Chase’s (2007) model of community assembly predicts that communities experiencing high stress will show lower species diversity and greater similarity than communities having low stress. The model predicts that stress gradients will cause communities to converge on a small number of stress-tolerant species, which will be similar among high stress sites. To test this, we studied lichen communities on rocky shorelines of the Boundary Water Canoe Area Wilderness in northern Minnesota. We hypothesized that wave action and ice formation near the shore establishes a stress gradient that attenuates with increasing distance from the shoreline. Lichen communities were sampled at 22 lakeshore sites by placing a 900 cm2 quadrat 1, 2 and 3 m from the shoreline, which approximated areas of high, medium and low stress, respectively.
Shoreline lichen assemblages showed a similar ranking of morphologies in terms of percent cover, such that crustose > foliose > fruticose species. However, with increasing distance from the shoreline the relative proportion of foliose species decreased as the proportion of fruticose species increased, while crustose cover remained constant across the gradient. We found that lichen species richness was lowest 1 m distant from shorelines (p < 0.002) and did not differ between 2 and 3 m. Community similarity was lowest 1 m from shorelines, but greatest at 2 m, which runs contrary to Chase’s (2007) model. The data suggest that wave and ice action near shorelines may constitute a disturbance that stochastically eliminates species rather than an environmental stressor that selects for stress-tolerant ones.