Mechanisms of dietary resource partitioning in large-herbivore assemblages: A plant-trait-based approach

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Arjun B. Potter, Matthew C. Hutchinson, Jonathan M. Levine and Robert M. Pringle, Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ, Johan Pansu, CNRS, Station Biologique de Roscoff, Roscoff, France, Bart Wursten, Meise Botanic Garden, Meise, Belgium, Ryan A. Long, Department of Fish and Wildlife Services, University of Idaho

Background/Question/Methods
Sympatric large mammalian herbivore species consistently differ in diet composition. Various theories proposed to explain these differences are not mutually exclusive, but are nonetheless difficult to reconcile and confront with data. Plant functional traits provide a novel window into herbivore diets and a means of testing multiple hypotheses in a unified framework. We used DNA metabarcoding to characterize the diets of 14 sympatric large-herbivore species in an African savanna in central Mozambique and analyzed diet composition in light of 27 functional traits that we measured locally for 204 plant species.
Results/Conclusions
Plant traits associated with the deep phylogenetic split between grasses and eudicots formed the primary axis of resource partitioning, affirming the generality and importance of the grazer-browser spectrum. A secondary axis comprised plant traits relevant to herbivore body size. Plant taxa in the diets of large-bodied species were lower on average in digestible energy and protein, taller on average (especially among grazers), and tended to be higher in tensile strength, stem-specific density, zinc, and potassium (and lower in sodium, copper, and stem dry matter content). These results are consistent with longstanding theories linking body size with forage quality and plant height, yet they also suggest the existence of undiscovered links between herbivore body size and a subset of rarely considered food-plant traits. For the first time, we tested the hypothesis that the leaf economic spectrum (LES), a major focus in plant ecology, is an axis of resource partitioning in large-herbivore assemblages; we found that although the LES was a minor axis of individual variation within a few species, it had little effect on interspecific dietary differentiation. These results identify key plant traits that underpin dietary niche partitioning in large-herbivore communities and suggest that accounting for multiple plant traits (and tradeoffs among them) will enable a deeper understanding of herbivore-plant interaction networks.