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

1) 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. 2) Synthesis of vegetation responses to
herbivore exclusion shows that herbivory decreased grass abundance by
57.0% and tree abundance by 30.6% across African savannas. 3) 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. 4) 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. 5) 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. 6) 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. 7) 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. --