Data from: Contrasting complexity of adjacent habitats influences the strength of cascading predatory effects

Although cascading effects of top predators can help structure communities, their influence may vary across habitats that differentially protect prey. Therefore, to understand how and to what degree habitat complexity can affect trophic interactions in adjacent habitats, we used a combination of a broad regional-scale survey, manipulative field trials, and an outdoor mesocosm experiment to quantify predator–prey interaction strengths across four trophic levels. Within estuaries of the southeastern USA, bonnethead sharks (Sphyrna tiburo) hunt blue crabs on mudflats and adjacent oyster reefs, two habitats with vastly different aboveground structure. Using 12-h tethering trials of blue crabs we quantified habitat-dependent loss rates of 37% on reefs and 78% on mudflats. We hypothesized that the sharks’ predatory effects on blue crabs would cascade down to release a lower-level mud crab predator, which subsequently would increase juvenile oyster mortality, but that the cascade strength would be habitat-dependent. We experimentally manipulated predator combinations in split-plot mesocosms containing reef and mudflat habitats, and quantified oyster mortality. Bonnetheads exerted strong consumptive and non-consumptive effects on blue crabs, which ceased eating oysters in the sharks’ presence. However, mud crabs, regardless of shark and blue crab presence, continued to consume oysters, especially within the structural refuge of the reef where they kept oyster mortality high. Thus, bonnetheads indirectly boosted oyster survival, but only on the mudflat where mud crabs were less active. Our work demonstrates how structural differences in adjacent habitats can moderate trophic cascades, particularly when mesopredators exhibit differential use of structure and different sensitivities to top predators.

尽管顶级捕食者的级联效应有助于构建群落结构，但其影响会因不同生境对猎物的保护程度差异而有所不同。为此，为明确生境复杂性可通过何种方式、在多大程度上调控相邻生境中的营养级互作，我们结合大范围区域尺度调查、控制性野外试验与室外中型实验生态系统试验（mesocosm experiment），对四个营养级的捕食者-猎物互作强度进行了量化。在美国东南部的河口区域内，窄头双髻鲨（Sphyrna tiburo）会在潮间带泥滩与邻近的牡蛎礁两类地上立体结构差异显著的生境中捕食蓝蟹。通过开展12小时蓝蟹拴系试验（tethering trials），我们量化得到生境依赖的捕食死亡率：牡蛎礁生境中死亡率为37%，泥滩生境中则达78%。我们提出如下假说：窄头双髻鲨对蓝蟹的捕食效应会通过级联下行效应，解除低营养级泥蟹的捕食压制，进而提升幼牡蛎的死亡率，但该级联效应的强度会因生境而异。我们在包含牡蛎礁与泥滩生境的裂区设计中型实验生态系统中对捕食者组合进行了人工调控，并量化了牡蛎死亡率。结果显示，窄头双髻鲨对蓝蟹具有显著的消耗性效应（consumptive effects）与非消耗性效应（non-consumptive effects），蓝蟹在鲨鱼存在时会停止捕食牡蛎。然而无论鲨鱼与蓝蟹是否存在，泥蟹都会持续捕食牡蛎，尤其是在牡蛎礁的结构庇护所（structural refuge）内，此时牡蛎死亡率始终维持在较高水平。由此可见，仅在泥滩生境中（该生境中泥蟹活动强度更低），窄头双髻鲨才会间接提升牡蛎的存活率。本研究阐明了相邻生境的结构差异可如何调控营养级联效应，尤其是当中捕食者（mesopredators）对生境结构的利用存在差异、且对顶级捕食者的敏感性各不相同之时。