Environmental stress model of community organization Relative importance Environmental stress competition predation stress alleviation associational defenses Bruno, Stachowicz & Bertness (2003) Environmental stress model diversity of basal species Overall richness / diversity Bruno, Stachowicz & Bertness (2003) Trends in Ecology and Evolution 18: Scrosati & Heaven (2007) Marine Ecology Progress Series 342: 1-14 Richness of secondary space holders (sessile and mobile associated species) Richness of primary space holders Vertical gradient Desiccation - Temperature Irradiance - Osmotic potential Horizontal gradient Wave exposure - Ice scour Test of ESM on the NW Atlantic coast Nova Scotia New England - Gulf of St. Lawrence coast - Atlantic coast Nova Scotia Methods Environmental stress gradients Vertical gradient (3 levels) High - Mid - Low intertidal Upper limit determined using barnacles Horizontal gradient (2-3 levels) Gulf (wave + ice): Sheltered - Exposed Atlantic (wave): Sheltered - Intermediate - Exposed dynamometers (wave exposure) cages (ice scour) Richness Sr of species Diversity (Simpson's index) 1 - D = 1 - (p i ) Evenness (Simpson's index) E = (D * S) -1 Methods Richness, Diversity & Evenness Summer Abundance of all seaweeds and invertebrates (% cover) 25 cm x 25 cm quadrats (n = 20 per Elevation x Exposure combination) 38 seaweeds - 29 invertebrates Results Gulf of St. Lawrence Atlantic 2 = 41 % 2 = 39 % Means SE Elevation Results - Richness across Elevation Elevation Wave - Ice Exposure Wave Exposure 2 = 10 % 2 = 12 % Results - Richness across Exposure Gulf of St. Lawrence Atlantic Means SE Test of ESM on the NE Atlantic coast Helgoland's elevation gradient (wave-sheltered shores) Test of ESM on the NE Atlantic coast (Helgoland Island) Overall richness and diversity (Helgoland Island) Zwerschke, Bollen, Molis & Scrosati (2013) Helgoland Marine Research 67: Environmental stress model (ESM) Menge & Sutherland (1987, American Naturalist 130: ) RICHNESS Predation HypothesisIntermediate Disturbance Hypothesis SESSILE SPECIES (PRIMARY SPACE HOLDERS)