Amundrud Sarah L, Srivastava Diane S, O'Connor Mary I
Department of Zoology & Biodiversity Research Centre, University of British Columbia, 6270 University Blvd., Vancouver, BC, V6T 1Z4, Canada.
J Anim Ecol. 2015 Jul;84(4):1092-102. doi: 10.1111/1365-2656.12350. Epub 2015 Mar 3.
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. 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. 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. 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. 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.
在众多生态系统中，食草动物群落可能对捕食压力变化（自上而下的影响）和资源可利用性变化（自下而上的影响）较为敏感。然而，目前尚不清楚这种自上而下和自下而上的影响是反映了捕食者和/或资源对食草动物的直接影响，还是间接影响，即反映了食草动物物种之间改变的相互作用。我们量化了自下而上和自上而下过程对鳗草（大叶藻）食草动物组合的直接和间接影响。在一项田间实验中，我们通过因子操纵水柱营养物质（使用奥绿肥（™）缓释肥料）和捕食压力（使用捕食者排除笼），并测量对食草动物丰度、丰富度和β多样性的影响。我们通过统计探索单个食草动物物种对营养级操纵的反应，研究了群落反应的可能机制。捕食者增加了食草动物的丰富度和总丰度，在这两种情况下都是通过群落组成的间接变化实现的。丰富度的增加是通过捕食者抑制常见的钩虾类物种（阿氏单角钩虾和短光钩虾），从而使更稀有的钩虾类物种（哥伦比亚奥罗钩虾和具喙原钩虾）得以包含进来。食草动物总丰度的增加反映了一种麦秆虫类钩虾物种（麦秆虫属）丰度的增加，同时其他常见物种的丰度下降。此外，捕食者通过降低麦秆虫属在栖息地斑块间丰度的变异性，降低了β多样性。奥绿肥（™）施肥使局部营养浓度增加，但营养物质在距肥料3米范围内消散至背景水平。添加营养物质对食草动物组合的影响较弱，既不影响丰富度，也只是通过增加一种食草动物物种（麦秆虫属）来增加总丰度。添加营养物质不影响β多样性。我们证明了营养级操纵对群落结构的组合水平影响是食草动物物种独特且通常是间接反应的结果。这些结果强调了在一个通常同时受到富营养化和过度捕捞影响的系统中理解食草动物 - 食草动物相互作用的重要性。
