2.2. Patterns of urchin abundance inside and outside C. filiformis

At each of the three locations, in water between 0 and 2 m, we compared the abundance of H. erythrogramma and the number of homing scars on rocky platforms within patches of C. filiformis to those in surrounding algal habitats (“outside”), which consisted primarily of geniculate coralline algae and sparsely distributed browns such as Sargassum spp. and E. radiata. Patches of C. filiformis were a minimum of ~3 m × 3 m and separated by at least 10 m. Within C. filiformis patches, urchins and the number of homing scars were also compared between two positions (“inside” and “edge”). Edges were defined as areas within 1 m of the border of a patch of C. filiformis, while “inside” areas were in central section of patches, >1.5 m from the border. All the urchins found were sitting in self‐bored homing scars or rock crevices. Mainly one large monospecific stand of C. filiformis dominates the locations with presence of secondary smaller patches. Abundances of H. erythrogramma and homing scars were counted in n = 5 replicate quadrats (N = 1 per patch; 50 × 50 cm) in each position with respect to patches (inside, edge and outside) at each location. Quadrats were haphazardly placed within patch positions. Quadrats were carefully searched for urchins and homing scars, which involved removing dense vegetation and, in some cases, feeling through sediment among C. filiformis to count the number of homing scars. It is possible that some homing scars were missed among dense C. filiformis.