Ungulate distributions in a rangeland with competitors, predators and pastoralists

摘要

African rangelands support diverse ungulate communities whose member species exhibit unique combinations of body morphology and behaviour that have evolved over millions of years to limit the effects of competition and predation on fitness, and more recently, to cope with people and livestock. The mechanisms by which native ungulates cope with the combined effects of competition, predation and human disturbance are poorly understood. Addressing this knowledge gap will help guide management and conservation plans for large mammal communities outside of strictly protected areas. We conducted animal counts on line transects and used a spatially explicit distance sampling model to test the influence of bottom-up effects (vegetation), top-down effects from African lion Panthera leo and spotted hyena Crocuta crocuta and human disturbance (livestock density, occupied human settlements) on native ungulate densities and distributions across a multiple-use rangeland in Kenya. We examined five species that varied in body morphology and foraging strategy, including two large-bodied grazers (zebra Equus quagga; wildebeest Connochaetes taurinus), two medium-bodied, mixed-feeders (Grant's gazelle Nanger granti; impala Aepyceros melampus) and one very large-bodied browser (giraffe Giraffa camelopardalis tippelskirchi). Densities and distributions of all species varied across available land-use types, with bottom-up effects (vegetation) and human disturbance having the strongest overall effects. Responses varied by ungulate foraging strategy. Distributions of grazers and the browser (giraffe) changed seasonally, while distributions of mixed-feeders and a grazer (zebra) changed diurnally. At broad spatial scales, ungulates did not respond to long-term variation in predation risk though they have been shown to respond behaviourally to short-term variation in risk.Synthesis and applications. Our results revealed that native ungulates in this multiple-use rangeland occur at densities comparable to many flagship protected area populations. We attribute observed densities to spatial heterogeneity across this landscape that included variation in human land use (community conservation area, seasonal settlements and livestock grazing, permanent settlements) and vegetation (grassland, bushland, woodland) that changed through time (seasonally, diurnally). This variation allowed ungulates to carve out ecological niches while reducing exposure to the potential limiting effects of competition and human disturbance. For some species, securing access to forage resources and avoiding human disturbance may be associated with costs through increased risk of predation. Our research reveals the trade-offs native ungulates make to cope with changes in forage availability, human disturbance and predation risk, providing important insights to help guide native ungulate conservation efforts in multiple-use rangelands.