Intriguing density dependence but problematic birth rates in Greaver et al.’s study of a black-rhinoceros population

Abstract

Greaver et al. (2014) presented evidence for density dependence in the Ithala population of black rhinoceros. Finding that they did not place their regression-based evidence in a modelling context, we recast their result as an example of the ramp model of density dependence that underlies black rhinoceros meta-population management. Greaver et al. concluded that the Ithala population did not reach carrying capacity, a conclusion we consider unwarranted since they did not conduct any analyses of trends in demographic parameters with population density. Our interpretation implies that the Ithala population did indeed reach carrying capacity. Where relevant, we compared their results for the Ithala population with those for another southern African black-rhinoceros population in order to provide a broader basis for evaluating black rhinoceros demography. We detail inconsistent presentation of data in their paper that plagued our efforts to understand their results and also draw attention to possible errors in some analyses. In particular, we argue that the results on birth rates reported by Greaver et al. appear dubious. Greaver et al. have presented important evidence for density dependence in a population of black rhinoceros but we suggest they have underutilized their data in interpreting this density dependence while misanalysing birth data.