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 predator)的级联效应可助力群落结构构建，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。
因此，为探明生境复杂度如何、在多大程度上影响相邻生境中的营养级相互作用，我们结合大尺度区域调查、操控性野外试验与室外中型实验生态系统(mesocosm)，对四个营养级的捕食者-猎物相互作用强度进行了量化。
在美国东南部的河口区域，窄头双髻鲨(Sphyrna tiburo)会在潮间带泥滩与邻近的牡蛎礁两种生境中捕食青蟹(blue crab)，二者的地表结构差异显著。通过开展12小时的青蟹拴系试验，我们量化得到不同生境下的猎物丢失率：牡蛎礁生境为37%，泥滩生境则达78%。
我们提出假说：窄头双髻鲨对青蟹的捕食效应会向下级联，释放出更低营养级的泥蟹捕食者，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，通过操控捕食者组合开展试验，并量化了牡蛎的死亡率。
窄头双髻鲨对青蟹展现出强烈的消耗性效应与非消耗性效应：在鲨鱼存在时，青蟹会停止捕食牡蛎。然而，无论鲨鱼与青蟹是否存在，泥蟹都会持续捕食牡蛎，尤其是在礁体的结构庇护区域内，该区域内的牡蛎死亡率始终维持在较高水平。
因此，窄头双髻鲨仅在泥滩生境中会间接提升牡蛎的存活率——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控营养级联效应，尤其是当中级捕食者(mesopredator)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的情况下。