Long-term changes in Serengeti-Mara wildebeest and land cover: Pastoralism, population, or policies?

Declines in habitat and wildlife in semiarid African savannas are widely re ported and commonly attributed to agropastoral population growth, livestock impacts, and subsistence cultivation. However, extreme annual and shorter- term variability of rainfall, primary production, vegetation, and populatio ns of grazers make directional trends and causal chains hard to establish i n these ecosystems. Here two decades of changes in land cover and wildebees t in the Serengeti-Mara region of East Africa are analyzed in terms of pote ntial drivers (rainfall, human and livestock population growth, socio-econo mic trends, land tenure, agricultural policies, and markets). The natural e xperiment research design controls for confounding variables, and our conce ptual model and statistical approach integrate natural and social sciences data. The Kenyan part of the ecosystem shows rapid land-cover change and dr astic decline for a wide range of wildlife species, but these changes are a bsent on the Tanzanian side. Temporal climate trends, human population dens ity and growth rates, uptake of small-holder agriculture, and livestock pop ulation trends do not differ between the Kenyan and Tanzanian parts of the ecosystem and cannot account for observed changes. Differences in private v ersus state/communal land tenure, agricultural policy, and market condition s suggest, and spatial correlations confirm, that the major changes in land cover and dominant grazer species numbers are driven primarily by private landowners responding to market opportunities for mechanized agriculture, l ess by agropastoral population growth, cattle numbers, or small-holder land use.