Salt marshes: biological controls of food webs in a diminishing environment

This essay reviews two important topics in coastal ecology: the work on the relative role of bottom–up and top–down controls in natural communities and the loss of wetlands worldwide. In salt marshes and other coastal wetlands, bottom–up and top–down mechanisms of control on natural communities are pervasive. Bottom–up effects through nutrient supply may propagate to upper trophic levels via better food quality, or indirectly by altering water and sediment quality. Top–down control by consumers alters lower trophic levels through consumption of primary producers, and indirectly by trophic cascades in which higher predators feed on grazers. The combined forcing of bottom–up and top–down controls govern assemblages of species in natural communities, mediated by physical and biogeochemical factors. Although there is much information about biological controls of coastal food webs, more information is needed. Even more important is that large losses of wetland are occurring along coastlines worldwide due to a variety of economic and social activities including filling, wetland reclamation, and sediment interception. Such losses are of concern because these wetlands provide important functions, including export of energy-rich material to deeper waters, nursery and stock habitats, shoreline stabilization, and intercept land-derived nutrients and contaminants. These important functions justify conservation and restoration efforts; barring such efforts, we will find it increasingly difficult to find coastal wetlands where we can continue to gain further understanding of ecology and biogeochemistry and lack the aesthetic pleasure these wetlands provide to so many of us.