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

Large herbivores are among the most ecologically influential and
extinction-prone animals. Megaherbivores in particular radically alter
vegetation. Few studies have tried to predict the impacts of herbivores—or
their loss—on plant species composition and community structure. First
principles suggest that preferred plants should be suppressed by
herbivores and released by herbivore removal, but this prediction may be
misleading if responses are strongly contingent on plant traits and
plant–plant interactions. We sought to predict responses of plant species
to size-selective herbivore exclusion in an African savanna, using data on
herbivore diets (from DNA metabarcoding) 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 different experimental treatments:
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 species’
diets were predicted by unique suites 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 (R2 = 0.24–0.30); taxa strongly favored by
elephants were ninefold more abundant inside exclosures. However,
herbivore selection failed to predict differences between fenced exclusion
treatments, suggesting that bottom-up effects of plant competition
intensify relative to consumptive effects as large-bodied herbivores are
removed. Third, including plant traits as covariates along with elephant
selectivity modestly improved predictability (R2 = 0.27–0.50). Despite
various sources of uncertainty and imprecision inherent to our approach,
including inability to distinguish selection for different plant
parts/stages, we show that elephant foraging decisions are a primary
determinant of plant community dynamics. Moreover, our findings indicate
that models based on readily attainable data can substantially predict
plant community responses to the extirpation or reintroduction of
megafauna. Future work can refine our approach by incorporating additional
traits associated with plant tolerance and competition, along with more
granular and mechanistic measurements of herbivore preferences and biomass
consumption, to predict even more accurately how large herbivore
population declines and extinctions will impact plant communities.