Habitat complexity and benthic predator-prey interactions in Chesapeake Bay

In Chesapeake Bay, the soft-shell clam Mya arenaria (thin-shelled, deep-burrowing) exhibits population declines when predators are active, and it persists at low densities. In contrast, the hard clam Mercenaria mercenaria (thick-shelled, shallow-burrowing) has a stable population and age distribution. We examined the potential for habitat and predators to control densities and distributions of bivalves in a field caging experiment (Mya only) and laboratory mesocosm experiments (both species). In the field, clams exposed to predators experienced 76.3% greater mortality as compared to caged individuals, and blue crabs were likely responsible for most of the mortality of juvenile Mya. In mesocosm experiments, Mya had lower survival in sand and seagrass than in shell hash or oyster shell habitats. However, crabs often missed one or more prey items in seagrass, shell, and oyster shell habitats. Predator search times and encounter rates declined when prey were at low densities, likely due to the added cost of inefficient foraging; however, this effect was more pronounced for Mya than for Mercenaria. Mercenaria had higher survival than Mya in mesocosm experiments, likely because predators feeding on Mercenaria spent less time foraging than those feeding on Mya. Mya may retain a low-density refuge from predation even with the loss of structurally complex habitats, though a loss of habitat refuge may result in clam densities that are not sustainable. A better understanding of density-dependent predator-prey interactions is necessary to prevent loss of food-web integrity and to conserve marine resources.

在切萨皮克湾（Chesapeake Bay）中，软壳蛤（soft-shell clam，*Mya arenaria*）具有壳薄、深潜穴的特征，当捕食者活跃时其种群数量会出现下降，且仅能以低密度维持种群存续。与之相对的是，硬壳蛤（hard-shell clam，*Mercenaria mercenaria*）壳厚、潜居较浅，其种群数量与年龄结构均保持稳定。本研究通过仅针对软壳蛤的野外笼控实验（field caging experiment），以及针对两种双壳类（bivalves）的实验室中型生态箱实验（laboratory mesocosm experiments），探究了栖息地与捕食者对双壳类种群密度及分布的调控潜力。野外实验结果显示，暴露于捕食者环境中的蛤类死亡率较笼控个体高出76.3%，且蓝蟹（blue crab）极有可能是造成幼年软壳蛤死亡的主要因素。在中型生态箱实验中，软壳蛤在沙质与海草生境中的存活率显著低于贝壳碎砾或牡蛎壳生境。但在海草、贝壳碎砾及牡蛎壳生境中，蓝蟹常会遗漏一只或多只猎物。当猎物处于低密度时，捕食者的搜寻时长与遭遇率均会下降，这大概率源于低效觅食所带来的额外能量成本；不过该效应在软壳蛤身上的表现要显著强于硬壳蛤。中型生态箱实验中，硬壳蛤的存活率高于软壳蛤，这可能是因为以硬壳蛤为食的捕食者，其觅食时长要短于以软壳蛤为食的捕食者。即便丧失结构复杂的栖息地，软壳蛤仍可能拥有低密度捕食避难所；但栖息地避难所的丧失，可能会导致蛤类种群密度无法维持可持续水平。为避免食物网（food-web）完整性受损并保护海洋资源，亟需深入理解密度依赖型捕食者-猎物相互作用（density-dependent predator-prey interactions）机制。