Positive feedback between large-scale disturbance and density-dependent grazing decreases resilience of a kelp bed ecosystem

We examined how large-scale disturbances that defoliate kelp beds (outbreaks of an invasive bryozoan, hurricanes) alter local-scale grazing dynamics of an abundant herbivore, the gastropod Lacuna vincta, on the Atlantic coast of Nova Scotia, Canada. From field observations and a 5 wk kelp-thinning experiment that simulated disturbance, we found that snail density and grazing intensity on the kelp Saccharina latissima increased non-linearly with decreasing kelp biomass, as it varied within a site. Grazing intensity on S. latissima also increased non-linearly with decreasing standing kelp biomass across 5 sites spanning 40 km (linear distance) of coast and 2 yr, but we did not find strong support for this relationship for the kelp Laminaria digitata. Intensification of grazing augments the indirect effect of L. vincta on S. latissima (increased blade erosion and fragmentation), and drives it beyond a threshold for further losses of kelp biomass with subsequent storms. This positive feedback between large-scale disturbances and local-scale grazing could reinforce the depletion of kelp and facilitate the establishment of turf-forming algae on Nova Scotian rocky reefs. We conclude that interactions of large external perturbations with local natural perturbations must be considered to under stand how drivers of ecosystem change collectively disrupt the balance of top-down and bottom-up forces to cause shifts to unexpected community states.