Temperature and nutrient availability alter consequences of phenological shifts in predatory-prey communities

While there is mounting evidence indicating that the relative timing of predator and prey phenologies shapes the outcome of trophic interactions, we still lack a comprehensive understanding of how important the environmental context (e.g. abiotic conditions) is for shaping this relationship. Environmental conditions not only frequently drive shifts in phenologies, but they can also affect the very same processes that mediate the effects of phenological shifts on species interactions. Thus, identifying how environmental conditions shape the effects of phenological shifts is key to predict community dynamics across a heterogenous landscape and how they will change with ongoing climate change in the future. Here I tested how environmental conditions shape effects of phenological shifts by experimentally manipulating temperature, nutrient availability, and relative phenologies in two predator-prey freshwater systems (mole salamander- bronze frog vs dragonfly larvae-leopard frog). This allowed me to (1) isolate the effect of phenological shifts and different environmental conditions, (2) determine how they interact, and (3) how consistent these patterns are across different species and environments. I found that delaying prey arrival dramatically increased predation rates, but these effects were contingent on environmental conditions and predator system. While both nutrient addition and warming significantly enhanced the effect of arrival time, their effect was qualitatively different: Nutrient addition enhanced the positive effect of early arrival while warming enhanced the negative effect of arriving late. Predator responses varied qualitatively across predator-prey systems. Only in the system with strong gape-limitation were predators (salamanders) significantly affected by prey arrival time and this effect varied with environmental context. Correlations between predator and prey demographic rates suggest that this was driven by shifts in initial predator-prey size ratios and a positive feedback between size-specific predation rates and predator growth rates. These results highlight the importance of accounting for temporal and spatial correlation of local environmental conditions and gape-limitation in predator-prey systems when predicting the effects of phenological shifts and climate change on predator-prey systems.

尽管已有越来越多的证据表明，捕食者与猎物的物候相对时序会调控营养相互作用（trophic interactions）的结果，但我们仍未能全面理解环境背景（例如非生物条件（abiotic conditions））在塑造这一关系中的重要性。环境条件不仅常常驱动物候的变化，还会影响那些介导物候变化对物种相互作用产生影响的核心过程。因此，明确环境条件如何调控物候变化的效应，是预测异质景观中的群落动态，以及未来持续的气候变化下群落动态将如何改变的关键所在。
本研究通过在两个淡水捕食者-猎物系统（钝口螈（mole salamander）-青铜蛙（bronze frog）、蜻蜓幼虫（dragonfly larvae）-豹蛙（leopard frog））中实验操控温度、养分有效性以及相对物候时序，探究了环境条件对物候变化效应的调控作用。这一实验设计使得我们得以：(1) 分离物候变化与不同环境条件的单独效应；(2) 解析二者的交互作用模式；(3) 明确这些模式在不同物种与环境间的一致性程度。
研究发现，延迟猎物抵达时间会显著提升捕食率，但该效应取决于环境条件与捕食者-猎物系统类型。尽管养分添加与增温均显著调控了猎物抵达时间的效应，但二者的作用性质存在显著差异：养分添加强化了猎物早抵达的正向效应，而增温则加剧了猎物晚抵达的负向效应。
不同捕食者-猎物系统的捕食者响应存在质的差异。仅在存在强烈口器限制（gape-limitation）的系统中，捕食者（钝口螈）才会显著受到猎物抵达时间的影响，且该效应随环境背景发生变化。
捕食者与猎物的种群统计速率间的相关性表明，这一现象源于捕食者与猎物初始体型比的变化，以及体型依赖型捕食率与捕食者生长率之间的正反馈环路。
本研究结果凸显了，在预测物候变化与气候变化对捕食者-猎物系统的影响时，需要考虑局域环境条件的时空相关性以及捕食者-猎物系统的口器限制特征的重要性。