Vegetation modification and resource competition in grazing ungulates

The prevalence of interspecific competition in animal communities is the subject of a long-running debate, chiefly because the underlying processes of resource exploitation and resource supply are often poorly understood. To provide some insight into these processes within a guild of grazing herbivores, two hypothetical mechanisms of exploitation competition were tested by measuring food intake of topi (Damaliscus lunatus) and wildebeest (Connochaetes murinus) when foraging on different sward structures in the Serengeti National Park. According to our bite quantity hypothesis. wildebeest, which have relatively wide mouths, can graze down vegetative swards to a height below that which can be tolerated by topi; and according to our bite quality hypothesis, the narrower-mouthed topi can reduce the leafy component of differentiated swards (i.e, swards in which seed-bearing stems have developed) through selective feeding to a level below that which can be tolerated by wildebeest. On differentiated swards with erect growth form, the topi selected 20% more green leaf in their diet, as measured by a calibrated visual technique, and also obtained higher short-term intake rates. Greater selectivity alone provided topi with a metabolisable energy intake estimated to be 16% higher than that of wildebeest. On vegetative swards, it was estimated that wildebeest could maintain positive energy balance on 2-cm swards, 1 cm shorter than the threshold height for topi. Our findings indicate the conditions under which each ungulate species map limit the other's use of natural pastures through interspecific competition: bite quantity competition may apply on short grazing lawns; bite quality competition is expected on differentiated swards with a limited supply of green leaf. We suggest that herbivory by one species can modify the vegetation in a way that makes it less profitable to competing species. In effect the vegetation is 'captured' as a resource by one species. Thus modification of vegetation is argued to be a critical component of resource competition in herbivores.