Data from: Indirect effects of predators control herbivore richness and abundance in a benthic eelgrass (Zostera marina) mesograzer community

1. Herbivore communities can be sensitive to changes in predator pressure (top-down effects) and resource availability (bottom-up effects) in a wide range of systems. However, it remains unclear whether such top-down and bottom-up effects reflect direct impacts of predators and/or resources on herbivores, or are indirect, reflecting altered interactions among herbivore species. 2. We quantified direct and indirect effects of bottom-up and top-down processes on an eelgrass (Zostera marina) herbivore assemblage. In a field experiment, we factorially manipulated water column nutrients (with Osmocote™ slow-release fertilizer) and predation pressure (with predator exclusion cages) and measured the effects on herbivore abundance, richness and beta diversity. We examined likely mechanisms of community responses by statistically exploring the response of individual herbivore species to trophic manipulations. 3. Predators increased herbivore richness and total abundance, in both cases through indirect shifts in community composition. Increases in richness occurred through predator suppression of common gammarid amphipod species (Monocorophium acherusicum and Photis brevipes), permitting the inclusion of rarer gammarid species (Aoroides columbiae and Pontogeneia rostrata). Increased total herbivore abundance reflected increased abundance of a caprellid amphipod species (Caprella sp.), concurrent with declines in the abundance of other common species. Furthermore, predators decreased beta diversity by decreasing variability in Caprella sp. abundance among habitat patches. 4. Osmocote™ fertilization increased nutrient concentrations locally, but nutrients dissipated to background levels within 3 m of the fertilizer. Nutrient addition weakly affected the herbivore assemblage, not affecting richness and increasing total abundance by increasing one herbivore species (Caprella sp.). Nutrient addition did not affect beta diversity. 5. We demonstrated that assemblage-level effects of trophic manipulations on community structure are the result of distinct and often indirect responses of herbivore species. These results underscore the importance of understanding herbivore–herbivore interactions in a system commonly subjected to both eutrophication and overfishing.1. 在众多生态系统中，草食动物群落对捕食压力变化（下行效应，top-down effects）与资源可获得性变化（上行效应，bottom-up effects）均较为敏感。然而，目前仍不清楚这类下行与上行效应究竟是捕食者和/或资源对草食动物的直接影响，还是间接通过改变草食动物物种间的相互作用所介导的间接效应。 2. 本研究量化了上行与下行过程对鳗草（Zostera marina）生境中草食动物类群的直接与间接效应。我们通过野外实验，采用双因素操控设计，分别操控水体营养盐水平（施加奥绿（Osmocote™）缓释肥）与捕食压力（使用捕食者排除笼），并测定其对草食动物丰度、物种丰富度及β多样性（beta diversity）的影响。此外，我们通过统计分析单个草食动物物种对营养级操控的响应，探究了群落响应的潜在机制。 3. 捕食者提升了草食动物的物种丰富度与总丰度，且二者均通过间接改变群落组成实现。物种丰富度的提升源于捕食者抑制了常见钩虾端足类物种（Monocorophium acherusicum与Photis brevipes），从而为稀有钩虾物种（Aoroides columbiae与Pontogeneia rostrata）的定殖创造了空间。总草食动物丰度的增加则源于某一麦秆虫端足类物种（Caprella sp.）的丰度提升，同时伴随其他常见物种丰度的下降。此外，捕食者通过降低不同生境斑块间Caprella sp.丰度的变异程度，降低了群落的β多样性。 4. 奥绿（Osmocote™）施肥可局部提升水体营养盐浓度，但施加点3米范围内的营养盐会在短期内回落至背景水平。营养盐添加对草食动物类群的影响较弱：未改变其物种丰富度，仅通过提升某一草食动物物种（Caprella sp.）的丰度来增加总丰度。此外，营养盐添加未对群落β多样性产生显著影响。 5. 本研究证实，营养级操控对群落结构的类群水平效应，实则是草食动物物种各自独特且多为间接的响应共同介导的结果。本研究结果凸显了在同时面临富营养化与过度捕捞压力的生态系统中，理解草食动物物种间相互作用的重要性。