Opposing bottom-up ‘resource-driven’ and top-down ‘consumer-driven’ forces interact to shape the structure of ecosystems. While these counteracting forces are well recognised, debate remains regarding which is more influential across space and time. Here we explore bottom-up versus top-down control of macroalgal communities for temperate rocky reef communities in highly urbanised Port Phillip Bay (PPB), southeast Australia. Field surveys show macroalgal cover to paradoxically decline with increasing ‘bottom-up’ nutrient inputs while the abundance of grazing sea urchins (Heliocidaris erythrogramma) increased. The mechanisms underpinning this pattern were examined by constructing urchin-exclusion plots using octocoral (Erythropodium hicksoni) colonies that grow on urchin barren grounds and form natural barriers to grazing sea urchins. Octocoral plots were constructed by cutting 200 mm by 200 mm squares to expose bare reef substratum within the centre of octocoral colonies, which enabled efficient replication of urchin-exclusions on barren grounds across three distinct zones of anthropogenic nutrient input in PPB. Octocoral plots successfully excluded urchins across zones and, in the absence of grazing, macroalgal production increased with increasing nutrient concentration as expected. This novel opportunity to efficiently replicate urchin-exclusions on high-density barren grounds across different zones of ‘bottom-up’ forcing demonstrates that urchin overgrazing can keep pace with and overwhelm increasing macroalgal productivity. Our findings also highlight that impacts of grazing can be greatest where bottom-up forces enable large abundances of herbivores to accumulate, which is counter to perceptions that impacts of herbivores will be greatest where macroalgal productivity is low.

中文翻译：

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跨生产力梯度的海胆控制大型藻类群落

反对自下而上的“资源驱动”和自上而下的“消费者驱动”的力量相互作用以塑造生态系统的结构。虽然这些反作用力已得到广泛认可，但关于哪种力量在时空上更具影响力的争论仍然存在。在这里，我们探讨了澳大利亚东南部高度城市化的菲利普港湾 (PPB) 温带岩礁群落的大型藻类群落自下而上与自上而下的控制。实地调查显示，随着“自下而上”的养分输入的增加，大型藻类覆盖率自相矛盾地下降，而放牧海胆（Heliocidaris erythrogramma）的数量却增加了。通过使用八珊瑚（Erythropodium hicksoni）群落构建海胆排除图来研究支持这种模式的机制，这些群落生长在海胆贫瘠的土地上并形成放牧海胆的天然屏障。八珊瑚地块是通过切割 200 毫米 x 200 毫米的正方形来构建的，以暴露八珊瑚群中心内的裸礁基质，这使得在 PPB 中三个不同的人为养分输入区的贫瘠土地上有效复制海胆排除物。八珊瑚地块成功地排除了跨区域的海胆，并且在没有放牧的情况下，大型藻类的产量随着营养浓度的增加而增加，正如预期的那样。这种在不同“自下而上”强迫区域的高密度贫瘠土地上有效复制海胆排除物的新机会表明，海胆过度放牧可以跟上并压倒不断增加的大型藻类生产力。我们的研究结果还强调，在自下而上的力量使大量食草动物得以积累的情况下，放牧的影响可能最大，