Experimental tests of the bacterial distance-decay relationship.

Community similarity declines with increasing geographic distance if species tend to be locally adapted or if they are dispersal limited. The distance-decay of similarity has been shown for bacteria previously, but distinguishing between these competing mechanisms is difficult from observational surveys. I found little evidence of a relationship between geographic distance and similarity in community composition in an aquatic bacterial community. When bacterial colonization occurred into initially identical sterile microcosms across a woodland, a strong distance-decay relationship was observed after 28 days, implying that dispersal limitation created a strong pattern in these communities in the absence of environmental differences. This conclusion was not supported by the results of a reciprocal transplant experiment. When microcosms at the extremities of the study area were transplanted to the opposite end of the study area, the communities converged on the community composition at the site to which they were transplanted. This convergence did not depend on whether colonization into the microcosms was prevented, implying a minor role for dispersal limitation over these spatial and temporal scales. Additional manipulations of colonization rates were consistent with the hypothesis that dispersal limitation structured these communities over short time scales (a few days), but that dispersal limitation had a minor role over longer time scales (>7 days).