Feedback loops in ecological hierarchies following urine deposition in tallgrass prairie

Although theory predicts that large-scale factors will constrain small-scale interactions, the potential for small-scale events to impact large-scale patch structure is less clear. We examined experimentally the effects of urine deposition, a highly localized event, on small- and large-scale vegetation structure and spatial variation in tallgrass prairie. The response by vegetation to urine deposition may be mediated by grazing. Thus, we also determined the probability of urine patches serving as foci for grazing events by bison. Likelihood of bison grazing was much higher on vs. off urine patches, and grazed areas initiated on urine patches expanded well beyond the area of urine deposition. Abundance of four common graminoid species, and of all graminoids combined, increased on urine patches in ungrazed prairie. while the abundance of Andropogon gerardii and total graminoids decreased on urine patches in grazed prairie. The abundance of Aster ericoides, and all forbs combined, increased on urine patches in ungrazed, but not in grazed, prairie. Species richness and Shannon-Weiner diversity increased on urine patches in ungrazed prairie but were not affected by urine treatment in grazed prairie. Total spatial variance of graminoids increased relative to controls in response to urine treatment, grazing, and the combination of these treatments. For forbs, total variance increased marginally compared to controls only in response to urine treatment in ungrazed prairie. In combination, urine patches plus gracing produced unique large-scale patch structure compared to urine patches in ungrazed prairie. The most important impact of urine patches on community structure resulted from preferential grazing of urine patches by bison, which increases both the size and severity of the grazed area. Urine patches are thus an trample of a small-scale perturbation that generates larger-scale patch structure in tallgrass prairie.