Resource selection by impala (Aepyceros melampus) in savanna systems, South Africa: which factors govern forage selection in a mixed feeder?

Abstract

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. I investigated fine scale resource selection in a mixed feeder, impala (Aepyceros melampus), as well as how their diet changed across seasons and which factors drove a change in diet. The study was conducted in two savanna reserves in South Africa, where I located foraging impala herds on a daily basis. At each feeding area I collected data at three spatial scales: species level, feeding station level and patch level. Some of the data included species identification, greenness, biomass (of the feeding station) and type of cover (in the patch). I also sampled an unused patch located 40 m away. I found that impala preferred grazing and especially grazing on Panicum maximum and Urochloa mosambicensis. Impala browsed infrequently and even though I expected them to make a switch from a grass dominated diet to a browse dominated diet, I only saw an expansion of their diet. Impala included more forbs, seed pods and grass stems in their diet during the late dry season. Greenness (of the species, feeding station and patch), biomass and type of cover were all important in driving selection. Because of their smaller bite sizes impala were able to select for single green stems located within mostly dry tufts of grass. Contrary to previous suggestions, impala did not switch from predominantly grazing to browsing when the 2 monthly running mean of rainfall fell below 30mm.