Data from: Functional response comparisons among freshwater amphipods: ratio-dependence and higher predation for Gammarus pulex compared to the non-natives Dikerogammarus villosus and Echinogammarus berilloni

The invasive Ponto–Caspian amphipod Dikerogammarus villosus strongly impairs the structure of recipient freshwater communities, mostly through predation of a wide range of invertebrates. Useful insights on the ecological impact of an invader can be provided by comparing its functional response (FR)—namely the relationship between resource use and resource availability—to that of trophic analogs from the recipient communities. We applied this method and derived the FR of D. villosus, the native Gammarus pulex, and Echinogammarus berilloni, another gammarid that is also undergoing range expansion, feeding on live dipteran larvae. We tested a range of predator densities to account for mutual interference among predators, assuming that predators do not forage alone in their natural environment. We also analysed the predators’ spatial distribution to investigate whether spatial behaviour might be at the origin of interference. For the three gammarid species, the per capita predation rate was a function of the prey-to-predator ratio and showed a decelerating rise to an asymptote. There was no difference in searching efficiency between the three species while prey handling time was significantly lower in G. pulex than in D. villosus and E. berilloni, leading to a higher food intake for the native at high prey densities. Differences in morphology and behaviour between the three gammarids might explain our results. Indeed, D. villosus tended to aggregate more than the other two gammarid species, which promotes conspecific interactions. Our findings suggest that mutual interference between conspecifics is a fundamental shaper of gammarid predation. Time spent interacting with conspecifics is likely to decrease the biotic pressure of the Killer shrimp D. villosus. From a broader perspective, in addition to higher predation risk, parasitism, habitat complexity or climatic variables, the presence of conspecifics of the predator (i.e. predator density) is another factor that has the potential to alter FR comparisons.入侵性蓬蒂-里海钩虾（Dikerogammarus villosus）会严重破坏入侵地淡水群落的结构，其主要干扰途径为捕食多种无脊椎动物。通过将入侵物种的功能反应（functional response，FR）——即资源利用量与资源可获得性之间的关联——与入侵地群落中营养级同类群的功能反应进行对比，可为解析入侵物种的生态影响提供极具价值的科学视角。本研究采用该方法，测定了斑尾刺钩虾（Dikerogammarus villosus，又称杀手虾）、本土钩虾（Gammarus pulex）以及另一种正处于分布扩张过程中的棘钩虾（Echinogammarus berilloni）以活体双翅目幼虫为猎物时的功能反应参数。考虑到捕食者在自然环境中并非单独觅食，本研究设置了一系列捕食者密度梯度，以探究捕食者间的相互干扰效应。此外，本研究还分析了捕食者的空间分布特征，以探究空间行为是否为干扰效应的潜在成因。对于三种钩虾物种而言，单位个体捕食率均随猎物-捕食者比值的变化而变化，且呈现出增速逐渐减缓、最终趋近渐近线的增长趋势。三种钩虾的搜索效率无显著差异；但本土钩虾（Gammarus pulex）的猎物处理时间显著短于斑尾刺钩虾与贝氏棘钩虾（Echinogammarus berilloni），这使得在高猎物密度下，本土钩虾的食物摄入量更高。三种钩虾在形态与行为上的差异或可解释本研究的实验结果。事实上，相较于另外两种钩虾，斑尾刺钩虾更倾向于集群，这会促进同种个体间的相互作用。本研究结果表明，同种个体间的相互干扰是调控钩虾捕食行为的核心驱动因素。与同种个体互动所耗费的时间，或可降低杀手虾（Dikerogammarus villosus）带来的生物胁迫压力。从更宏观的视角来看，除了更高的捕食风险、寄生作用、栖息地复杂性以及气候变量之外，捕食者同种个体的存在（即捕食者密度）是另一个可能会改变功能反应对比结果的关键因素。