Community richness, diversity, and evenness across rocky intertidal environmental stress gradients in Atlantic Canada

An environmental stress model (ESM) developed by Menge and Sutherland and improved by other researchers predicts how local-scale richness and diversity (used interchangeably) should vary with environmental stress. We tested model predictions by surveying all benthic producers and consumers across vertical (elevation) and horizontal (wave/ice exposure) stress gradients in rocky intertidal habitats from the Gulf of St. Lawrence (which freezes in winter) and open Atlantic (which does not freeze) coasts of Nova Scotia, Canada. Since local winter conditions are harsher than on most temperate shores studied previously, we made predictions for an intermediate-to-high range of stress: richness and diversity would be lowest in highly stressful habitats and would increase with decreasing stress. Results matched predictions across vertical gradients on both coasts (richness and diversity were negatively related to elevation) and across horizontal gradients on the Gulf coast (richness and diversity were negatively related to wave/ice exposure) but not entirely on the Atlantic coast (richness was negatively related to wave exposure, but diversity showed an opposite trend). The spatial changes in evenness explained such differing trends in richness and diversity. Richness and diversity were higher on the Atlantic than on the Gulf coast, consistent with the stronger physical stress (winter ice scour) on the Gulf coast. Our study indicates that richness and diversity may respond differently to local-scale environmental stress, contrary to common assumptions. To determine the conditions under which such differences might occur, future ESM studies should investigate both variables and also evenness, which together with richness determines diversity.