Environmental context shapes the relationship between grass consumption and body size in African herbivore communities

Though herbivore grass dependence has been shown to increase with body
size across herbivore species, it is unclear whether this relationship
holds at the community level. Here we evaluate whether grass consumption
scales positively with body size within African large mammalian herbivore
communities and how this relationship varies with environmental context.
We used stable carbon isotope and community occurrence data to investigate
how grass dependence scales with body size within 23 savanna herbivore
communities throughout eastern and central Africa. We found that dietary
grass fraction increased with body size for the majority of herbivore
communities considered, especially when complete community data were
available. However, the slope of this relationship varied, and rainfall
seasonality and elephant presence were key drivers of the variation—grass
dependence increased less strongly with body size where rainfall was more
seasonal and where elephants were present. We found also that the
dependence of the herbivore community as a whole on grass peaked at
intermediate woody cover. Intraspecific diet variation contributed to
these community-level patterns: common hippopotamus (Hippopotamus
amphibius) and giraffe (Giraffa camelopardalis) ate less grass where
rainfall was more seasonal, whereas Cape buffalo (Syncerus caffer) and
savanna elephant (Loxodonta africana) grass consumption were parabolically
related to woody cover. Our results indicate that general rules appear to
govern herbivore community assembly, though some aspects of herbivore
foraging behavior depend upon local environmental context.