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

Sympatric large mammalian herbivore species differ in diet composition,
both by eating different parts of the same plant and by eating different
plant species. Various theories proposed to explain these differences are
not mutually exclusive, but are difficult to reconcile and confront with
data. Moreover, whereas several of these ideas were originally developed
with reference to within-plant partitioning (i.e., consumption of
different tissues), they may analogously apply to partitioning of plant
species; this possibility has received little attention. 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 and analyzed diet composition in light of 27 functional
traits that we measured locally for 204 plant species. 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, zinc, stem-specific density, and potassium
(and lower in sodium, stem dry matter content, and copper). These results
are consistent with longstanding hypotheses linking body size with forage
quality and plant height, yet they also suggest the existence of
undiscovered links between herbivore body size and a set of rarely
considered food-plant traits. We also tested the novel 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 the
LES was a minor axis of individual variation within a few species, but had
little effect on interspecific dietary differentiation. Synthesis. These
results identify key plant traits that underpin the partitioning of
food-plant species 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.