How do small browsers respond to resource changes? Dietary response of the Cape grysbok to clearing alien Acacias

Abstract

Summary 1. The responses of small ruminants to changing food availability may vary from a broadening of the diet with declining availability, as predicted by optimality theory, to the maintenance of a restricted, selected diet, as predicted by the body-size constraint hypothesis. 2. We test these conflicting predictions by assessing the dietary responses of the Cape grysbok Raphicerus melanotis, a near-endemic to the Cape Floristic region, South Africa, to changes in the availability of a key forage species (i.e. Australian Acacia sp.). We predicted that the removal of alien Acacias would cause a broadening of the diet, and the consumption of previously avoided species, despite the selectivity imposed by their small body size. We used faecal analyses to describe grysbok diet and assessed diet quality through faecal quality analyses. 3. Results show that grysbok are highly selective browsers, able to change their diet and dietary preferences in response to changes in food availability. Animals included additional species in the diet in the absence of alien Acacias, altered their principal dietary items, broadened foraging strategies to include grass and some previously avoided species became preferred in the absence of Acacias. These dietary changes were effective in maintaining dietary protein intake, and caused a reduction in fibre intake in the absence of Acacias. 4. The data do not support the body size constraint hypothesis which is thought to impose a limit on the ability of small ruminants to select additional dietary species. 5. These data represent the first quantification of extensive grazing (up to 51% of the diet) by a species considered a browser. These findings support the prediction of optimality theory, whereby animals faced with a loss of important food items broaden their diet to include previously avoided species.