Will tree euphorbias (Euphorbia tetragona and Euphorbia triangularis) survive under the impact of black rhinoceros (Bicornis diceros minor) browsing in the Great Fish River Reserve, South Africa? Linda C. Heilmann 1,  , Karen de Jong 1,  , Peter C. Lent 2 and Willem F. de Boer 1, * 1 Resource Ecology Group, Wageningen University, Bornsesteeg 69, 6708 PD Wageningen, The Netherlands and 2 Faculty of Agriculture and Environmental Science, University of Fort Hare, Alice 5700, South Africa Abstract The impact of black rhinoceros (Bicornis diceros minor) on the tree euphorbias Euphorbia tetragona and Euphorbia triangularis was studied in the Great Fish River Reserve, South Africa. Black rhinoceros pushed over about 5–7% of the trees in a 2-month period. There was a preference of rhinos for smaller trees, however this preference did not guarantee euphorbia survival in the larger size classes. This means that tree euphorbias could very well disappear from all areas accessible to rhinos. Rhino feeding choices were correlated with higher plant mois- ture content, higher nitrogen content, and a higher digestibility. Key words: damage, diet choice, food quality, moisture content, size preference, survival Re ´sume ´ L’impacte du rhinoce ´ros noir (Bicornis diceros minor) sur les euphorbias Euphorbia tetragona et E. triangularis fut e ´tudie ´ dans la Re ´serve de la Great Fish River, en Afrique du Sud. Les rhinoce ´ros noirs de ´fonce `rent 5 a ` 7% des arbres environ pendant une pe ´riode de deux mois. Malgre ´ la pre ´fe ´rence des rhinoce ´ros pour les plus petits arbres, la survie des euphorbias plus grands ne fut pas garantie. Ceci veut dire que les euphorbias pourraient bien disparaı ˆtre de tous les endroits accessibles aux rhinoce ´ros. Les choix alimentaires furent lie ´s au contenu d’eau et de nitroge `ne chez les plantes, et la digestibilite ´. Introduction Black rhinoceros (Bicornis diceros minor L.) and the tree euphorbias Euphorbia tetragona (Haw.) and Euphorbia tri- angularis (Desf.) are on the CITES-list of endangered species (Inskipp & Gillett, 2003). Black rhinos are in appendix I and both tree euphorbias in appendix II of this list. However, it is not known if the two species can co-exist. The Great Fish River Reserve (GFRR) in South Africa contains a rapidly growing population of about 70 black rhinos, established after their first re-introduction in 1986 (Ausland et al., 2002). Black rhinos push over E. tetragona and E. triangularis, which usually results in the uprooting and death of the trees, even if B. diceros only takes a small bite (L.C. Heilmann & K. De Jong, Pers. obs.). This study was initiated because of concern by reserve management about declining densities of E. tetragona and E. triangularis. Megaherbivores often have dramatic influences on their environment, not only because they have a large biomass intake, but also because of mechanical destruction (Owen- Smith, 1988). Penzhorn, Robbertse & Olivier (1974) showed that the average total biomass outside an elephant (Loxodonta africana Blum.) exclosure was only 45% of the biomass inside this exclosure. Especially Portulacaria afra (Jacq.) and Aloe africana (Mill.) suffered from the elephant damage. In the Rwenzori National Park, elephants were responsible for a tree mortality rate of 5.7% per annum with Euphorbia candelabrum (Tre ´m.) as the dominant spe- cies (Eltringham, 1980). Other studies showed mortality rates for Acacia tortilis (Fors.), which ranged from 6% to 8% per annum (Lamprey et al., 1967; Pellew, 1983; Mwalyosi, *Correspondence: E-mail address: fred.deboer@wur.nl  The first two authors contributed equally to this article. Ó 2006 The Authors Journal Compilation Ó 2006 East African Wild Life Society, Afr. J. Ecol., 44, 87–94 87