Top-Down Control of Primary Production in Atlantic Salt Marshes

Brown University, Providence RI

Investigators

Abstract

Top?Down Control of Primary Production in Atlantic Salt Marshes Understanding the relative importance of consumers (top?down forces) and resource supply (bottom?up forces) in controlling the production and distribution patterns in natural communities has long been one of the fundamental questions of community ecology. Many aquatic communities have been shown to have strong top?down control, while most systems dominated by vascular plants have been assumed to be controlled by bottom?up forces. Salt marshes are generally used as textbook examples of ecosystems dominated by vascular plants that are controlled by edaphic factors and resource supplies. Here we present data that suggests grazing by the snail Littoraria irrorata controls primary production of the dominant macrophyte in western Atlantic salt marshes, the cordgrass Spartina alterniflora. This snail is common in marshes from New Jersey to the Gulf of Mexico and often is found at high densities at intermediate marsh elevations associated with dying and senescing plants. While it has been assumed that these areas represent patches of dying plants that attract snail detritivores, our data suggest that snail grazing may generate these patterns and thus that these marshes are potentially under top?down control. In this study, we will examine the general hypothesis that the marsh snail Littoraria can exert top?down control over the primary production of Spartina alterniflora on marshes in the southeastern United States. In particular, we examine the generality of our preliminary results on Georgia marshes where the paradigm that marshes are controlled by bottom?up forces was originally developed over 30 years ago. Specifically, we propose to examine the following hypotheses to elucidate the causes and consequences of grazer control in these systems. 1) Where does top?down control by snail grazing occur on southeastern marshes? We will test the hypothesis that snail densities sufficient to control cordgrass production occur where high larval delivery and predator refuges coincide across marsh landscapes. 2) Where is top?down control by snail grazing possible in southeastern marshes? We will examine the hypothesis that grazer control of cordgrass production is possible anywhere on the marsh landscape that high snail densities can be maintained. 3) The high production of cordgrass in southeastern salt marshes is the consequence of a trophic cascade, where snail predators, including crabs, fish and turtles, limit the densities of grazing snails that are capable of limiting cordgrass production. The results of this study may change the way we view one of the most dominant shoreline ecosystems in North America. If these marshes are potentially consumer controlled, as hypothesized, how we manage, restore and think about these communities ecologically and evolutionarily will need to be rethought.