Data from: Prey responses to fine-scale variation in predation risk from combined predators

While it is well documented that organisms can express phenotypic plasticity in response to single gradients of environmental variation, our understanding of how organisms integrate information along multiple environmental gradients is limited in many systems. Using the freshwater snail Helisoma trivolvis and two common predators (water bugs Belostoma flumineum and crayfish Orconectes rusticus), we explored how prey integrate information along multiple predation risk gradients (i.e. caged predators fed increasing amounts of prey biomass) that induce opposing phenotypes. When exposed to single predators fed increasing amounts of prey biomass, we detected threshold responses; intermediate amounts of consumed biomass induced phenotypic responses, but higher amounts induced little additional induction. This suggests that additional increases in predator-induced traits with greater predator risk offer minimal increases in fitness or that a limit in the response magnitude was reached. Additionally, the response thresholds were contingent on the predator and focal trait. For shell width, responses were generally detected at a lower amount of consumed biomass by water bugs compared to crayfish. Within the crayfish treatments, we found that the shell thickness response threshold was lower than the shell width response threshold. When we combined gradients of consumed biomass from both predators, we found that the magnitude of response to one predator was often reduced when the other predator were present. Interestingly, these effects were often detected at consumed biomass levels that were lower than the threshold concentration necessary to elicit a response in the single-predator treatments. Moreover, our combined predator treatments revealed that snails shifted from discrete responses to more continuous (i.e. graded) responses. Together, our results reveal that organisms experiencing multiple environmental gradients can integrate this information to make phenotypic decisions and demonstrate the novel result that an exposure to multiple species of predators can lower the response threshold of prey.尽管已有充分研究证实，生物可针对单一环境变异梯度表现出表型可塑性（phenotypic plasticity），但当前在多数研究系统中，我们对生物如何整合多条环境梯度的信息仍存在认知局限。本研究以淡水螺旋尾螺（Helisoma trivolvis）与两种常见捕食者——负子蝽（Belostoma flumineum）及锈色螯虾（Orconectes rusticus）为实验材料，探究猎物如何整合多条捕食风险梯度（predation risk gradients，即投喂递增猎物生物量的笼养捕食者所介导的风险）的信息，而这些梯度可诱导出相反的表型。当暴露于投喂递增猎物生物量的单一捕食者环境时，我们检测到了阈值响应：中等水平的捕食者摄食生物量可诱导表型响应，但更高水平的摄食生物量仅引发极微弱的额外诱导。这提示，随捕食者风险升高而增强的捕食诱导性状，对适合度的提升幅度极小，或是已达到响应幅度的上限。此外，响应阈值因捕食者类群与目标性状而异。针对壳宽性状，相较于螯虾处理组，负子蝽处理组中的猎物会在更低的捕食者摄食生物量水平下触发响应。在螯虾处理组内部，我们发现壳厚的响应阈值低于壳宽的响应阈值。当结合两种捕食者的摄食生物量梯度开展实验处理时，我们发现，当另一种捕食者存在时，猎物对某一种捕食者的响应强度通常会被削弱。有趣的是，这些效应往往出现在低于单一捕食者处理组中引发响应所需的阈值浓度的摄食生物量水平。此外，复合捕食者处理组还显示，螺类的响应模式从离散响应转变为更连续的（即梯度式）响应。综上，本研究结果表明，身处多条环境梯度中的生物可整合这些信息以做出表型决策，并证实了一项新发现：暴露于多种捕食者物种可降低猎物的响应阈值。