The influence of rainfall on zebra population dynamics: implications for management

1. Where wildlife is not formally protected in Africa, abundant species are often consumptively managed, for benefit, control, or both. Such management should preferably be based on some understanding of the ecological processes regulating populations, the extent to which wildlife conflicts with livestock, and the rates at which wildlife can be harvested on a sustainable basis. 2. We report the results of a simulation modelling analysis of the dynamics of a plains zebra population in Laikipia District, Kenya. This largely unfenced, non-protected area of 9666 km(2) is comprised of privately, publicly, and communally owned properties supporting livestock and wildlife. We examined the influence of rainfall on zebra abundance, and evaluated the sustainability of alternative harvesting regimes. 3. The model was stage- and sex-structured, corresponding with age/sex classes recognizable in the field. Vital rates and age at first reproduction were adjusted annually using two alternative approaches, both involving annual re-calculation of carrying capacity as a function of rainfall. The first adjustment factor was simply a function of carrying capacity - a solely 'Rainfall-Dependent' (RD) mechanism. The second adjustment factor was proportional to the ratio of carrying capacity to population size - a Rainfall-Mediated Density-Dependent (RMDD) mechanism. 4. Both versions of the model reconstructed known zebra dynamics over the last 16 years within the precision afforded by sample counting. However, RMDD gave a better fit, yielded parameter settings that were meaningful ecologically, and better predicted the impact of a severe drought than did RD. We infer that rainfall strongly influences the abundance of Laikipia's zebras by a mechanism involving adjustments in the strength of density dependence. 5. Simulations using RMDD suggested that previously allocated annual harvest quotas in Laikipia (up to 15%) might not have been sustainable, but a 6% annual harvest would be a sustainable harvest. 6. Synthesis and applications: The model captures a fundamental feature of the mechanism by which rainfall limits large-bodied savanna ungulates: populations typically decline faster during dry phases than they can increase during wet phases. As a result, the greater the variation in annual rainfall, the greater the proportion of time the population spends below carrying capacity. The model has been adopted as the basis for zebra management in Laikipia. To our knowledge, this is the first population-specific model to be used to guide the consumptive management of a wild ungulate in Kenya.