Influence of acute and chronic disturbance on macrophyte landscape zonation

摘要

Although the roles of physical disturbance and successional recovery from such disturbances in structuring natural communities are well known, recent studies have begun to uncover the potential for alternate outcomes or climax states in a number of systems. Here, we examine the distribution of tropical macrophytes at a site with heavy wave exposure and explore equilibrium (micro-habitat) and non-equilibrium (patch-dynamic) hypotheses to explain the observed pattern. The existence of a large and distinct zone of prolific macroalgae, undescribed 30 yr earlier, situated between 2 zones with a relatively high abundance of seagrass, challenges the classic successional regime within Caribbean macrophyte beds. Significantly smaller mean size and lower frequency of acute disturbances within the macroalgal-dominated zone, as compared to the outer, mixed macrophyte zone, appears to contradict the highly disturbed environment of classic colonizers. The dominance of macroalgae in the mid-shore zone may be enhanced by the presence of large sediment sizes that are accumulated through the effect of chronic wave stress. Potential causes of overall changes in the hydrodynamic forces at the study site include rise in sea level, increased wave exposure and associated erosion of reefs protecting the coast. We propose a model of macrophyte bed development and distribution that includes the important chronic 'stress' category of wave energy and potential dominance by macroalgae. Increased average wave energy can impose an elevated level of stress leading to modified patterns of macrophyte distributions and changes in the deterministic endpoint of the successional sequence.