Resource selection in a mixed feeder: Which factors drive switching between diets?

Background/Question/Methods

Herbivores can be divided into three general groups: grazers, browsers and mixed feeders. Mixed feeders switch between grazing and browsing throughout the year, mostly due to changes in environmental conditions. Most studies investigating resource partitioning and the browser/grazer continuum in mammalian herbivores have focused on possible digestive differences between grazers and browsers, but few studies have investigated the factors that cause mixed feeders to switch between grazing and browsing. Impala (Aepyceros melampus) is a mixed feeder that eats a variety of grasses, dicots and plant parts. We conducted a study in two savanna reserves in South Africa to investigate the factors that influenced diet switching by impala. Over a five-month period, from May to October 2009, we located and observed impala in feeding areas. We identified all plant species, used or unused by impala. We ranked greenness of each species, estimated biomass at the feeding station, and ranked basal cover of each plant species. We also collected fresh feces where available to assess diet composition through microhistology.

Results/Conclusions

Impala preferred grazing, specifically on Panicum maximum and Urochloa mosambicensis. They browsed infrequently, even during the late dry season when grasses were dry and scarce. We established that a decrease in greenness decreased selection of grass blades, but increased selection of forbs and seed pods. Surprisingly 30-70% of their diet in the late dry season composed of single green grass stems in large tufts of brown grass. Unlike larger herbivores, such as buffalo, impala’s small bite size enabled them to be more selective to fulfill their high energy requirements. There was also a positive relationship between average weekly greenness and the presence of grass in feces. Although it has been suggested that impala switch from grazing to browsing when rainfall drops below a certain level, we suggest that it is a drop in overall greenness that governs switching. Hence, despite the lack of diet switching, the expansion of their diet during the late dry season supports ungulate foraging theory and echos the diet expansion of sable antelope during the dry season.