Biology and management affecting the decline of a black rhinoceros, Diceros bicornis minor (Linnaeus, 1758), population in Ndumo Game Reserve, KwaZulu-Natal, South Africa

Abstract

Black rhinoceros (Diceros bicornis) are classified as critically endangered on the IUCN red data list (IUCN 2010). In Ndumo Game Reserve in northern KwaZulu-Natal, South Africa, a black rhino population has seen a decline in numbers from nearly 50 rhino in 1988 to only eight in 2006. This study aimed to determine the factors or events responsible for the decline in black rhino numbers in Ndumo. The first part of this study focuses on the historical data and literature on Ndumo’s black rhino population. Specifically, I examined historical data regarding Ndumo’s black rhino population estimates, recruitment patterns, mortality rates, number of removals, introductions, densities and other large herbivore population estimates. Results suggest that a combination of high removals due to inaccurate estimates and competition from other herbivore populations, specifically nyala (Nyala angasi) and impala (Aepyceros melampus), played a role in this population’s decline. Also, a change in the Usuthu River course negatively affected the population’s social structure, movement patterns and recruitment success. The introduction of five black rhino from Hluhluwe-iMfolozi Park in 2008 seems to have been a success, and should increase reproductive success. Secondly, we determined the amount of browse currently (2008) available to black rhino in Ndumo, the proportional species composition of this browse and its suitability for black rhino. The results show that browse availability is fairly high (average of 17.8%) in Ndumo but the suitability is low. Croton menyhartii is the single biggest contributor to browse availability in Ndumo, contributing 25% of all available browse. This plant species is unfortunately rejected by black rhino and most other browsers. Furthermore, two invasive alien plant species (Chromolaena odorata and Lantana camara) are amongst the four most abundant plant species in Ndumo and are also rejected by black rhino. An effective alien plant clearing programme is needed to eradicate invasive alien species in Ndumo, which will help alleviate the poor browse conditions. A population viability analysis (PVA) was done during the last part of the study to predict the possibility of extinction or survival of this black rhino population in future. The VORTEX model was used in this study. Data and trends as actually observed in Ndumo since 1988 was used as the first simulation’s input parameters, to test whether the model will predict a similar decline in black rhino numbers as observed in Ndumo. Furthermore, sensitivity analyses with different input parameters were done to test the probability of extinction or survival under all possible circumstances. The model predicted a high probability of survival, even with most of the sensitivity analyses, suggesting that small populations of less than 50 individuals are viable if managed correctly. Parameters that impacted negatively on the growth rate of this population were density dependant breeding, a low recruitment rate and a decline in carrying capacity.