Impacts of large herbivores on savanna plant communities: Predictive models of herbivore selectivity and plant response

Abstract

Large herbivores are among the most ecologically influential and extinction‐prone animals. Megaherbivores, in particular, radically alter vegetation. Yet, few studies have tried to predict the impacts of herbivores—or their extinction—on plant species composition and community structure. First principles suggest that preferred food plants should be most strongly suppressed by herbivores and released by herbivore loss, but this intuition may be misleading if plant responses are strongly contingent on functional traits and/or plant–plant interactions. We sought to predict the responses of plant species to size‐selective herbivore‐exclusion treatments in an African savanna, using data on herbivore diets and plant functional traits. Our analysis had three stages. First, we identified plant traits that predicted selectivity (use relative to availability) by the dominant herbivore species excluded by each experimental treatment: megaherbivores (elephant, giraffe; ≥1000 kg), mesoherbivores (buffalo, zebra, impala; 40–600 kg), and dik‐dik (5 kg). Several plant traits predicted selectivity across multiple herbivore species, but each species' diet was best predicted by a distinctive suite of traits. Second, we tested whether herbivore selectivity alone predicted plant responses. Elephant selectivity uniquely predicted plant responses in exclosures relative to unfenced control plots ( R 2 = 0.24–0.30); plant taxa strongly favored by elephants were ninefold more abundant inside exclosures. However, herbivore selectivity failed to predict differences in community structure between the different fenced exclusion treatments, suggesting that bottom‐up effects of competition may intensify relative to consumptive effects as smaller herbivores are removed. Third, we show that including plant traits as covariates alongside elephant selectivity modestly improved predictability ( R 2 = 0.27–0.50). Despite various sources of uncertainty and imprecision inherent in our approach, we show that elephant foraging decisions are a major determinant of plant community dynamics. Our findings indicate that models based on readily attainable data can substantially predict plant community responses to the loss or reintroduction of megafauna. Future work can refine our approach by incorporating additional plant traits associated with tolerance and competition, along with mechanistic measurements of herbivore preferences and biomass consumption, to predict even more accurately how large‐herbivore population declines and extinctions will impact plant communities.

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