Communities and food web structures of aquatic ecosystems can be strongly affected by the establishment of alien macroinvertebrate species. In many European waters, the invasion of the Ponto-Caspian amphipod Dikerogammarus villosus has led to displacement of other macroinvertebrates. Predation by D. villosus is often assumed to be the key driver of the displacement based on results of laboratory studies, but this has not been verified in the field. Here, we report our investigation of the relevance of D. villosus predation in the River Rhine system using both stable isotope analyses of d 13 C and d 15 N, and molecular analyses of D. villosus gut contents with group-specific primers aiming at macroinvertebrate prey taxa. Stable isotope analyses of D. villosus from ten sites showed mean d 15 N values comparable to those of primary consumers. Overall, only approximately 1% of all tested primer/gut content combinations revealed DNA of the respective taxa. Both indicate minor importance of predation by D. villosus as a driver of the observed macroinvertebrate displacement. Conceivably, competitive strength due to opportunistic feeding, indicated by different niche widths between and a strong intraspecific variation of d 13 C values of D. villosus within sites of our study, is much more important for its invasion success.

Summary Biodiversity is globally threatened by the replacement of native species by invasive species and ensuing changes in ecosystem functioning. Although trophic linkages between aquatic and terrestrial ecosystems have received attention, effects of aquatic invasive species on the flow of resource subsidies have been considered only recently. We examined how the effects of one of the most invasive macroinvertebrate species in European waterways, the amphipod Dikerogammarus villosus, extend from streams to the terrestrial food web. We quantified aquatic emergence and the contribution of aquatic resources to the diets of two riparian spider taxa in relation to the density of D. villosus. Our results indicated that the effects of this invasive species carry over to the terrestrial system via cross‐ecosystem flow of resource subsidy. The contribution of aquatic resources to the diet of the terrestrial web‐building spider Tetragnatha decreased from 60% at low densities of D. villosus to 10% at a D. villosus density >5000 individuals m−2. This correlates with a decreasing emergence rate of merolimnic midges (species with an aquatic larval phase) from 12 to <3 mg dry biomass m−2 day−1 at the respective densities of D. villosus. The magnitude of biomass flow from the aquatic to the terrestrial ecosystem is most likely decreased by D. villosus, and this decrease extends to the diet of riparian web‐building spiders. Effects of this aquatic invader may also extend to a decoupling of the terrestrial ecosystem from the aquatic ecosystem in terms of subsidy flux.

A good understanding of the mechanisms and magnitude of the impact of invasive alien species on ecosystem services and biodiversity is a prerequisite for the efficient prioritisation of actions to prevent new invasions or for developing mitigation measures. In this review, we identified alien marine species that have a high impact on ecosystem services and biodiversity in European seas, classified the mechanisms of impact, commented on the methods applied for assessing the impact and the related inferential strength, and reported on gaps in available information. Furthermore, we have proposed an updated inventory of 87 marine species in Europe, representing 13 phyla, which have a documented high impact on ecosystem services or biodiversity.Food provision was the ecosystem service that was impacted by the greatest number of alien species (in terms of both positive and negative impacts). Following food provision, the ecosystem services that were negatively affected by the highest number of alien species were ocean nourishment, recreation and tourism, and lifecycle maintenance, while the ecosystem services that were most often positively impacted were cognitive benefits, water purification, and climate regulation. In many cases, marine aliens were found to impact keystone/protected species and habitats. Thirty percent of the assessed species had an impact on entire ecosystem processes or wider ecosystem functioning, more often in a negative fashion. Forty-nine of the assessed species were reported as being ecosystem engineers, which fundamentally modify, create, or define habitats by altering their physical or chemical properties.The positive impacts of alien species are probably underestimated, as there is often a perception bias against alien species. Among the species herein assessed as high-impact species, 17 had only negative and 7 only positive impacts; for the majority (63 species), both negative and positive impacts were reported; the overall balance was often unknown. Although there is no doubt that invasive species have modified marine ecosystems, evidence for most of the reported impacts is weak, as it is based on expert judgement or dubious correlations, while only 13% of the reported impacts were inferred via manipulative or natural experiments. A need for stronger inference is evident, to improve our knowledge base of marine biological invasions and better inform environmental managers.

No abstract

Up to now our knowledge of water mite diet has been fragmentary. It is derived from observations in the field and laboratory or from a few selective laboratory experiments on food choice. In the present study, we were able to detect chironomid DNA in water mite bodies for the first time using molecular methods. Prey DNA was detected in virtually all Hygrobates fluviatilis (Hygrobatidae) that were fed on chironomid larvae after a starvation period of up to approximately 1 week. From the shortest interval (1 h after feeding) to the longest period after feeding (50 h) the relative amount of detected prey DNA was significantly reduced. In addition, there was a relationship between the relative amount of prey DNA and the assumed amount of the ingested prey (classified in categories of the dead prey which reflect the increasing ingestion of the mites and the decreasing body content of the prey individuals). The results of our study indicate that similar molecular analyses will be a powerful tool for diet investigations of mites from the field on various taxonomic resolutions of prey taxa. Moreover, the results of food selection experiments from the laboratory could be compared to evidence of predation by individuals from the field. For many mite taxa, especially ones which turned out to be difficult to breed in the laboratory (e.g. by unknown diet), the new methods might enable us to gain the first ever data on diet and thus may help us to consider the role of water mites in food webs more adequately in the future.