Biogeographical variation in the distribution, abundance, and interactions among key species on rocky reefs of the northeast Pacific

Abstract

Macroecological research over large latitudinal gradients can reveal broad-scale patterns that provide context for local-scale studies and insight into relevant scales of variation in community structure. Grazing by sea urchins is one of numerous physical and biotic factors that leads to geographic variation in community structure on rocky reefs worldwide. We describe patterns of abundance, distribution, and trophic interactions of urchins and their predators on subtidal rocky reefs across much of their range in the northeast Pacific Ocean. Surveys and tethering experiments were conducted between Point Conception, California, USA, and Bahía Asunción, Baja California Sur, México, including both fished areas and marine protected areas (MPAs). Variability in abundance was greatest at the smallest spatial scales (transects and sites) rather than the largest spatial scale (regions); however, we found correlative evidence of region-wide top-down control in which higher densities of predators (primarily spiny lobsters) led to increased abundances of kelp Macrocystis pyrifera. Similarly, urchin survival varied regionally, with survival rates increasing from south to north. The effect of MPAs was less clear: species abundances varied between individual MPAs, but there were no clear differences in urchin survival between MPAs and fished areas. Community structure of rocky reefs in this region varies substantially among sites and appears to be driven in some locations by predators and in others by factors that obscure the importance of trophic interactions. Our study provides empirical evidence of the variability in top-down forcing in these communities and cautions against making broad generalizations based on inferences from local-scale studies.