The impact of invasive crayfish on stream ecosystems

Invasive crayfish are a serious threat and ecosystem engineers that compete with native species for shelter and food resources, show combative interactions against native species, and negatively affect species diversity. In this work, I used two North American invasive crayfish species that have successfully spread across Europe spinycheek crayfish (Faxonius limosus) and signal crayfish (Pacifastacus leniusculus) as models to assess their potential Impacts on native benthic fish, stone loach (Barbatula barbatula) and bullhead (Cottus gobio), which are among the most common benthic fish in Europe. I have investigated the competition for shelter and agonistic interactions between these invasive crayfish species and the native benthic fish species under laboratory conditions have employed a multi-object tracking algorithm to monitor and visualize the fish's and crayfish's activity inside the experimental tank. Spinycheek and signal crayfish successfully displaced both benthic fish species from their shelter. Both crayfish species attected the behaviour of stone loach, reducing its activity and increasing its hiding outside the shelter. Although bullheads did not reduce shelter use, they displayed similar behavioural changes, if less intense, In addition, I demonstrated remarkable aggressive interactions by both crayfish species against stone loaches and bullheads. Further investigations were performed to assess how variations in crayfish density influence the structure of invertebrate communities. I conducted comparative analyses of community composition across nine stream sites with varying densities of signal crayfish in Wied Stream, Germany. These findings revealed a correlation between crayfish density and diversity and evenness of invertebrates, suggesting that signal crayfish pose a substantial threat to Invertebrate biodiversity In conclusion, this thesis reveals that these invasive crayfish species not only compete with native biota for essential resources but also fundamentally alter the ecological dynamics of freshwater habitats. This thesis emphasises the urgent need to manage and mitigate the potential consequences of crayfish invasion and to preserve native aquatic ecosystems.