Mortality and female fecundity in an expanding black rhinoceros (Diceros bicornis minor) population

Abstract

Identifying factors that affect demographic parameters and how those factors act is vital for understanding population dynamics, especially of endangered species. Moreover, specific ideas in the population dynamics of large herbivores underpin the management of the critically endangered black rhinoceros (Diceros bicornis). We studied an expanding black rhinoceros population since its establishment in 1986 in the Great Fish River Reserve, South Africa, through 2008 when managed removals interrupted natural dynamics. During the study, only 13 animals died, of which nine were subadults. In a linear modelling context, we used information-theoretic methods to evaluate the influence of independent variables expected to affect demographic parameters. For females, age at first reproduction (AFR) increased with abundance, but there was no effect of abundance on adult fecundity as measured by inter-birth intervals (IBIs). We evaluated these results in the theoretical context of population dynamics of large herbivores, in particular, Eberhardt’s proposal of a specific sequence in which demographic parameters first respond to increasing density. Our observations are consistent with Eberhardt’s prediction that immature individuals are impacted before adults, but the relative timing and magnitude of density effects on immature individuals was unclear. Rainfall did not influence AFR or IBIs. Maternal age influenced IBIs but much of the observed variation in IBIs was not accounted for by structural variation. Studies of populations more nearly approaching a stable age distribution and carrying capacity are needed to resolve remaining uncertainties and ambiguities in the life history of the black rhinoceros in particular and megaherbivores in general.