Gruner Daniel S, Smith Jennifer E, Seabloom Eric W, Sandin Stuart A, Ngai Jacqueline T, Hillebrand Helmut, Harpole W Stanley, Elser James J, Cleland Elsa E, Bracken Matthew E S, Borer Elizabeth T, Bolker Benjamin M
Department of Entomology, University of Maryland, College Park, MD 20742, USA.
Ecol Lett. 2008 Jul;11(7):740-55. doi: 10.1111/j.1461-0248.2008.01192.x. Epub 2008 Apr 25.
Nutrient availability and herbivory control the biomass of primary producer communities to varying degrees across ecosystems. Ecological theory, individual experiments in many different systems, and system-specific quantitative reviews have suggested that (i) bottom-up control is pervasive but top-down control is more influential in aquatic habitats relative to terrestrial systems and (ii) bottom-up and top-down forces are interdependent, with statistical interactions that synergize or dampen relative influences on producer biomass. We used simple dynamic models to review ecological mechanisms that generate independent vs. interactive responses of community-level biomass. We calibrated these mechanistic predictions with the metrics of factorial meta-analysis and tested their prevalence across freshwater, marine and terrestrial ecosystems with a comprehensive meta-analysis of 191 factorial manipulations of herbivores and nutrients. Our analysis showed that producer community biomass increased with fertilization across all systems, although increases were greatest in freshwater habitats. Herbivore removal generally increased producer biomass in both freshwater and marine systems, but effects were inconsistent on land. With the exception of marine temperate rocky reef systems that showed positive synergism of nutrient enrichment and herbivore removal, experimental studies showed limited support for statistical interactions between nutrient and herbivory treatments on producer biomass. Top-down control of herbivores, compensatory behaviour of multiple herbivore guilds, spatial and temporal heterogeneity of interactions, and herbivore-mediated nutrient recycling may lower the probability of consistent interactive effects on producer biomass. Continuing studies should expand the temporal and spatial scales of experiments, particularly in understudied terrestrial systems; broaden factorial designs to manipulate independently multiple producer resources (e.g. nitrogen, phosphorus, light), multiple herbivore taxa or guilds (e.g. vertebrates and invertebrates) and multiple trophic levels; and - in addition to measuring producer biomass - assess the responses of species diversity, community composition and nutrient status.
养分有效性和食草作用在不同生态系统中对初级生产者群落生物量的控制程度各不相同。生态理论、许多不同系统中的个体实验以及针对特定系统的定量综述表明：（i）自下而上的控制普遍存在，但相对于陆地系统，自上而下的控制在水生生境中更具影响力；（ii）自下而上和自上而下的力量相互依存，存在统计上的相互作用，这些相互作用会增强或减弱对生产者生物量的相对影响。我们使用简单的动态模型来回顾产生群落水平生物量独立响应与交互响应的生态机制。我们用析因荟萃分析的指标校准了这些机制预测，并通过对191次食草动物和养分的析因操纵进行全面的荟萃分析，测试了它们在淡水、海洋和陆地生态系统中的普遍性。我们的分析表明，尽管在淡水生境中增加幅度最大，但施肥会使所有系统中的生产者群落生物量增加。去除食草动物通常会增加淡水和海洋系统中的生产者生物量，但在陆地上的影响并不一致。除了海洋温带岩礁系统显示出养分富集和去除食草动物的正协同作用外，实验研究对养分和食草作用处理对生产者生物量的统计相互作用的支持有限。食草动物的自上而下控制、多个食草动物类群的补偿行为、相互作用的时空异质性以及食草动物介导的养分循环，可能会降低对生产者生物量产生一致交互效应的概率。后续研究应扩大实验的时间和空间尺度，特别是在研究较少的陆地系统中；拓宽析因设计，以独立操纵多种生产者资源（如氮、磷、光）、多种食草动物类群或类属（如脊椎动物和无脊椎动物）以及多个营养级；并且——除了测量生产者生物量之外——评估物种多样性、群落组成和养分状况响应。
