New species from the Pacific : Establishment and dispersal of two invasive crabs (genus Hemigrapsus) in German coastal waters
Human-mediated introductions of species crossing oceans and continents gain increasing scientific and public attention as a major biological consequence of globalisation. Marine ecosystems are facing particularly high numbers of biological invasions. Gaining knowledge about factors promoting invasion success and how they interact with each other is crucial to assess impacts of invasions, as well as future invasion events and range extensions. The first study in this thesis gives an introductory overview on this topic, highlighting the importance of vectors, species traits, interactions and invasion history as promoting factors. The scope of this thesis is an investigation of such promoting factors and their effects in a recent marine invasion in Europe, the notable case of the northwestern Pacific crabs Hemigrapsus takanoi and H. sanguineus. To gain an in-depth understanding of the investigated factors, a comprehensive approach, combining observational, experimental and genetic data, is used. The thesis contains the first records of H. takanoi from the Baltic Sea (2nd study), and evidence for successful reproduction of H. takanoi in the Baltic Sea, by reconstructing its full larval cycle (3rd study). It further identifies two early life-history-traits as potential promoters of establishment success for Hemigrapsus spp. despite the presence of a native competitor (4th study), and contains a genetic analysis of European H. takanoi populations, providing evidence for multiple introductions from the native ranges, and secondary transport within Europe. Overall, this thesis shows the invasion of Hemigrapsus spp. in Europe to be a still dynamic and ongoing process, illustrated by the reported introduction and establishment of H. takanoi in the Baltic Sea. It furthermore shows that combinations of different aspects of a species' biology, in this case ecological adaptations and interactions as well as invasion history, jointly contribute to invasion success in marine species.