Department of Ecology and Evolutionary Biology, University of California, Irvine, California, USA.
Ecology. 2024 Nov;105(11):e4439. doi: 10.1002/ecy.4439. Epub 2024 Oct 2.
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.
消费者的损失威胁着生态系统的完整性，但消费者对社区和生态系统的影响的机制仍然难以预测。这在一定程度上是由于消费者在这些系统中扮演的复杂角色。在这里，我们通过量化两种机制来突出这种复杂性，这些机制表明软体动物食草动物 - 通常被认为是其藻类资源的消费者 - 促进了岩石海岸上的藻类。在高区潮池中的初步观察表明，含有较高密度食草动物的水池中的水柱氨浓度和光合作用生物量都较高，这表明食草动物的局部尺度营养回收可能会增强藻类生物量。我们通过在整个潮池水平上实验性地操纵食草动物的丰度来评估这种可能性，但要控制这些食草动物进入每个池内实验区的通道。与预测相反，当池塘中的食草动物丰度增加时，食草动物排除区内的藻类生物量会增加，我们发现食草动物排除围栏内的藻类生物量不受食草动物丰度的影响。相反，在实验性操纵食草动物丰度增加的情况下，食草动物在低食草动物丰度下表现出的消耗作用转变为促进作用。这一发现表明，这些正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“黏液”（即软体动物在基质上爬行时留下的足腺粘液痕迹）在促进藻类繁殖方面的潜在作用，从而介导了新有机物质向系统中的空间补贴。此外，不同的食草动物群体对氨排泄（即，头巾蜗牛）与黏液产生（即，滨螺）的贡献不成比例，这表明食草动物多样性可能发挥作用。我们的工作突出了消费者影响其资源的复杂方式，包括这些食草动物促进其消耗的藻类的多种互补机制。
