Herbivory on woody plants and induced responses in two similar species of Acacia in the Kruger National Park, South Africa

Abstract

I studied browsing patterns and the effects of browsing by different groups of herbivores on woody plants in Kruger National Park in South Africa. I determined how the browsing pressure varied with distance to water and measured inducible responses on two species of Acacia (A. exuvialis and A. grandicornuta). The experimental approach involved excluding herbivores by the use of fences. Three treatments were used; a full exclosure (FE, excluding all mammals larger than hares), a partial exclosure (PE, excluding elephants and giraffes) and a control area (NE, no exclusion). Two vegetation types were distinguished, related to the crest (further from the river) and the footslope (closer to the river) within the study site. Thirty transects were randomly placed in each treatment (15 on each vegetation type) and the woody plants in each transect were counted and measured. Spines, leaves and shoots were measured on one selected Acacia individual in each transect. In NE and PE browsing severity was estimated on all individuals < 150 cm. Results show that browsing was greater on footslope than on crest, and greater where all herbivores had access to the plants compared to where elephants and giraffes were excluded. Effects of elephant exclusion were greater on crest than on footslope. Some evidence of inducibility of plant responses was found. Spines were 39 % shorter in FE compared to NE for A. exuvialis, but leaf lengths seemed to increase with exclusion of herbivores. A strong positive correlation was found between shoot length and spine abundance, but because of heavy rain near the end of fieldwork, shoot lengths and spine abundances were greatest in PE. The effects of elephants on the woody plants of the savanna ecosystem demonstrated in this study are alarming when considering the rapidly growing populations of elephants and the potential damage they can cause. The greater effect of elephant exclusion I found on the crest is a result which would be interesting to incorporate into elephant management. Long spines and short leaves make plants less attractive to mammalian herbivores and this is very important to consider in the widespread and growing utilization of savanna vegetation as food for livestock, because if the sizes of elephant populations continue to increase, the competition between wildlife and livestock for the savanna ecosystem will do the same