Prior to examining Clathromorphum nereostratum bioerosion at each focal study site, we characterized the ecological status of the site by quantifying the abundance of its benthic constituents (i.e., kelps, understory algae, coralline algae, etc.) using the same methods that have been employed by us and others over the past 30 years (Estes et al. 2010). We characterized two types of sites: (1) those that have long persisted as urchin barrens (“habitat.type” = “Barren”) and (2) urchin barrens that are situated immediately adjacent to shallow, remnant kelp stands, and thereby receive urchin food subsidies (“habitat.type” = “Barren + kelp subsidy”). At these latter sites, we also surveyed the adjacent kelp stand (“habitat.type” = “Shallow kelp”).

At each site, a diver sampled twenty 0.25-m^2 quadrats at the target depth (21-37 feet) and along its contour, taking a random number of kicks between quadrats. Patches of unconsolidated substrate were rare on the rocky reef; however, if one was encountered while sampling, the diver took an additional random number of kicks and again deployed the quadrat. In each quadrat, s/he counted the stipe density of all kelps (by species) and estimated the abundance (percent cover) of fleshy red algae, sessile invertebrates, encrusting coralline algae (almost exclusively C. nereostratum), branching coralline algae, and other algae such as Desmarestia or Codium species. Percent cover was visually estimated on a scale of 1-6, where 1 = 0-5%, 2 = 6-25%, 3 = 26-50%, 4 = 51-75%, 5 = 76-95%, and 6 = 96-100 % cover.