Herbivore population regulation and resource heterogeneity in a stochastic environment

• Published in: Ecology (Durham) 2015-08, Vol.96 (8), p.2170-2180
• Main Authors: Hempson, G. P., Illius, A. W., Hendricks, H. H., Bond, W. J., Vetter, S.
• Format: Article
• Language: English
• Subjects: African semi-arid grazing system; Animal Husbandry; Animals; Body Composition; Body condition; capital-income breeder; Conservation of Natural Resources; consumer resource dynamics; density dependence; dry season; Dry seasons; Ecosystem; Environment; environmental stochasticity; equilibrium; Female; Fertility; Forage; Foraging behavior; Goats; Goats - physiology; Herbivores; Humans; key resource; life history strategy; Male; National parks; Natural resources; nonequilibrium; Population density; Population dynamics; Population ecology; Population size; Pregnancy; rangeland debate; South Africa; Stochastic models; Stochastic Processes; wet season
• ISSN: 0012-9658; 1939-9170
• DOI: 10.1890/14-1501.1

Large-mammal herbivore populations are subject to the interaction of internal density-dependent processes and external environmental stochasticity. We disentangle these processes by linking consumer population dynamics, in a highly stochastic environment, to the availability of their key forage resource via effects on body condition and subsequent fecundity and mortality rates. Body condition and demographic rate data were obtained by monitoring 500 tagged female goats in the Richtersveld National Park, South Africa, over a three-year period. Identifying the key resource and pathway to density dependence for a population allows environmental stochasticity to be partitioned into that which has strong feedbacks to population stability, and that which does not. Our data reveal a density-dependent seasonal decline in goat body condition in response to concomitant density-dependent depletion of the dry-season forage resource. The loss in body condition reduced density-dependent pregnancy rates, litter sizes, and pre-weaning survival. Survival was lowest following the most severe dry season and for juveniles. Adult survival in the late-dry season depended on body condition in the mid-dry season. Population growth was determined by the length of the dry season and the population size in the previous year. The RNP goat population is thereby dynamically coupled primarily to its dry-season forage resource. Extreme environmental variability thus does not decouple consumer resource dynamics, in contrast to the views of nonequilibrium protagonists.