Using models in the management of Black rhino populations

A study was undertaken, modeling Black rhino, Diceros bicornis, populations from Hluhluwe-Umfolozi Game Reserve and Mkuzi Game Reserve, Republic of South Africa. This study had two specific goals. Firstly, we tried to find a simple model to predict the population number in the following year and give insight in the number of animals that can be removed from the population. Secondly, the use of different models gave us the opportunity to evaluate the effect of small differences in model structure on the outcome of the models. Five different models were tested, mainly focusing on the structure of density dependence. The density dependent model, that incorporated stronger density dependence at high densities, fitted best to the observed census values. The study, furthermore, indicated that the population in Hluhluwe-Umfolozi is stabilizing. The situation in Mkuzi was not that clear. Based on these results, managers should therefore be more careful with removing animals from this population. Finally the results clearly showed that different model structures can result in very different model outcomes. Besides in the values of the model parameters, this was also shown by different values for the population number where the yearly increase is maximal. Based on these results it does not seem wise to base management decisions solely on the outcome of one model. Therefore, we strongly encourage managers to compare the outcome from different model structures, based on different biological assumptions.