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自下而上调节群落的典型例子，群落由相对难吃的植物主导，受物理条件和营养供应的控制。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，PI发现常见的沼泽蜗牛Littoraria能够对沼泽植物施加强大的自上而下的控制。除非受到捕食者的控制，否则这种蜗牛可以将一个高生产力的生态系统转变为贫瘠的泥滩。在过去的十年里，美国东南部的大片盐沼(100‘S平方公里)经历了前所未有的死亡。从2002年至2006年，对20个死亡区的调查显示，死亡区边界的钉螺密度超过每平方米1000只。在GA、FL和LA的排除实验中，蜗牛积极地将盐沼转变为贫瘠的泥滩。这些发现和过去十年间蓝蟹密度的同时急剧下降(20%-80%)表明，在小规模实验中发现的级联消费者效应可能适用于更大的营养级联，这是导致南部盐沼死亡的原因之一。然而，还没有关于商业上重要的蓝蟹如何通过抑制蜗牛密度来调节沼泽死亡的研究。这一知识差距将通过以下问题得到解决：1)蓝蟹在调节蜗牛种群方面发挥了什么作用？这个问题将通过蓝蟹包含和排除的多地点实验来评估。2)沼泽景观中蓝蟹密度的空间差异是否决定了捕食、随后的食草动物暴发和低蟹密度地区沼泽死亡的空间差异？为了评估这个问题，广泛的调查(20个地点)将确定青蟹和蜗牛密度、放牧强度和植物生物量之间的关系。3)在近代历史上，青蟹数量的下降是否通过释放植物食草性蜗牛而间接导致湿地死亡？这个问题将使用一种相关的方法进行评估，使用长期(~30年)蓝蟹收成、蜗牛密度和存档螺壳上疤痕发生率的数据(FWS，LTER)。智力价值：这项研究将为盐沼控制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。有证据表明，特定物种、更高营养水平的植物生长控制和最近的群落死亡将迫使生态学家重新评估盐沼生态中已有60年历史的自下而上的模式。广泛的影响：沼泽死亡的后果对人类影响深远，因为沼泽草提供许多基本服务(例如，保护海岸线)。这项拟议研究的结果将使管理者能够预测捕食者(即蓝蟹)枯竭的影响，并制定有效的湿地保护战略(例如减少捕捞)。为了促进利益相关者之间就消费者在沼泽死亡中的作用进行沟通，结果将在一系列研讨会和问答会议上向主要利益攸关方介绍：公众、渔民和沼泽管理者(TNC，FWS)。此外，教育和外展活动将与高中教师合作，通过1)实地课程、2)研究和3)海洋专题讨论会(例如，与以前的UF 2008一样)，吸引在实践领域科学和潜在职业机会方面出类拔萃(童子军)和有风险(成绩/出勤率较低)的学生。

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).