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

Spatial dominance is maintained by the balance of three processes: mor tality, 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 growt h may ameliorate the effects of predation and physical disturbance on spatial dominance. In the model, growth, mortality, and recruitment ar e depicted as Vectors on a set of isoclines for log percent cover. The 100% isocline is analogous to the self-thinning line. Self-thinning d ata for the mussel Mytilus edulis are used to establish the log-percen t isoclines. A review of published growth and mortality data suggests mussels can use growth to offset the effects of mortality. Small musse ls can easily double in length in 1 yr, and this is sufficient to with stand 60% mortality per year without a loss in percent cover. Since bo th growth and mortality tend to be size specific, the point at which g rowth can no longer offset mortality depends on the environmental cond itions that affect rates of growth and mortality. The model and analys is can be easily extended to other species and suggest that species wi th rapid growth and large variation in body size need not rely on recr uitment to maintain spatial dominance, even when faced with high rates of predation.