An index for assessing effectiveness of plant structural defences against mammal browsing

Plant structural defences against mammals play an important role in ecosystem functioning as they simultaneously mediate the fitness of both animals and plants. The efficiency of structural defences can be described by the amount of plant material an animal can remove in one bite. Quantifying bite size by direct observation is difficult requiring controlled access to both the plants of interest and the herbivores. A method called the bite size index (BSI) has been proposed using human bites to simulate the bite size of medium-sized mammalian herbivores. In this paper, we evaluated the utility of the BSI for assessing the efficacy of plant structural defences. We analysed the intra- and inter-specific variability of the BSI in an African savanna (Hluhluwe–iMfolozi, South Africa) rich in herbivores, its repeatability between recorders, how the results reflect known properties of plant structural defences and how well the index correlates with the bite size of goats and its efficacy for explaining species distribution on a fire–herbivory gradient. The BSI was readily applied to a large set (n = 55) of plant species and proved to be consistent among recorders (8 of the 9 recorders obtained similar estimates). Bite sizes of goats and human observers were strongly correlated for 14 plant species (R 2 = 0.70) with divergent leaf and branch types. An analysis of BSI for 55 plant species disclosed two main contributory factors: leaf dimensions and spinescence. The BSI was significantly related to plant species distribution on a fire–herbivore gradient in an African savanna.