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Dr. Thomas Sisk

Professor of Biology and Environmental Science

Research Location Site: Reed Lake (Mogollon Rim) and Sonoita Valley (SE Arizona)

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Livestock grazing on public lands has become one of the most contentious issues in the southwestern United States during the 1990s. On one side of the debate are environmentalist activists that perceive the impact of livestock on native, intact ecological communities as extreme and unacceptable. On the other side of the debate are traditional ranchers, many of whom are long-time residents contending that the overall condition of rangelands has improved in their lifetimes. Somewhere in the middle are an increasing number of ranchers and land managers who argue that while traditional grazing practices can result in rangeland degradation, grasslands can actually be improved through the use of high-intensity, short duration grazing rotation strategies.

Is there a role for science amidst this debate? We believe that the answer is yes, and that scientists can help to clarify the biological resource issues at stake and, once clarified, design experiments involving the ideas and participation of different constituencies in the debate. The discussion of livestock grazing on arid lands is often muddled by the use of ill-defined terms such as "ecosystem health" or "rangeland condition." We believe that the discussion can be clarified and moved forward if such general terms are replaced by two distinct, measurable ecosystem characteristics: biological diversity and ecosystem productivity. We have initiated a research project that will examine how plant and arthropod diversity and net primary productivity of arid grasslands vary as functions of livestock grazing intensity. The four treatments include: 1) livestock exclosure, 2) traditional low-intensity, long-duration grazing rotation, 3) high density, short-duration, HRM-style grazing and 4) extremely high density, short-duration grazing (to simulate herd-impact).

We have established one pilot site and two long-term study sites in north, central and SE Arizona. The pilot site (called Post Canyon) was established in 1997, and was chosen because the first three types of range management mentioned above were well established. Data from the pilot site demonstrate patterns of variation in arthropod orders as a function of livestock management. After two consecutive drought years, aboveground net primary productivity was significantly higher at the ungrazed site compared to the traditional or HRM managed ranches. In contrast, plant species diversity did not significantly vary as a function of livestock management.

In central Arizona we established one long-term study site in 1997 and another in the fall of 1998. Both sites are comprised of replicated, 1-ha plots on grasslands managed under the 4 different management treatments listed above. Blocks of these four treatments are replicated three times at the study sites. Unlike most other studies that have compared the effects of different stocking densities on soils and vegetation, this study stresses practical implications by working on operating ranches. Ranchers are responsible for deciding animal stocking densities and grazing durations while researchers examine changes in plant and arthropod diversity, net primary productivity as well as factors that underlie diversity and/or productivity, such as soil moisture dynamics, associative nitrogen fixation, standing biomass, decomposition, and cycling of nitrogen and phosphorus.

Our research program has developed a strong educational component involving graduate and undergraduate students at two institutions, as well as ranchers, environmentalists, and other interested citizens. A fundamental objective of this study is to bridge the existing gulf between research science, ranchers and other land managers, and the public.