Density vs. Trait-Mediated Interactions Between Predators and Prey: Their Influence on Rocky Shore Algal Diversity and Community Structure

营养级联的概念深刻地提高了我们对捕食者-猎物相互作用如何塑造生态群落的结构和多样性的理解。营养级联研究通常集中在捕食者对猎物密度的影响如何通过食物链传播来塑造物种多样性和群落结构(即密度中介的间接相互作用)。最近，人们对相互作用的改变越来越感兴趣：系统中第三个物种(捕食者)的存在改变了其他两个物种(猎物及其资源)之间的相互作用。特征介导的间接相互作用(TMII)是由于捕食者对猎物特征的影响而不是对猎物密度的影响而发生的。例如，在水生系统中，水媒捕食者的风险线索可以诱导猎物行为的变化，如摄食率，从而改变猎物对较低营养水平的影响。因此，TMII强调非致命的捕食者效应，而不是更普遍的强调致命的间接影响(DMII)。尽管生态学家开始意识到TMII的重要性，但对密度和特征介导的捕食者效应的相对重要性以及它们如何塑造生态群落的多样性和结构的因素仍知之甚少。关于食草性蜗牛对岩石海岸大型藻类群落的多样性和结构的重要性的经典研究给人留下了持久的印象。Luchenco证明，这些群落的多样性和演替与钉螺(Littorina Littorea)的密度密切相关，这种关系的功能性质在潮汐池塘和新生底质之间是不同的。此外，她还假设，青蟹(Carcinus Maenas)的捕食可能通过调节斜纹夜蛾的密度对藻类群落产生间接影响。这一假设在概念上很吸引人，但20多年过去了，它还没有得到实验验证。最近的数据表明，青蟹风险线索对蜗牛放牧的抑制可以对岩岸藻类群落产生重大影响。这项研究通过提出以下问题来考察青蟹和斜纹夜蛾之间的密度和特征介导的相互作用对岩质海岸藻类多样性和结构格局的相对重要性：什么因素决定了密度和特征中介效应的相对重要性？--ii.DMIIs和TMIIs的相对重要性如何影响岩石潮间带海岸长期的藻类多样性和群落结构？--iii.DMIIs和TMIIs相对重要性的不同生境能否解释两种突出的岩石海岸栖息地：浮生基质和潮汐池塘中藻类多样性和钉螺密度之间功能关系的差异？这项研究将对经验和理论生态学有广泛的影响，明确地检验更高级别的相互作用在塑造长期社区动态中所起的作用。通过在一个(A)被广泛认为是密度调节效应作用的经典示范和(B)对现代生态学的发展做出重大贡献的系统中工作，拟议的研究将提供对岩质海岸和其他群落多样性和结构形成过程的更好理解。由于这些食物链相对简单和容易操纵，这些栖息地为探索这些问题提供了一个极好的机会。

The trophic cascade concept has profoundly improved our understanding of how predator-prey interactions shape the structure and diversity of ecological communities. Trophic cascade studies typically focus on how predator effects on prey density propagate through food chains to shape species diversity and community structure (i.e., Density-Mediated Indirect Interactions). Recently, there has been growing interest in interaction modifications: the presence of a third species (predator) in the system alters the interaction between two other species (prey and their resource). Trait-Mediated Indirect Interactions (TMIIs) are those that occur because of predator effects on prey traits rather than prey density. For example, in aquatic systems water-borne predator risk cues can induce changes in prey behavior, such as feeding rates, thus altering the prey.s impact on lower trophic levels. Hence, TMIIs emphasize nonlethal predator effects in contrast to the more prevalent emphasis on lethal indirect effects (DMIIs). Although ecologists are beginning to appreciate the importance of TMIIs, there is still a limited understanding of the factors governing the relative importance of density and trait-mediated predator effects and how they shape the diversity and structure of ecological communities. Classic work on the importance of herbivorous snails to the diversity and structure of macro-algal communities on rocky shores has left a lasting impression. Luchenco demonstrated that diversity and succession in these communities is closely tied to snail (Littorina littorea) density and that the functional nature of this relationship differs between tidal pools and emergent substrata. Moreover, she hypothesized that green crab (Carcinus maenas) predation may exert an indirect influence on algal communities by regulating L. littorea density. This hypothesis is conceptually appealing, but over 20 years have passed without it being tested experimentally. Recent data suggest that the suppression of snail grazing by green crab risk cues can have a significant effect on rocky shore algal communities. This research examines the relative importance of density and trait-mediated interactions between green crabs and L. littorea to patterns of algal diversity and structure on rocky shores by asking the following questions: --I. What factors shape the relative importance of density and trait-mediated effects? -- II. How may the relative importance of DMIIs and TMIIs influence long-term algal diversity and community structure on rocky intertidal shores? -- III. Can habitat-specific differences in the relative importance of DMIIs and TMIIs explain differences in the functional relationship between algal diversity and snail density in two prominent rocky shore habitats: emergent substrata and tide pools? This research will have broad implications to empirical and theoretical ecology by explicitly examining the role higher order interactions play in shaping long term community dynamics. By working in a system (a) widely held as a classic demonstration of the role of density-mediated effects and (b) that has contributed significantly to the development of modern ecology, the proposed research will provide a better understanding of the processes shaping diversity and structure in rocky shore and other communities. Because these food chains are relatively simple and easily manipulated, these habitats afford an excellent opportunity to explore these issues.