DISSERTATION RESEARCH The Mediating Effects of Food Web Structure and Habitat Complexity on the Occurrence of Terrestrial Trophic Cascades

摘要食物网结构、野生捕食和栖息地复杂性通常被认为是自上而下效应的重要媒介，但很少有实验研究专门测试这些因素对营养级联发生的影响（天敌对食草动物的抑制导致初级产量的增加）。复杂的食物网，比如那些在野外捕食的食物网，可能会排除营养级联，因为敌人的影响会通过物种相互作用的网状网络减弱。然而，栖息地的复杂性可能为捕食者提供了避难所，从而简化了食物网，增强了自上而下效应的强度。通过操纵中观实验，我们提出：1)确定食物网复杂性（捕食者物种丰富度和捕食者功能群组成）对营养级联发生的影响；2)评估栖息地复杂性在减少捕食者之间网状相互作用和增强自上而下效应强度中的作用。通过确定这些因素如何影响自上而下效应的强度，可以对可能发生级联的系统的组成进行概括。这种预测能力对生物防治（在农业环境中引发营养级联）和物种保护（在物种丧失或入侵后最大限度地减少对整个社区的影响）都有更广泛的影响。

ABSTRACTFood-web structure, intraguild predation, and habitat complexity have often been implicated as important mediators of top-down effects, yet little experimental work has been done to specifically test the impact of these factors on the occurrence of trophic cascades (natural-enemy suppression of herbivores which results in enhanced primary production). Complex food webs, such as those with intraguild predation, may preclude trophic cascades because enemy impacts attenuate through a reticulate network of species interactions. However, habitat complexity may provide refuges for predators from intraguild predation, thereby simplifying the food web and enhancing the strength of top-down effects. By conducting manipulative mesocosm experiments, we propose to 1) determine the effects of food web complexity (predator species richness and predator functional group composition) on the occurrence of trophic cascades and 2) assess the role of habitat complexity in reducing reticulate interactions among predators and enhancing the strength of top-down effects. By determining how these factors influence the strength of top-down effects, generalizations can be made about the composition of systems in which cascades may occur. This predictive ability has broader impacts for both biological control (eliciting a trophic cascade in an agricultural setting) and species conservation (minimizing community-wide impacts following species loss or invasion).