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

生态学的一个基本目标是了解控制自然群落中植物生产和分布模式的物理和生物过程。60多年来，盐沼植物群落一直被认为是一个由物理条件和营养供应控制的相对难吃植物主导的自下而上调节群落的典型例子。然而，最近在美国东南部盐沼的研究挑战了这一范式，并表明强大的营养相互作用是同样重要的机制。在弗吉尼亚州、路易斯安那州和佐治亚州的盐沼中，PI发现常见的沼泽蜗牛Littoraria能够对沼泽植物施加强有力的自上而下的控制。除非受到捕食者的控制，否则这种蜗牛会把一个高产的生态系统变成贫瘠的泥滩。在过去的十年里，美国东南部大片的盐沼（100平方公里）经历了前所未有的死亡。2002-2006年对20个死区进行调查，发现死区边界蜗牛密度超过1000只/m2。在GA、FL和LA的排除试验中，蜗牛积极地将盐沼转化为贫瘠的泥滩。这些发现和过去十年蓝蟹密度的急剧下降（20-80%）表明，小规模实验中发现的级联消费者效应可能适用于导致南部盐沼死亡的更大的营养级联。然而，目前还没有研究表明，具有重要商业价值的蓝蟹是如何通过抑制蜗牛密度来调节沼泽死亡的。这一知识差距将通过以下问题来解决：1)蓝蟹在调节蜗牛种群中的作用是什么？这个问题将通过蓝蟹包含物和排除物的多地点实验来评估。2)沼泽景观中蓝蟹密度的空间变化是否决定了低密度地区蓝蟹捕食、随后的食草动物爆发和沼泽死亡的空间变化？为了评估这一问题，将进行大规模调查（20个站点），以确定蓝蟹和蜗牛密度、放牧强度和植物生物量之间的关系。3)在近代史上，蓝蟹数量的减少是否间接导致了沼泽的死亡？这个问题将使用相关方法进行评估，使用长期（~30年）的数据（FWS， LTER）集，包括蓝蟹收成、蜗牛密度和存档蜗牛壳上疤痕的发生率。智力优势：这项研究将为控制盐沼提供一种新的、自上而下的机制，并揭示蓝蟹是否是营养级联中的关键捕食者。物种特异性、更高营养水平控制植物生长和近期群落死亡的证据将迫使生态学家重新评估60年来盐沼生态学中自下而上的范式。更广泛的影响：沼泽死亡对人类的影响是深远的，因为沼泽草提供了许多基本服务（例如，海岸线保护）。这项拟议研究的结果将使管理人员能够预测捕食者（即蓝蟹）枯竭的影响，并制定有效的沼泽保护战略（例如减少捕鱼）。为了促进利益攸关方之间就消费者在沼泽死亡中的作用进行沟通，将在一系列研讨会和问答会议上向主要利益攸关方：公众、渔民和沼泽管理者（TNC， FWS）介绍结果。此外，与高中教师合作的教育外展活动将通过以下方式吸引在实地科学和潜在职业机会方面表现优异（童子军）和有风险（成绩低/出勤率低）的学生：1)实地课程，2)研究和3)海洋研讨会（例如，如以前在UF 2008）。

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).