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（top predators）的级联效应（cascading effects）可帮助构建群落结构，但其影响会因对猎物具有差异化保护作用的生境而异。为此，为阐明生境复杂性如何、在多大程度上影响相邻生境中的营养级相互作用（trophic interactions），我们结合大范围区域尺度调查（regional-scale survey）、控制性野外试验（manipulative field trials）与室外中型实验生态系统（mesocosm）实验，对4个营养级（trophic levels）间的捕食者-猎物相互作用强度进行了量化。在美国东南部的河口湾（estuaries）中，窄头双髻鲨（学名：Sphyrna tiburo）会在潮滩（mudflats）与相邻牡蛎礁（oyster reefs）这两种地上结构差异显著的生境中捕食蓝蟹（blue crabs）。通过开展蓝蟹12小时栓系试验（tethering trials），我们量化得到生境依赖的死亡率：礁区为37%，潮滩则为78%。我们提出如下假说：鲨类对蓝蟹的捕食效应会向下级联释放，使下层泥蟹捕食者得以繁盛，进而提升幼牡蛎的死亡率，但该级联效应的强度会随生境不同而产生差异。我们在包含礁区与潮滩生境的裂区设计中型实验生态系统（split-plot mesocosms）中对捕食者组合进行了操控，并量化了牡蛎死亡率。窄头双髻鲨对蓝蟹表现出显著的消耗性效应（consumptive effects）与非消耗性效应（non-consumptive effects），蓝蟹在鲨类存在时会停止捕食牡蛎。但无论鲨类与蓝蟹是否存在，泥蟹都会持续捕食牡蛎，尤其是在礁区的结构庇护所内，它们会维持较高的牡蛎死亡率。由此可见，窄头双髻鲨仅在潮滩生境中间接提升了牡蛎的存活率，因为该生境中泥蟹的活动强度更低。本研究阐明了相邻生境的结构差异如何调节营养级联效应，尤其当中捕食者（mesopredators）对生境结构的利用程度以及对顶级捕食者的敏感性存在差异时，这种调节作用会更为突出。