The dispersal of organisms is primarily a natural process, but in the course of globali-zation it is being extremely intensified by anthropogenic influences. Marine ecosystems in particular are experiencing a large number of biological invasions. Once established, alien species interact with the abiotic and biotic environment of the new ecosystem. Most introduced species become inconspicuous members of the ecosystem, but some have the potential to influence intraspecific interactions. A good understanding of the interactions within an ecosystem is necessary to understand the effects of a biological invasion. This knowledge contributes to a better understanding of how the marine environment is influenced by human activities, which can form the basis for sustainable conservation efforts. The objective of this thesis is to investigate the interactions between native and non-native organisms, ranging from effects on population dynamics to ecosystem functions, by examining the interactions between the two introduced Pacific shore crabs, Hemigrapsus takanoi and Hemigrapsus sanguineus, and native organisms in the oyster reef community in the Wadden Sea. This thesis investigates the populations dynamics of three intertidal crab species at four study sites along the Wadden Sea coastline and how the seasons may affect the population (Part 2), as well as the distribution of the species along a tidal gradient (Part 2). Abundance studies revealed an increase of mean Hemigrapsus spp. densities on mixed reefs of native blue mussels (Mytilus edulis) and Pacific oysters (Magallana gigas) from 2011 to 2020 (Part 2). As the Asian brush-clawed shore crab H. takanoi represents one of the most abundant brachyuran crab species, the feeding habits of H. takanoi, its effects on prey populations, and on the associated community in the newly invaded habitat were studied (Part 3). Experiments revealed that H. takanoi affects the recruitment success of epifauna, while no effect on endofauna were observed. The higher density of H. takanoi compared to the native C. maenas results in a larger impact on their prey community, even though individual consumption rates are lower for H. takanoi. Predators may also have non-consumptive effects on other organisms, therefore the native parasite-host system of Himastala elongata was studied. (Part 4). These experi-ments revealed a complex interaction between non-consumptive predation risk effects on parasite transmission which may constitute an important indirect mechanism affecting prevalence and distribution patterns of parasites across their life cycles. Finally, the thesis investigated parasite infection of the three crab species, as a fundamental biological factor that can influence community ecology (Part 4). The study on infection levels in introduced and native crabs confirms that the native crab indeed experience a higher infection level than their introduced counterparts, H. takanoi and H. sanguineus (Part 5). In addition, the number of parasites is higher in introduced crab species in comparison to the native crab species, which is also evident from the first detection of entoniscid Portunion maenadis (Part 4). Overall, the results show that the two introduced crab species interact with the native biota in a variety of ways and thus influence the biological environment. This thesis also demonstrates that an assessment of the potential effects of an introduced species on the existing ecosystem must be based on a wide variety of investigations, as the potential possible interactions between residents and introduced species are very complex.