Nutrient Regimes Determine the Strength of Herbivore-Mediated Stabilizing Feedbacks in Barrens

The last few decades have seen a rapid spread of barrens on temperate rocky reefs as sea urchin overgrazing of marine forests causes habitats to shift to more depauperate stable states. Reversing these trends requires a proper understanding of the novel ecological feedbacks that maintain rocky barrens and the conditions under which they operate. In this study, we explored the role of a secondary herbivore (a grazing limpet) in reinforcing the stability of barrens formed under different nutrient conditions. Combining comparative and field experimental studies in two Mediterranean regions characterized by contrasting nutrient regimes, we assessed: (i) if the creation of barren areas by sea urchins enhances limpet abundance, (ii) the grazing impact by limpets through foraging marks (halos), and (iii) the ability of limpets to maintain barrens in the absence of sea urchins. Our results show that sea urchin overgrazing of erect macroalgae enhanced limpet abundance in both nutrient regimes. The effects of limpet grazing on macroalgal assemblages varied with nutrient regimes being up to six times more intense under oligotrophic conditions. Finally, we found that limpets were able to maintain barrens in the absence of the sea urchins only under low nutrient regimes. Our results suggest greater vulnerability of subtidal forests to overgrazing in oligotrophic regions and demonstrate the importance of environmental settings in regulating ecosystem feedbacks mediated by plant-herbivore interactions