Ecological Archives E088-053-A1

Julie C. Ellis, Myra J. Shulman, Megan Wood, Jon D. Witman, and Sara Lozyniak. 2007. Regulation of intertidal food webs by avian predators on New England rocky shores. Ecology 88:853–863.

Appendix A. Additional methods for the large-scale gull exclusion experiment as well as some photographs from the experiment.

Our statistical model examining C. borealis densities as the response variable included two random effects [island and site nested within island], three main fixed effects [tide (high, low); depth (0m, -1m); and the number of crab carapaces per low tide (continuous)], and second and third-order interaction terms. The initial analysis by GLIMMIX ANOVA showed that island and site(island) had no significant effect on C. borealis densities. Therefore, we excluded these factors and re-analyzed the data. There were no significant interaction effects among gull density, depth and tide. We excluded all interaction effects with a P > 0.2, leaving only four fixed effects in the final analysis: depth, tide, gull predation rate, and gull predation rate × depth.

Photos from the experiment: (a) C. borealis eating snail in tidepool, (b) C. borealis in algal canopy at low tide, (c) emergent C. borealis in algal canopy at low tide, and (d) emergent crabs.

(a) A1a
(b) A1b
(c) A1c

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