It is well known that small-scale disturbances such as gopher mounds can influence plant community composition. Effects of gopher mounds on particular plant species could be mediated by abiotic factors, release from competition with other plants, or interactions with herbivores, disease or pollinators. Although there is a wealth of literature on the importance of disturbance for plant communities, there are few studies addressing mechanisms driving the responses of individual plant species to these disturbances, and fewer still that consider alternative mechanisms through which disturbance may act on a particular plant species. We used several approaches in the field to explore how a rare endemic plant in N. California (Mimulus angustatus) is influenced by gopher mound disturbances. We followed M. angustatus density and flower size and the density of likely competitors on experimental and natural soil disturbances across years since disturbance, we measured pollen receipt for naturally occurring M. angustatus with different levels of disturbance, and we conducted a field pollination experiment to determine the importance of pollination for M. angustatus reproduction.
We found that M. angustatus density was strongly positively affected by gopher mounds, but this effect was not mediated by competition with other plant species. Competitors were also influenced by gopher disturbance, but competitor densities were not related to M. angustatus density. We also found that M. angustatus density and flower size both declined with time since disturbance; M. angustatus density was near zero by three years post disturbance and flower size declined more than 25% in two years. Mimulus angustatus plants had more pollen on their stigmas when their flowers were large, and when there were more Mimulus flowers in the immediate neighborhood, both of which occur with more disturbance. Mimulus angustatus also benefits from outcrossed pollen (sets more seeds when pollinated). Taken together these results suggest that gopher mounds benefit M. angustatus recruitment not through the commonly considered competition pathway, but through increased pollination along with abiotic factors not measured in this study. Understanding the effects of small-scale disturbances on the population growth of individual plant species may thus require considering pollination and other biotic interactions besides competition.