Reproduction and population density: trade-offs for the conservation of rhinos in situ

As the human population increases, wildlife is becoming restricted to protected remnants of ecosystems with boundaries that limit dispersal. The effect on large herbivores generally is an increase in population density. We investigated the relationship between population density and reproduction in white rhinos (Ceratotherium simum) in Matobo National Park, Zimbabwe. Two lines of evidence were examined: ( 1) a contrast in reproduction between a high-density and a low-density population; and ( 2) a longitudinal analysis of changes in reproductive parameters of a fenced population as density increased by > 200%. As density increased within the fenced area, the rates of population growth and recruitment of calves decreased, and the age at first reproduction for females increased significantly. Females in the low-density population produced their first calves at significantly younger ages than those in the high-density population. Loss of body condition was related to both population density and female reproductive status. Because most African rhinos exist in relatively small reserves, an understanding of the implications of restricted dispersal and increased population density on their demography is critical to their conservation in situ. Managers need to consider trade-offs between enhanced safety from poaching and density-dependent effects on reproduction in developing conservation strategies for rhinos and other rare, large species confined to reserve systems.