Browsing in a heterogeneous savanna

Large herbivores generally depend on and interact with a food resource that is heterogeneous at different spatial scales. Plants allocate resources to rapid growth or to defence mechanisms depending on the availability of res ources relative to loss of resources from herbivory. Herbivores select food and feeding habitats in order to maximize intake rate of nutrients and dig estible energy, while avoiding chemical and structural deterrents. To optim ize foraging, herbivores select habitats and food items in a hierarchical w ay, and different attracting and deterring factors may govern selection at different scales. We studied the impact of twig biting by a guild of indige nous browsers in three vegetation types in a semi-arid savanna in Botswana. The heaviest browsing pressure was in the vegetation type richest in prefe rred plant species, although that type was also richest in defended species . There were large differences in relative utilization between plant specie s, and ranking of species was roughly similar in the different vegetation t ypes. Browsing pressure varied between species from almost 0-30%. Overall, spinescent trees were less browsed than non-spinescent ones, and evergreen species were less browsed than deciduous ones. In two of the three vegetati on types there was a negative correlation between browsing pressure on a sp ecies and its frequency. there was a high incidence of rebrowsing, and once a tree had been browsed, the probability that it would be browsed again in creased. The results largely agree with predictions based on the resource a vailability hypothesis, the scarcity accessibility hypothesis and recent th eories on the significance of plant defences and on plant's response to bro wsing and the subsequent response by herbivores on the plant's responses.