Population regulation of sea urchins in a rocky subtidal ecosystem

Abstract

Herbivory by sea urchins is an important control of seaweed biomass worldwide. For my doctoral thesis, I investigate biological processes that govern the dynamics of a kelp bed ecosystem on the Atlantic coast of Nova Scotia: increases in sea urchin (Strongylocentrotus droebachiensis) density that trigger a phase shift to urchin barrens, and an amoebic disease of sea urchins that triggers the reverse shift to kelp beds. I demonstrate experimentally that a phase shift from kelp beds to barrens can occur through formation of destructive grazing aggregations of sea urchins within kelp beds. However, recurrent outbreaks of disease are preventing the establishment of urchin grazing aggregations and stabilizing the kelp bed state. These epizootics have increased in frequency over the last 35 years in association with increasing strong storms and peak sea temperatures, conditions that may favour introduction of the pathogenic agent (Paramoeba invadens) from possible offshore source populations and its spread along the coast. P. invadens has remained functionally and physiologically stable over this period, suggesting that environmental change likely is responsible for an increase in disease. Predation on juvenile sea urchins by cancrid crabs may limit recovery of urchin populations within a kelp bed following disease outbreaks, and this is mediated in part by the availability of spatial refuges to juveniles, such as spaces within the holdfasts (anchoring structures) of kelp. Globally, a reduction in sea urchin grazing pressure due to epizootics has led to profound changes in the structure and functioning of coastal marine ecosystems, with shifts from sea urchin barrens to kelp beds in Nova Scotia and California, and shifts from coral- to macroalgal-dominance on reefs in the Caribbean and tropical western Atlantic. My research underscores the importance of longitudinal studies to monitor changes in the frequency and extent of sea urchin epizootics, environmental correlates that may explain these events, and the attendant impacts of sea urchin die-offs on the ecology of coastal ecosystems.