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

Abstract

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 modelled 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 CO 2 . 7. Synthesis. Herbivore biomass determines the magnitude of their impacts on savanna vegetation, with effect sizes based on direct observation that outstrip | 2805

Seasonal strategies differ between tropical and extratropical herbivores

“…By contrast, in the tropics, where the distribution of these behaviours is more restricted, herbivores may be less able to respond to changing resource distributions (Abraham et al, 2019;Cardillo et al, 2005;Ripple et al, 2015). These results add to a growing body of literature emphasizing the precarious position of large-bodied tropical herbivores in a human-dominated future (Abraham et al, 2019;Cardillo et al, 2005;Estes et al, 2011;Ripple et al, 2015;Staver et al, 2021).…”

“…Here we find that comparatively fewer tropical herbivores possess strategies for coping with variable resource availability (see also Abraham et al., 2019), which puts tropical herbivores lacking such strategies at risk of decline. Widespread population declines that have already been documented among tropical herbivores (Ripple et al., 2015) are therefore likely to continue, with cascading effects on vegetation as plants are freed from widespread herbivory pressure (Estes et al., 2011; Staver et al., 2021). All in all, these results suggest diverging trajectories for tropical and extratropical herbivores in the face of global change.…”

“…All data compiled for this study are published alongside the paper in Supporting Information and are also available from the Dryad Digital Repository https://doi.org/10.5061/dryad.31zcr jdnb (Abraham et al, 2021).…”

“…Herbivore population sizes determine the magnitude of their ecological impacts, their own conservation status and their sensitivity to global change (Cardillo et al., 2005; Estes et al., 2011; Ripple et al., 2015; Staver et al., 2021). Two behaviours play an outsized role in allowing mammalian herbivores to achieve large populations by enabling them to circumvent seasonal resource bottlenecks (Illius & O'Connor, 2000): seasonal migration and seasonal dietary shifts (Staver & Hempson, 2020).…”

Qualitative Modeling for Bridging Expert-Knowledge and Social-Ecological Dynamics of an East African Savanna

“…The last two scenarios are related to the presence of herbivore functional groups. Indeed, herbivores are known as a major driver of vegetation change, with grazers and browsers having contrasting effects on vegetation dynamics [72]. Therefore, we defined the "no grazers" and "no browsers" scenarios to simulate the effect of each group's local extinction.…”