1. Environmental stress model of community organization

2. Relative importance Environmental stress competition predation stress alleviation associational defenses Bruno, Stachowicz & Bertness (2003) Environmental stress model diversity of basal species

3. Overall richness / diversity Bruno, Stachowicz & Bertness (2003) Trends in Ecology and Evolution 18: 119-125 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

4. Vertical gradient Desiccation - Temperature Irradiance - Osmotic potential Horizontal gradient Wave exposure - Ice scour Test of ESM on the NW Atlantic coast

5. Nova Scotia New England - Gulf of St. Lawrence coast - Atlantic coast Nova Scotia

6. 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)

7. 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)

9. 38 seaweeds - 29 invertebrates Results

10. Gulf of St. Lawrence Atlantic  2 = 41 %  2 = 39 % Means ± SE Elevation Results - Richness across Elevation Elevation

11. Wave - Ice Exposure Wave Exposure  2 = 10 %  2 = 12 % Results - Richness across Exposure Gulf of St. Lawrence Atlantic Means ± SE

12. Test of ESM on the NE Atlantic coast

13. Helgoland's elevation gradient (wave-sheltered shores) Test of ESM on the NE Atlantic coast (Helgoland Island)

14. Overall richness and diversity (Helgoland Island) Zwerschke, Bollen, Molis & Scrosati (2013) Helgoland Marine Research 67: 663-674

15. Environmental stress model (ESM) Menge & Sutherland (1987, American Naturalist 130: 730-757) RICHNESS Predation HypothesisIntermediate Disturbance Hypothesis SESSILE SPECIES (PRIMARY SPACE HOLDERS)