Does the magnitude of wildlife subsidies influence production, stability, and trophic cascades in a large African river?

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This project will address food web structure and dynamics in the Mara River, a trans-boundary river that provides water for the Serengeti-Mara Ecosystem, nearly one million rural poor, and 5% of the river flow into Lake Victoria and the Nile Basin. The Mara River is the only permanent source of water for wildlife and is therefore considered the 'life blood' of the Serengeti. This proposal focuses on the effects of nutrient addition to the river ecosystem from natural and exceedingly large inputs of nutrients from hippopotamus and wildebeest. The key question is how do large and irregularly timed nutrient inputs affect the way river ecosystems function? This project will conduct a series of experiments in the Mara river system to address this question. Results of this project will produce estimates of the distribution and abundance of hippos, and the location and magnitude of wildebeest drowning events that will provide important information for the management of hippos and wildebeest in the Serengeti-Mara Ecosystem. Hippos and wildebeest are extremely important to the tourism economy of Kenya and Tanzania and are species of considerable conservation concern. A collaboration with the National Museums of Kenya will provide important data on the diversity and biomass of insects and fish in the Mara River. This project will train a postdoctoral associate, a graduate student, and multiple undergraduate students, and broaden participation of under-represented groups in science by involving underrepresented minority undergraduate students in summer research. This project will also provide field-based short courses in Kenya for US students. Finally, results from this research will be shared with global audiences through web-based outreach initiatives using real-time discharge data, live photo streams from the field, and Twitter and blog updates.The movement of energy, materials, and organisms across ecosystem boundaries is a common feature of many landscapes. These spatial subsidies have strong but variable effects on communities and ecosystems. The magnitude of spatial subsidies may be one of the most important drivers of the variability in community and ecosystem level effects. Theory suggests that low levels of inputs stabilize food webs and increase community persistence, particularly when complementing patterns of local productivity, and high levels of inputs decrease food web stability and increase the strength of trophic cascades. Very little empirical research has addressed the impacts of the magnitude of subsidies, raising the central question of this proposal: how does the magnitude of subsidies influence community and ecosystem processes? The Mara River sustains the largest remaining overland migration of around 1.2 million wildebeest and a population of over 4,000 hippopotamus (hippos). Hippos, through excretion and egestion during daily feeding migrations, and wildebeest, through carcass inputs during mass drownings, create a natural gradient of increasing subsidy inputs from upstream to downstream in the Mara River. This project utilizes basin-wide comparisons, in situ comparative experiments, and small-scale artificial stream experiments to test how the magnitude of spatial subsidies influences primary and secondary production, the strength of trophic cascades, and food web stability.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.