Top-Down Control of Primary Production in Atlantic Salt Marshes

大西洋盐沼初级生产自上而下的控制了解消费者(自上而下的力量)和资源供应(自下而上的力量)在控制自然群落生产和分配格局中的相对重要性，长期以来一直是群落生态学的基本问题之一。许多水生群落被证明具有很强的自上而下的控制力，而大多数由维管植物主导的系统被认为是受自下而上的力控制的。盐沼通常被用作由维管植物主导的生态系统的教科书例子，维管植物受土壤因素和资源供应控制。在这里，我们提供的数据表明，蜗牛的放牧控制着大西洋西部盐沼中优势大型植物--互花米草的初级产量。这种蜗牛在从新泽西到墨西哥湾的沼泽中很常见，通常在与死亡和衰老的植物相关的中等沼泽海拔处发现密度较高的蜗牛。虽然人们一直认为这些地区代表着成片死亡的植物，吸引了蜗牛的有害动物，但我们的数据表明，蜗牛放牧可能会产生这些模式，因此这些沼泽可能处于自上而下的控制之下。在这项研究中，我们将检验沼泽蜗牛Littoraria可以对美国东南部沼泽上的互花米草的初级产量施加自上而下的控制的一般假设。特别是，我们考察了我们对佐治亚州沼泽的初步结果的普遍性，在那里，沼泽由自下而上的力量控制的范例最初是在30多年前发展起来的。具体地说，我们建议检验以下假设，以阐明这些系统中放牧控制的原因和后果。1)东南部沼泽地上，蜗牛放牧的自上而下控制发生在哪里？我们将检验这样一种假设，即足以控制线草产量的蜗牛密度出现在整个沼泽地的高幼虫传播量和捕食者避难所重合的地方。2)在东南部沼泽地，通过蜗牛放牧来自上而下控制的可能性在哪里？我们将检验这样一种假设，即在沼泽地貌的任何地方，放牧控制线草生产的可能性都是可能的，即可以维持高蜗牛密度。3)东南部盐沼中线草的高产量是营养级联的结果，在那里，包括螃蟹、鱼和海龟在内的蜗牛捕食者限制了能够限制线草生产的食草性蜗牛的密度。这项研究的结果可能会改变我们对北美最主要的海岸线生态系统之一的看法。如果这些沼泽地像假设的那样潜在地由消费者控制，那么我们如何从生态和进化的角度来管理、恢复和思考这些社区，将需要重新考虑。

Top?Down Control of Primary Production in Atlantic Salt MarshesUnderstanding 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.