Patterns and effects of disturbance in Caribbean macrophyte communities

This thesis examines a number of natural and anthropogenic disturbances within marine macrophyte habitats of the Caribbean. Understanding the effects of disturbance and the patterns associated with such dynamics is fundamental to ecological studies. Dynamics of interest included: interactions between populations; interactions between life history strategies; successional regimes; and alterations of community structure including loss of trophic heterogeneity and the possibility of "alternate" states. First I explored natural physical disturbance and succession. The dominance of macroalgae in the mid-shore, between areas of seagrass, challenged "classic" succession in such communities. I therefore proposed a model that included chronic "stress" by wave energy that could lead to a reversal in the climax state. Next, I investigated the importance of other grazers (i.e. trophic heterogeneity) in mediating the strength of trophic cascades (e.g. overgrazing). The enclosure experiments used suggested that different life history strategies respond differently to experimental conditions and that interference competition between specialist (conch) and generalist (urchins) grazers results in urchins switching to alternate resources and displaying lower condition. This dynamic may indirectly "buffer" the community against population expansions of urchins and overgrazing of diversity enhancing detritus. Under high nutrient enrichment, urchins maintained themselves, the trophic cascade and low diversity by switching to "expanded" autochthonous and "new" allochthonous resources. I continued to examine the effects of increasing nutrient enrichment, which correlated well with increasing human density, by examining eleven seagrass beds. The patterns of increasing consumer density and decreasing consumer diversity corresponded well to increasing enrichment and loss of autochthonous detritus. At high levels of enrichment, the community was dominated (> 90%) by stress tolerant/competitive producers (Thalassia testudinum) and generalist echinoids (> 98%). This again was thought possible through a subsidy to opportunistic producers. Finally, I set out to understand the nature of the proposed subsidy using stable isotopes (13C, 15N). The general pattern indicated phytoplankton, and associated detritus, increased with enrichment substantiated by significant shifts in delta13C of consumers and sediment organic material (SOM) to lighter carbon sources and change in the contribution of particulate organic material to SOM mixture from 7% to 44%. Although overgrazing of macrophyte dominated systems has often been linked to predator and competition release of urchins due to over-fishing, the contribution of nutrient enrichment should also be considered.