Burkepile Deron E, Hay Mark E
School of Biology, Georgia Institute of Technology, Atlanta, Georgia 30332-0230, USA.
Ecology. 2006 Dec;87(12):3128-39. doi: 10.1890/0012-9658(2006)87[3128:hvncom]2.0.co;2.
Pervasive overharvesting of consumers and anthropogenic nutrient loading are changing the strengths of top-down and bottom-up forces in ecosystems worldwide. Thus, identifying the relative and synergistic roles of these forces and how they differ across habitats, ecosystems, or primary-producer types is increasingly important for understanding how communities are structured. We used factorial meta-analysis of 54 field experiments that orthogonally manipulated herbivore pressure and nutrient loading to quantify consumer and nutrient effects on primary producers in benthic marine habitats. Across all experiments and producer types, herbivory and nutrient enrichment both significantly affected primary-producer abundance. They also interacted to create greater nutrient enrichment effects in the absence of herbivores, suggesting that loss of herbivores produces more dramatic effects of nutrient loading. Herbivores consistently had stronger effects than did nutrient enrichment for both tropical macroalgae and seagrasses. The strong effects of herbivory but limited effects of nutrient enrichment on tropical macroalgae suggest that suppression of herbivore populations has played a larger role than eutrophication in driving the phase shift from coral- to macroalgal-dominated reefs in many areas, especially the Caribbean. For temperate macroalgae and benthic microalgae, the effects of top-down and bottom-up forces varied as a function of the inherent productivity of the ecosystem. For these algal groups, nutrient enrichment appeared to have stronger effects in high- vs. low-productivity systems, while herbivores exerted a stronger top-down effect in low-productivity systems. Effects of herbivores vs. nutrients also varied among algal functional groups (crustose algae, upright macroalgae, and filamentous algae), within a functional group between temperate and tropical systems, and according to the metric used to measure producer abundance. These analyses suggest that human alteration of food webs and nutrient availability have significant effects on primary producers but that the effects vary among latitudes and primary producers, and with the inherent productivity of ecosystems.
对消费者的普遍过度捕捞以及人为养分负荷正在改变全球生态系统中自上而下和自下而上作用力的强度。因此，确定这些作用力的相对和协同作用，以及它们在不同栖息地、生态系统或初级生产者类型之间的差异，对于理解群落结构如何形成变得越来越重要。我们对54个野外实验进行了析因荟萃分析，这些实验正交操纵了食草动物压力和养分负荷，以量化消费者和养分对底栖海洋栖息地初级生产者的影响。在所有实验和生产者类型中，食草作用和养分富集均显著影响初级生产者的丰度。它们还相互作用，在没有食草动物的情况下产生更大的养分富集效应，这表明食草动物的丧失会使养分负荷产生更显著的影响。对于热带大型藻类和海草而言，食草动物的影响始终比养分富集的影响更强。食草作用对热带大型藻类有强烈影响，但养分富集的影响有限，这表明在许多地区，尤其是加勒比地区，食草动物种群的抑制在推动从珊瑚主导型礁到大型藻类主导型礁的阶段转变中所起的作用比富营养化更大。对于温带大型藻类和底栖微藻，自上而下和自下而上作用力的影响随生态系统的固有生产力而变化。对于这些藻类群体，养分富集在高生产力系统中似乎比在低生产力系统中具有更强的影响，而食草动物在低生产力系统中发挥更强的自上而下的作用。食草动物与养分的影响在藻类功能组（壳状藻类、直立大型藻类和丝状藻类）之间、温带和热带系统内的功能组之间以及根据用于测量生产者丰度的指标也有所不同。这些分析表明，人类对食物网和养分可用性的改变对初级生产者有重大影响，但这些影响在不同纬度和初级生产者之间以及随生态系统的固有生产力而有所不同。
