The past, present, and future of herbivore impacts on savanna vegetation

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Carla Staver and Allison T Karp, Ecology and Evolutionary Biology, Yale University, New Haven, CT, Joel O. Abraham, Dept. of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, Gareth Hempson, Centre for African Ecology, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa, J. Tyler Faith, Natural History Museum of Utah, University of Utah, Salt Lake, UT, J. Tyler Faith, Anthropology, University of Utah, Salt Lake, UT

Background/Question/Methods
Herbivory is a key process structuring vegetation in savannas, especially in Africa where large mammal herbivore communities remain intact. Exclusion experiments consistently show that herbivores impact savanna vegetation, but effect size variation has resisted explanation, limiting our understanding of the past, present, and future roles of herbivory in savanna ecosystems. We synthesized experimental responses to herbivore exclusion across African savannas, evaluated response magnitudes in relation to variation in environment and in herbivore densities, and used results to evaluate the possible past, present and future impacts of herbivores on savannas.
Results/Conclusions
Herbivory decreased grass abundance by 57% and tree abundance by 31% across African savannas. The magnitude of herbivore exclusion effects scaled with herbivore abundance: more grazing herbivores resulted in larger grass responses and more browsing herbivores in larger tree responses. However, existing experiments are concentrated in semi-arid savannas (400-800 mm rainfall) and soils data are mostly lacking, which makes disentangling environmental constraints a challenge and priority for future research. Observed herbivore impacts were ~ 2.1× larger than existing estimates modeled based on consumption. Wildlife metabolic rates may be higher than are usually used for estimating consumption, which offers one clear avenue for reconciling estimated herbivore consumption with observed herbivore impacts. Plant-soil feedbacks, plant community composition, and the phenological or demographic timing of herbivory may also influence vegetation productivity, thereby magnifying herbivore impacts.
Because herbivore abundance so closely predicts vegetation impact, changes in herbivore abundance through time are likely predictive of the past and future of their impacts. Grazer diversity in Africa has declined from its peak 1 million years ago and wild grazer abundance has declined historically, suggesting that grazing likely had larger impacts in the past than it does today. Current wildlife impacts are dominated by small-bodied mixed feeders, which will likely continue into the future, but the magnitude of top-down control may also depend on changing climate, fire, and atmospheric CO2.
Synthesis. Herbivore biomass determines the magnitude of their impacts on savanna vegetation, with effect sizes based on direct observation that potentially outstrip existing modeled estimates across African savannas. Findings suggest substantial ecosystem impacts of herbivory and allow us to generate evidence-based hypotheses of the past and future impacts of herbivores on savanna vegetation.