Do elephants eat more trees when less grass is available? A field study in Kruger National Park, South Africa.

Although African bush elephants are often responsible for the majority of herbivore-driven savanna tree mortality, confusion remains regarding the factors that influence their diet. Some elephants either browse or graze almost exclusively, while others balance the two, and the determinants of this variation remain poorly understood. I seek to determine whether grass availability controls the proportion of woody plants in elephant diet and, therefore, the amount of damage elephants do to trees while foraging. Preliminary analysis using already-available data on grass biomass, elephant density, and elephant damage to trees suggest that tree damage is in fact negatively correlated with grass availability. However, there is a negative correlation between grass biomass and elephant density and a positive one between elephant density and tree damage, which may be sufficient to explain this pattern. This finding, while interesting, does not therefore answer the question of the direct effect of grass availability on tree damage. To resolve this issue, I intend to use my own data to separate the effect of population density from that of grass availability. I gathered data in Kruger National Park, South Africa, on elephant diet composition (by taking dung samples that I will analyze for carbon isotope ratios) and grass biomass (using a disk pasture meter). I will perform the required isotope analyses and evaluate the combination of datasets over the course of this fall.