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.已有大量文献证实，生物可响应单一环境变异梯度表现出表型可塑性（phenotypic plasticity），但目前在多数类群中，我们对生物如何整合多条环境梯度的信息仍缺乏足够认识。本研究以淡水螺平扁盘螺（Helisoma trivolvis）以及两种常见捕食者——水蝽（Belostoma flumineum）和螯虾（Orconectes rusticus）为研究对象，探究猎物如何整合多条捕食风险梯度（即笼养捕食者取食的猎物生物量递增）的信息，这类梯度可诱导出对立的表型。当仅暴露于取食猎物生物量递增的单一捕食者环境时，我们检测到了阈值响应：中等水平的被捕食猎物生物量可诱导表型响应，但更高水平的捕食者暴露并未引发额外的响应提升。这表明，随捕食者风险升高而增强的捕食诱导性状，对适合度的提升贡献极小，或是响应强度已达到上限。此外，响应阈值因捕食者类群和目标性状而异。就壳宽性状而言，相较于螯虾，水蝽处理组中较低的被捕食猎物生物量即可诱导出响应。在螯虾处理组内部，我们发现壳厚性状的响应阈值低于壳宽性状的响应阈值。当同时施加两种捕食者的取食生物量梯度时，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。值得注意的是，这类效应通常在低于单一捕食者处理组中引发响应所需的阈值生物量水平时就已显现。此外，双捕食者处理组还显示，螺类的响应从离散型转向了更为连续的（即梯度型）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，并证实了一个全新的结论：暴露于多种捕食者物种下，可降低猎物的响应阈值。