Grazing ungulates select for grasses growing beneath trees in African savannas

In savannas, isolated large trees can form ‘islands of fertility’, referring to their elevated soil nutrients and their effect on light and water availability in their direct surroundings. Consequently, a quality difference between understorey grasses and open grassland can develop, creating patches of highly nutritious forage for grazing ungulates. Grass species composition beneath and outside of tree canopies was determined in a savanna system of Kruger National Park, South Africa. Direct observations were used to test whether grazing ungulates, i.e., impala, Burchell's zebra and blue wildebeest graze relatively more often beneath than outside of large tree canopies. Additionally, it was investigated whether they selected feeding locations according to allometric scaling and to their sex, and if feeding behaviour was influenced by weather conditions. Instantaneous scan sampling showed that ungulates preferred beneath-canopy grasses, independently of weather conditions. Grass species composition differed beneath and outside tree canopies, and beneath-canopy grasses exhibited more bite marks than outside canopy grasses. Blue wildebeest grazed least often beneath canopies compared to the other species. Females of impala were found to feed on beneath-canopy forage more often than males. Thus, large isolated trees strongly influenced the feeding behaviour of grazing ungulates. The current decline in large savanna trees should therefore be retarded and protective measures should be taken.