Resource overlap within a guild of browsing ungulates in a South African savanna.

Abstract

Food selection by free-ranging black rhinoceros, eland, giraffe and kudu as well as the utilisation of vegetation types by the latter three browsers were investigated over an entire seasonal cycle, from June 1998 to July 1999, at Weenen Nature Reserve, KwaZulu-Natal. The study was aimed at determining the extent of resource overlap within this browser guild. Feeding habits of eland, giraffe and kudu were studied by direct observations, while a plant-based technique was used for black rhinoceros. Dung counts were conducted to monitor selection for vegetation types. Overlap was estimated by measuring the similarities in resource utilisation patterns. Giraffe were exclusively browsers, feeding mostly on woody foliage, over the complete seasonal cycle. The bulk of the annual diet of kudu also consisted of woody browse, although forbs were important and their use increased from early summer to winter. The annual diet of eland consisted of approximately equal proportions of grass and browse, with pods making up almost a third of the diet. Similar to kudu, forbs were more prominent in the winter diet, while grass use decreased. During winter, overlap in forage types generally increased and was considerable because the browsers did not resort to distinct forage 'refuges'. Overlap in the utilisation of woody plant species, however, decreased as animals diversified their diets. Nonetheless, overlap was extensive, primarily owing to the mutual utilisation of Acacia karroo and Acacia nilotica. The quantity of woody foliage decreased during winter, as indicated by phenological differences, but numerous individual plants still carried leaves. Based on current evidence, food quality was assumed to decline. Under prevailing conditions, eland, giraffe and black rhinoceros suffered no mortalities indicating that they were not food limited, possibly owing to the nutritional advantages conferred by their large body size, and that competition among them was unlikely. By comparison, kudu mortalities were great which may signify that they were constrained by food supply and that the larger browsers exerted a pronounced competitive effect on them. Based on the current study it is hypothesised that during periods of resource scarcity the abundance of high quality foods are limited and if interspecific competition does prevail, which will further limit the availability of these resources, it is the smaller bodied herbivores that will be most affected and suffer the greatest mortalities. Consequences of competitive interactions among these browsers have important management implications, especially in small reserves, which are a key stone for the conservation of mammalian herbivores.