Collaborative Research: TERRG: Thresholds in ecosystem responses to rainfall gradients

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The biology and chemistry of streams change in response to changes in temperature and rainfall patterns. Changes in temperature and precipitation may lead to predictable changes in the biology and chemistry of streams, while other changes are abrupt and unpredictable. In the face of environmental change, we need to understand the environmental conditions that result in predictable and abrupt responses to both the biology and chemistry of streams. This award will support research on understanding biological controls and environmental constraints on the structure and function of streams in South Texas. The research will take advantage of a semi-arid to mesic rainfall gradient that features an abrupt shift in stream community composition and ecosystem functioning at 85 cm rainfall/yr. The research will consist of a mix of field observations, manipulative experiments, and modeling. Training of undergraduate (including minority) and graduate students and outreach via an established K-12 program will contribute to the project?s broader impacts.This project will advance our understanding of why subtle shifts in the environment can result in the significant and abrupt reorganization of ecological communities. Spatial patterns in structure and function of ecosystems along precipitation gradients indicate that threshold effects are common and widespread. Therefore, understanding the mechanisms that drive these patterns is essential for forecasting which regions of the world will undergo rapid ecosystem shifts under future environmental change. Stream ecosystems are particularly sensitive to changes in precipitation, but multiple mechanisms can be invoked to explain observed threshold effects in streams. These mechanisms include: 1) Changes in the quantity, quality, temporal stability, and predictability of the resource base, which cascade up to higher trophic levels; and conversely, 2) Changes in the temporal stability, predictability, and average environmental conditions impacting community structure, which then cascade down to ecosystem processes. By exploring these alternate and potentially simultaneously operating mechanistic hypotheses in a set of streams along a precipitation gradient in South Texas, the research will capitalize on an ideal natural experiment along with a strong foundation of preliminary data collected by the principal investigators. The results of this study will in turn inform our understanding of how similar processes operate in stream ecosystems around the world, and the basic principles will enhance our understanding of these processes in all ecosystems. The award will support training of undergraduate, graduate, postdoctoral scholars, and early career faculty at both a Hispanic Serving Institution and a primarily undergraduate institution serving 40% minorities. Undergraduates will conduct independent research and present the results at professional meetings. K-12 outreach and adult education will be conducted through the SIU Center for Ecology and Graduate Research Assistants Supporting Science (GRASS) program.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.