Impact assessment of alien macroinvertebrates in Flanders (Belgium)

Besides habitat fragmentation, the introduction of invasive species is considered to be one of the greatest threats to global biodiversity. Due to increased global trade, habitat degradation and climate change the number of species introductions has increased spectacularly during the last decades. This has led to changes in structure and functioning of ecosystems worldwide. In this study, the impact and spread of alien macroinvertebrates in surface waters in Flanders was investigated. To this end, a large database consisting of biological and physical-chemical data was used, which was collected by the Flemish Environment Agency and supplemented with own sampling campaigns. An integrated approach was aimed for, where the results from laboratory studies, long-term field data analysis and data-driven modelling were combined in order to gain insight in the ecology of alien macroinvertebrate species and the drivers that cause changes in macroinvertebrate community composition. A detailed study on the distribution of alien macroinvertebrates in Flanders revealed that in total, 65 alien macroinvertebrates are established of which 40 are regularly encountered in fresh and slightly brackish inland waters. Most alien taxa belonged to the crustaceans and molluscs originating from North America and the Ponto-Caspian region. Many alien species were first discovered in the east of Flanders from where they started the colonisation of the central and western parts of Flanders. Changes in the macroinvertebrate composition were discovered during the last two decades as a result of changing environmental conditions and the introduction of alien species. When analysing the factors that favoured the establishment and spread of alien macroinvertebrates it was found that shipping, hydro-morphological and physical-chemical factors were detrimental for the success of alien macroinvertebrates. Canals, harbours and the polder waters were hot spots for alien species introductions. Small streams were less invaded by alien macroinvertebrates probably because of a higher biotic resistance and the lack of proper vectors. Case studies of different aquatic ecosystems helped understanding the different factors contributing to successful invasions. The case study on the harbour of Ghent indicated that previously degraded ecosystems are favourable for the early establishment by alien macroinvertebrates. With improving chemical water quality due to the installation of wastewater treatment plants and a stricter environmental legislation not only indigenous, but also alien macroinvertebrates colonised the harbour of Ghent. In the polder waters the indigenous brackish water species decreased in abundance, whereas the American amphipod Gammarus tiginus quickly took in a dominant position after its establishment. These observed changes are probably caused by a decrease in salinity which coincided with an increase of freshwater asselids and the euryhaline species G. tigrinus. It was not only the introduction of the invasive amphipod, but the combination with changing environmental conditions that caused the changes in macroinvertebrate composition. The case study on the Belgian coastal harbours confirmed earlier findings that brackish waters are characterised by a low natural species diversity and a relatively high number of alien species. The harbour of Zeebrugge, which received most international ships, had the highest diversity of alien macroinvertebrates and was also characterised by a high site specific biocontamination index. The knowledge gathered during the case studies was used when making predictions on the future distribution of alien macrocrustaceans in Flanders. Based on data-driven classification and regressions trees it was found that alien macrocrustaceans prefer large rivers and canals with a good chemical water quality and that with increasing conductivity the abundance and species richness of alien macrocrustaceans increases in the brackish water environment. When incorporating the improvements in water quality, it was found that the number of alien species (alien species diversity) will increase in the future, but that the fraction of alien species (alien species abundance) will remain stable. In the last step, an integrated model coupling a habitat suitability model, a water quality model and a migration model was developed to predict the future distribution of a highly invasive alien amphipod species, Dikerogammarus villosus. It was found that D. villosus will invade more large watercourses in Flanders during the next fifteen years as a result of decreases in COD, nitrogen and phosphorous loads and an increase in oxygen concentration. It was calculated that D. villosus spreads with an average speed of five km per year and that given the relatively small size of Flanders, migration speed will not limit its maximal dispersal. The here developed model could be applied as an efficient tool by decision makers to perform risk analysis for (potential) invasive macroinvertebrate species to determine the future distribution range as this could help to reduce the number of species introductions and the impact they have on ecosystem functioning. Besides performing risk assessment several management measures, such as ballast water control, regulations regarding the trade of aquatic alien species and further insight in invaded ecosystems are necessary to reduce the further spread and minimise the impact of invasive alien species