Are blue crab declines leading to a trophic cascade and massive loss of U.S. southern marshes?

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A fundamental goal of ecology is to understand the physical and biological processes that control plant production and distribution patterns in natural communities. For over 6 decades, salt marsh plant assemblages have been considered a classic example of a bottom-up regulated community dominated by relatively unpalatable plants controlled by physical conditions and nutrient supplies. Recent research in southeastern U.S. salt marshes, however, challenges this paradigm and suggests that powerful trophic interactions are equally important mechanisms. In Virginia, Louisiana, and Georgia salt marshes, the PI found that the common marsh snail, Littoraria, is capable of exerting strong, top-down control of marsh plants. Unless kept in check by its predators, this snail can transform a highly productive ecosystem to barren mudflat.Over the past decade, large expanses of southeastern U.S. salt marshes (100's of km2) have experienced unprecedented die-back. From 2002-2006, surveys of 20 die-off areas revealed snail densities exceeding 1000/m2 on die-off borders. In exclusion experiments in GA, FL and LA, snails actively converted salt marshes to barren mudflats. These findings and the concurrent, steep decline (20-80%) of blue crab densities over the past decade suggest that cascading consumer effects discovered in small-scale experiments may apply to a larger trophic cascade that contributes to die-off in southern salt marshes. There have, however, been no studies of how commercially important blue crabs regulate marsh die-off via suppression of snail densities. This knowledge gap will be address with the following questions:1) What is the role of blue crabs in regulating snail populations? This question will be evaluate with multi-site experiments of both blue crab inclusions and exclusions.2) Does spatial variation in blue crab densities across marsh landscapes determine spatial variation in predation, subsequent grazer outbreaks, and marsh die-off in areas of low crab density? To evaluate this question, broad-scale surveys (20 sites) will identify relationships among blue crab and snail density, grazing intensity and plant biomass.3) Have blue crab population declines indirectly contributed to marsh die-off by releasing plant-grazing snails from top-down control in recent history? This question will be evaluated using a correlative approach, with long-term (~30 yrs) data (FWS, LTER) sets of blue crab harvests, snail densities, and incidence of scarring on archived snail shells.Intellectual Merit: This research would contribute a new, top-down mechanism for control of salt marshes, and reveal if blue crabs are keystone predators in the trophic cascade. Evidence for species-specific, higher trophic level control of plant growth and recent community die-off would compel ecologists to reevaluate the 60-yr-old bottom-up paradigm in salt marsh ecology.Broader Impacts: The consequences of marsh die-off are far-reaching for humans, since marsh grasses provide many essential services (e.g., shoreline protection). Results from this proposed research would allow managers to predict effects of predator (i.e., blue crabs) depletion and to formulate effective strategies for marsh conservation (e.g. reduced fishing). To facilitate inter-stake-holder-communication on the role of consumers in marsh die-off, results will be presented in a series of seminars and Q&A sessions to key stakeholders: the public, fishermen, and marsh managers (TNC, FWS). In addition, educational, outreach activities, in collaboration with high school teachers, will engage students that are excelling (boy scouts) and at-risk (low grades/attendance) in hands-on field science and potential career opportunities through: 1) field classes, 2) research and 3) a marine symposium (e.g., as previously at UF 2008).