Fire ecology and spatial heterogeneity in an East African savanna: The importance of grazer body size

The high diversity of larger grazers in savanna systems has been attributed to the inherent spatial and temporal heterogeneity related to forage quantity and quality. Specifically, allometric scaling laws suggest that smaller animals are constrained to feed on rarer, higher quality items, while large species can tolerate poor quality food which is more abundant. Recent theoretical work further suggests that smaller bodied species are able to perceive heterogeneity in grass quality at finer scales, suggesting that the spatial attributes of forage may also promotes coexistence among grazers. We used replicated prescribed burns to modify plant quality and quantity in a Kenyan savanna where an entire guild of grazers is present, varying in body size from hare to elephant. We varied the extent (1, 9, and 81 ha) and grain (continuous and patchy) of burns in order to test whether the spatial attributes of burned patches affected grazer use. Near infrared reflectance spectroscopy (NIRS) was used to predict forage quality and plant cover was estimated to address whether grazer selectivity altered plant community composition. We found a strong negative relationship between preference for burn sites and body mass, with smaller grazers preferentially selecting burned areas while the largest species avoided burns. Plots with higher quality forage were preferred more than lower quality plots, but the magnitude varied by grazer body size. As predicted by scaling theory, smaller bodied animals had higher dung densities in the smallest patch sizes of highest quality, while the larger species either had uniform dung distributions or, as for the elephant, had highest dung densities in the largest patches of low quality. Themeda triandra was the only grass species to regain its original biomass in burned sites by the end of the experiment. The percentage of T. triandra, as a component of total forage abundance, increased with burning and was positively correlated with total dung densities. These results represent the fruitfulness of combining insights from fire ecology, spatial ecology, and ecophysiology toward the goal of understanding the processes that maintain larger herbivore diversity in savanna systems.