Impact of the black rhinoceros (Diceros bicornis minor)on a local population of Euphorbia bothae in the Great Fish River Reserve, South Africa Bodina L. Luske1, Tonja Mertens1 , Peter C. Lent2, Willem F. de Boer1* and Herbert H. T. Prins1 1Resource Ecology Group, Wageningen University, Droevendaalsesteeg 3a, 6708 PB Wageningen, The Netherlands and 2Faculty of Science and Agriculture, University of Fort Hare, Alice 5700, South Africa grande partie d’une population locale d’Euphorbia bothae. Abstract Le maintien du rhino noir en danger et de la population In the Great Fish River Reserve, South Africa, black rhi- prote´ge´ed’E. bothae sont deux priorite´sdelare´serve en noceros (Diceros bicornis minor) feed extensively on a local matie`re de conservation. C’est pourquoi on a investigue´ la population of Euphorbia bothae. Maintaining the endan- durabilite´ de cette interaction plante-animal en compa- gered black rhinoceros and the protected E. bothae popu- rant les caracte´ristiques des populations, l’incidence lation are both conservation priorities of the reserve. et l’intensite´ de la consommation du rhino dans la re´serve Therefore, the sustainability of this plant–animal interac- et dans un enclos adjacent d’ou` les rhinos sont exclus. Des tion was investigated by comparing population charac- photographies prises d’un point fixe ont montre´ que, sur teristics, browsing incidence and intensity within the une pe´riode de deux mois, 36.6% des 213 plantes suivies reserve and in an adjacent exclosure without access to avaient e´te´ broute´es, avec une perte moyenne de biomasse rhino. Fixed-point photographs showed that over a de 13%, et un pour cent avait e´te´ de´truit. Des 26 plantes 2-month period 36.6% of 213 monitored plants were qui avaient e´te´ rephotographie´es apre`s environ trois ans, browsed, with an average biomass loss of 13%, and 1% 70% montraient une diminution de la biomasse, qui e´tait were destroyed. Of 26 plants re-photographed after de 37.8% en moyenne pour cette pe´riode. Pendant ce laps approximately 3 years, 70% showed a decrease in bio- de temps, 19% des plantes suivies sont mortes. Les petites mass, averaging 37.8% over this period. In this time span, plantes (<45 cm) e´taient surrepre´sente´es dans la zone 19% of the monitored plants died. Small plants (<45 cm) broute´e par les rhinos, alors que la fraction des plantes were over-represented in the rhino-browsed area, whereas actives au point de vue reproduction et la densite´ ge´ne´rale the fraction of reproductively active plants and overall des plantes se sont ave´re´es plus faibles que dans l’enclos plant density were found to be lower than in the adjacent adjacent. On n’a pu mettre en e´vidence aucune croissance exclosure. No evidence of short-term compensatory growth compensatoire d’E. bothaea` court terme en re´ponse au in response to browsing was found for E. bothae. This study broutage des rhinos. Cette e´tude indique que, vu la taille indicates that, with the current population size, rhinos are actuelle de leur population, les rhinos surexploitent la overexploiting the E. bothae population and special mea- population d’E. bothae et qu’il faut prendre des mesures sures should be taken to prevent local extinction. spe´ciales pour empeˆcher l’extinction locale de cette der- nie`re. Key words: browsing, compensation, megaherbivores, population structure, thicket, reproduction Re´sume´ Introduction Dans la Great Fish River Reserve, en Afrique du Sud, le Megaherbivores such as black rhinoceros (Diceros bicornis rhinoce´ros noir (Diceros bicornis minor) se nourrit en tre`s Linnaeus) or African elephant (Loxodonta africana Blu- menbach) were abundant in precolonial times in the *Correspondence: E-mail: [email protected] Eastern Cape Province of South Africa (Boshoff et al., Deceased 2002) but were extirpated or, in the case of the elephant, Ó 2009 The Authors. Journal compilation Ó 2009 Blackwell Publishing Ltd, Afr. J. Ecol. 1 2 Bodina L. Luske et al. nearly extirpated, in the 19th century. Because of their second scenario is the one in which regrowth and considerable biomass, they played an important role in recruitment of plants are not fast enough to compensate ecosystem structure (Cowling, Proches & Vlok, 2005). To for the biomass removed by browsing, illustrated by a restore the original biome and its functioning, black rhinos lowered recruitment rate or a decreased generative activity (Diceros bicornis minor Drummond) were reintroduced into of the plants (Pfab & Witkowski, 1999; Russell & Fowler, the Great Fish River Reserve (GFRR, Eastern Cape Prov- 2004). Incessant browsing may also cause a shift in size ince, South Africa), starting in 1986. The reserve distribution of the plants (scenario three), as has been (450 km2) now hosts a population of over 100 black rhi- recorded for similar interactions (Motta, 2003; Siipilehto & noceros and a few individuals have been translocated al- Heikkila¨, 2005). The fourth scenario is that plants over- ready to other reserves. This successful reintroduction may compensate browsing, so that browsing promotes rapid have a negative impact on plant biodiversity, which poses shoot regrowth (Paige & Whitham, 1987; Du Toit, Bryant dilemmas to nature reserve management (Johnson, Cowl- & Frisby, 1990). It has been suggested that E. bothae may ing & Phillipson, 1999. The rhinos feed extensively on a respond to browsing by an over-compensatory growth local population of Euphorbia bothae Lotsy and Goddijn. mechanism resulting in more side-shoots and stems (D. This succulent plant is included in Appendix II of the Brown, pers. comm.). Convention on International Trade in Endangered Species In other words, the impact of browsing by the popula- of Wild Fauna and Flora (CITES) list, while the black rhino tion of black rhino on this species of Euphorbia in the GFRR is included in Appendix I. Maintaining both the critically may be either negative or positive. During this study, we endangered black rhinoceros and the E. bothae population focused on measuring specific population characteristics are the conservation priorities of the reserve. of E. bothae in the reserve such as size distribution, Euphorbia bothae occurs in the xeric succulent thicket, plant growth and browsing incidence and intensity to which contains a high plant diversity and endemism determine the impact of black rhino on the E. bothae especially among succulents and geophytes (Cowling & population. Hilton-Taylor, 1994). According to Low & Rebelo (1996), large browsers historically played a part in limiting the Methods geographic distribution of this vegetation type. Since the early 1900s, the xeric succulent thicket was mainly used Study site and species for farming goats (Mills & Martin, 2004). The removal of large indigenous browsers from the system may well have The research was conducted in the Great Fish River Re- led to the expansion of E. bothae in the Eastern Cape serve (33°07¢S; 26°38¢E) in the Eastern Cape Province of (B. Fike pers. comm.; Low & Rebelo, 1996). South Africa (45.000 ha). The area receives an average Previous studies have shown the importance of E. bothae annual rainfall of 420 mm (Fabricius, Palmer & Burger, in the diet of rhinos in the GFRR and its status as a pre- 2002). ferred browse species, with estimates of the contribution of Only a few studies are available on E. bothae (Fig. 1). E. botha in the rhino’s diet varying between 4% and 41% Although in most literature this species is considered as an (Ausland et al., 2002; Brown et al., 2003; Ganqa, Scogings independent species, it was described by Rodriguez (1993) & Raats, 2005). As has been suggested for other species of as a naturally occurring hybrid of Euphorbia coerulescens Euphorbia, E. bothae may also be a seasonally important Haw and Euphorbia tetragona Haw. Similar to most source of water for the rhinos (Dudley, 1997; Heilmann Euphorbia species, E. bothae produces a potentially toxic et al., 2006). As has been reported for African elephant latex, which apparently does not harm the rhinos. The feeding on rare plant species in the Eastern Cape (Johnson plants bear distinctive inflorescences, called cyathia, and et al., 1999), the high density of rhinos may lead to pairwise thorns, which protect them from herbivores. The unsustainable consumption of the local E. bothae popula- colour of the individual stems varies from green to grey. tion. Measurements were made in the ‘short Euphorbia thicket’ Several scenarios for plant–herbivore interactions are (SET), a vegetation type dominated by E. bothae, which known from the literature. The worst-case scenario is a only occurs in the south-western part of the reserve. SET is predator–prey interaction where the animals kill plants part of the xeric succulent thicket vegetation type, which during one feeding event (Heilmann et al., 2006). The contains mainly evergreen, drought-resistant woody Ó 2009 The Authors. Journal compilation Ó 2009 Blackwell Publishing Ltd, Afr. J. Ecol. Impact of black rhino on Euphorbia bothae 3 Data collection Twenty-five line transects of 100 m each were selected in a stratified random manner in both the browsed area and the reference area. Every 10 m, several characteristics (distance to the transect, height, crown diameter, per- centage dead plant material, presence of cyathia or fruits, number of stem tips, number of browsed stem tips) of the plant nearest to the transect line were recorded. Plant cover was calculated as a projection of the crown area, and plant density by using the plotless distance method (Bon- ham, 1989). To estimate browsing pressure, a stratified random method was used to select 40 sites in the browsed area and 20 in the reference area. From 16 November 2005 to 2 December 2005, a total of 332 plants, 213 in the browsed area and 112 in the reference area, were photographed from fixed points using a digital camera and tripod (see Fig 1 Specimen of Euphorbia bothae Fig.