 habitat types across all census regions. I obtained plant community diversity and structure, and soil and plant samples to assess water infiltration capacity and nutrient cycling. I also used high-resolution unmanned aerial vehicle (UAV) habitat mapping to extrapolate plot results and assess plant community heterogeneity.
Our results suggest that a positive relationship exists between abundance of T. gelada and plant community diversity, heterogeneity, and structural heterogeneity in afroalpine habitat patches. However, this relationship was strongly mediated by the abundance of domestic herbivores, which had a negative relationship with plant community diversity, heterogeneity, and structural heterogeneity. Our findings reinforce findings of the continued ecological degradation of the Ethiopian highland ecosystem and anecdotal differences in ecological structure and function between areas of varying faunal abundance and diversity. Such differences suggest a complex relationship between endemic grazer and plant communities, one that may hold key adaptive consequences for the ecosystem as a whole. The results of this study will be crucial for an updated conservation status assessment of this endemic species, and will provide much needed ecological information for the development of conservation and restoration strategies in the increasingly degraded Ethiopian highlands.