Declines in habitat and wildlife in semiarid African savannas are widely reported 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 populations of grazers make directional trends and causal chains hard to establish in these ecosystems. Here two decades of changes in land cover and wildebeest in the Serengeti-Mara region of East Africa are analyzed in terms of potential drivers (rainfall, human and livestock population growth, socio-economic trends, land tenure, agricultural policies, and markets). The natural experiment research design controls for confounding variables, and our conceptual model and statistical approach integrate natural and social sciences data. The Kenyan part of the ecosystem shows rapid land-cover change and drastic decline for a wide range of wildlife species, but these changes are absent on the Tanzanian side. Temporal climate trends, human population density and growth rates, uptake of small-holder agriculture, and livestock population trends do not differ between the Kenyan and Tanzanian parts of the ecosystem and cannot account for observed changes. Differences in private versus state͞communal land tenure, agricultural policy, and market conditions 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, less by agropastoral population growth, cattle numbers, or small-holder land use.T he extent to which conservation areas can successfully coexist with local users in developing countries is hotly debated, as are the conditions for environmental, social, and economic sustainability of any such coexistence (refs. 1-5 and 44). In East African savannas, habitat loss and wildlife decline are widely perceived and generally attributed to rapid population growth and the spread of subsistence cultivation. Directional trends and causal chains are hard to establish in semiarid lands, however, because rainfall, primary production, grazer populations, and vegetation formations show major unpredictable fluctuations between seasons and years. The 100,000-km 2 Serengeti-Mara Ecosystem (SME) serves as a natural experiment allowing analysis of the long-term outcomes of different policies for conservation on the one hand and community development on the other. The SME comprises contrasting land-use zones with different tenure arrangements, ranging from state-controlled ''fortress'' conservation areas to private and nonprivate tracts with multiple land uses, some with community-based conservation initiatives, superimposed on a rangeland where ecological, microeconomic, and ethnic continuities make it possible to control for many confounding variables. The SME is bisected by the Kenya͞Tanzania border, allowing comparative analysis of the implications of contrasting economic, political, and...