The loss of consumers threatens the integrity of ecological systems, but the mechanisms underlying the effects on communities and ecosystems remain difficult to predict. This is, in part, due to the complex roles that consumers play in those systems. Here, we highlight this complexity by quantifying two mechanisms by which molluscan grazers—typically thought of as consumers of their algal resources—facilitate algae on rocky shores. Initial observations in high‐zone tide pools revealed that both water‐column ammonium concentrations and photosynthetic biomass were higher in pools containing higher densities of grazers, suggesting that local‐scale nutrient recycling by the grazers could be enhancing algal biomass. We assessed this possibility by experimentally manipulating grazer abundances at the level of whole tide pools but controlling access of those grazers to experimental plots within each pool. Contrary to predictions that algal biomass inside grazer exclusions would increase as grazer abundances in the pools increased, we found that algal biomass inside grazer‐exclusion fences was unaffected by grazer abundances. Instead, the consumptive effects of grazers that were evident at low grazer abundances transitioned to facilitative effects as experimentally manipulated grazer abundances increased. This finding suggested that these positive interactions were associated with the physical presence of grazers and not just grazers' effects on nutrient availability. Subsequent experiments highlighted the potential role of “slime”—the pedal mucous trails left behind as the mollusks crawl on the substratum—in promoting the recruitment of algae and thereby mediating a spatial subsidy of new organic matter into the system. Furthermore, different grazer groups contributed disproportionately to ammonium excretion (i.e., turban snails) versus slime production (i.e., littorine snails), suggesting a potential role for grazer diversity. Our work highlights the complex ways in which consumers affect their resources, including multiple, complementary mechanisms by which these grazers facilitate the algae they consume.

中文翻译：

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揭示消费者对海洋初级生产者的多重便利效应


消费者的流失威胁着生态系统的完整性，但对社区和生态系统产生影响的潜在机制仍然难以预测。这在一定程度上是由于消费者在这些系统中扮演着复杂的角色。在这里，我们通过量化软体动物放牧动物（通常被认为是藻类资源的消费者）促进岩石海岸上的藻类的两种机制来强调这种复杂性。对高区潮汐池的初步观察表明，在包含较高密度放牧动物的水池中，水柱铵浓度和光合生物量都较高，这表明放牧动物的局部规模养分回收可能正在增加藻类生物量。我们通过在整个潮汐池水平上实验操纵放牧者的丰度来评估这种可能性，但控制这些放牧者对每个池内实验地块的访问。与放牧者排除内的藻类生物量会随着池中放牧者丰度的增加而增加的预测相反，我们发现放牧者排除栅栏内的藻类生物量不受放牧者丰度的影响。相反，随着实验操纵的放牧动物丰度的增加，在低放牧动物丰度下明显的放牧动物的消耗效应转变为促进效应。这一发现表明，这些积极的相互作用与食草动物的物理存在有关，而不仅仅是食草动物对养分可用性的影响。随后的实验强调了“粘液”——软体动物在基质上爬行时留下的脚踏粘液痕迹——在促进藻类招募方面的潜在作用，从而介导新有机物进入系统的空间补贴。 此外，不同的食草动物群体对铵态氮排泄（即头巾蜗牛）与粘液产生（即山毛蜗牛）的贡献不成比例，这表明食草动物多样性的潜在作用。我们的工作强调了消费者影响其资源的复杂方式，包括这些放牧者促进他们消耗藻类的多种互补机制。