Data from: Synergistic impacts by an invasive amphipod and an invasive fish explain native gammarid extinction

Background: Worldwide freshwater ecosystems are increasingly affected by invasive alien species. In particular, Ponto-Caspian gobiid fishes and amphipods are suspected to have pronounced effects on aquatic food webs. However, there is a lack of systematic studies mechanistically testing the potential synergistic effects of invasive species on native fauna. In this study we investigated the interrelations between the invasive amphipod Dikerogammarus villosus and the invasive fish species Neogobius melanostomus in their effects on the native amphipod Gammarus pulex. We hypothesized selective predation by the fish as a driver for displacement of native species resulting in potential extinction of G. pulex. The survival of G. pulex in the presence of N. melanostomus in relation to the presence of D. villosus and availability of shelter was analyzed in the context of behavioural differences between the amphipod species. Results: Gammarus pulex had a significantly higher susceptibility to predation by N. melanostomus compared to D. villosus in all experiments, suggesting preferential predation by this fish on native gammarids. Furthermore, the presence of D. villosus significantly increased the vulnerability of G. pulex to fish predation. Habitat structure was an important factor for swimming activity of amphipods and their mortality, resulting in a threefold decrease in amphipods consumed with shelter habitat structures provided. Behavioral differences in swimming activity were additionally responsible for higher predation rates on G. pulex. Intraguild predation could be neglected within short experimental durations. Conclusions: The results of this study provide evidence for synergistic effects of the two invasive Ponto-Caspian species on the native amphipod as an underlying process of species displacements during invasion processes. Prey behaviour and monotonous habitat structures additionally contribute to the decline of the native gammarid fauna in the upper Danube River and elsewhere.

背景：全球淡水生态系统正日益受到外来入侵物种的影响。其中，蓬蒂-里海（Ponto-Caspian）区域的鰕虎鱼科（gobiid）鱼类和端足类（amphipod）被认为会对水生食物网产生显著影响。然而，目前尚缺乏从机制层面系统性探究入侵物种对本土动物区系潜在协同效应的研究。本研究针对入侵端足类大叶卷叶钩虾（Dikerogammarus villosus）与入侵鱼类黑口新鰕虎（Neogobius melanostomus）二者对本土端足类淡水钩虾（Gammarus pulex）的影响，探究二者间的相互关联。我们提出假说：该鱼类的选择性捕食是本土物种被排挤的驱动因素，可能导致淡水钩虾（G. pulex）走向灭绝。结合两种端足类的行为差异，本研究分析了在黑口新鰕虎存在的条件下，大叶卷叶钩虾的存在以及遮蔽物可获得性对淡水钩虾存活率的影响。
结果：所有实验中，淡水钩虾相较于大叶卷叶钩虾，对黑口新鰕虎捕食的易感性显著更高，这表明该鱼类会优先捕食本土鰕虎类动物。此外，大叶卷叶钩虾的存在会显著提升淡水钩虾面临鱼类捕食时的脆弱性。生境结构是影响端足类游泳行为与死亡率的重要因素，当提供遮蔽生境结构时，被捕食的端足类个体数较对照组降低至原先的1/3。游泳行为的差异进一步导致了淡水钩虾更高的被捕食率。在较短的实验周期内，种内集团捕食（intraguild predation）可忽略不计。
结论：本研究结果提供了证据，表明两种蓬蒂-里海入侵物种对本土端足类存在协同效应，这一过程是入侵进程中本土物种被排挤的潜在机制。猎物行为与单一化的生境结构进一步加剧了多瑙河上游及其他区域本土鰕虎类动物区系的衰退。