THE ROLE OF GROWTH IN MAINTAINING SPATIAL DOMINANCE BY MUSSELS (MYTILUS-EDULIS)

Spatial dominance is maintained by the balance of three processes: mortality, recruitment, and growth. Patches of mussels facing high rates of mortality may show no change in percent cover (i.e., dominance) as long as growth by the remaining survivors can fill in the vacant space. A graphical model is developed to examine the possibility that growth may ameliorate the effects of predation and physical disturbance on spatial dominance. In the model, growth, mortality, and recruitment are depicted as Vectors on a set of isoclines for log percent cover. The 100% isocline is analogous to the self-thinning line. Self-thinning data for the mussel Mytilus edulis are used to establish the log-percent isoclines. A review of published growth and mortality data suggests mussels can use growth to offset the effects of mortality. Small mussels can easily double in length in 1 yr, and this is sufficient to withstand 60% mortality per year without a loss in percent cover. Since both growth and mortality tend to be size specific, the point at which growth can no longer offset mortality depends on the environmental conditions that affect rates of growth and mortality. The model and analysis can be easily extended to other species and suggest that species with rapid growth and large variation in body size need not rely on recruitment to maintain spatial dominance, even when faced with high rates of predation.