Pathways for effects of small-scale disturbances on a rare plant: HowMimulus angustatus benefits from gopher mounds

Small-scale soil disturbances such as soil mounds produced by gophers are known to influence local plant communities. A variety of mechanisms might account for the influence of gopher disturbances on individual plant success, but understanding of these mechanisms is not well developed. Disturbances are often assumed to affect plants through changing competition or the abiotic environment, but disturbances might also influence plant size, which in turn influences other biotic interactions, including pollination. In this study, we tested for effects of soil disturbance on the density, flower size, and reproduction of an annual plant, Mimulus angustatus. We used soil disturbance manipulations, observational data, and a pollination experiment to examine two possible pathways for effects of disturbance on M. angustatus: competition and pollination. We also considered how effects of gopher mounds change with years since initial disturbance. We found that disturbance strongly increased the local density and flower size of M. angustatus, but the strength of these effects decays quickly. We found no support for these effects being mediated by competition with other plant species. We also found that M. angustatus with larger flowers receive more natural pollination. Our pollination experiment suggests that M. angustatus benefits from pollination (seed set increases with hand pollination or access to pollinators), and suggests that pollen limitation may occur. Taken together, our results indicate that increased pollination and seed set is an alternative response to disturbance that should receive further consideration. Regardless of the exact mechanism, it appears that in the absence of frequent small-scale soil disturbance, M. angustatus would not persist at our field site, as a large majority of plant recruitment and seed set occurs on recent disturbances.