Dr. Martin Bader

Aim: Biological invasions are a major threat to biodiversity in aquatic and terrestrial habitats. Insects represent an important group of species in freshwater and terrestrial habitats, and they constitute a large proportion of non-native species. However, while many non-native insects are known from terrestrial ecosystems, they appear to be less represented in freshwater habitats. Comparisons between freshwater and terrestrial habitats of invader richness relative to native species richness are scarce, which hinders syntheses of invasion processes. Here, we used data from three regions on different continents to determine whether non-native insects are indeed under-represented in freshwater compared with terrestrial assemblages. Location: Europe, North America, New Zealand.
Methods: We compiled a comprehensive inventory of native and non-native insect species established in freshwater and terrestrial habitats of the three study regions. We then contrasted the richness of non-native and native species among freshwater and terrestrial insects for all insect orders in each region. Using binomial regression, we analysed the proportions of non-native species in freshwater and terrestrial habitats. Marine insect species were excluded from our analysis, and insects in low-salinity brackish water were considered as freshwater insects.
Results: In most insect orders living in freshwater, non-native species were under-represented, while they were over-represented in a number of terrestrial orders. This pattern occurred in purely aquatic orders and in orders with both freshwater and terrestrial species. Overall, the proportion of non-native species was significantly lower in freshwater than in terrestrial species.
Main conclusions: Despite the numerical and ecological importance of insects among all non-native species, non-native insect species are surprisingly rare in freshwater habitats. This is consistent across the three investigated regions. We review hypotheses concerning species traits and invasion pathways that are most likely to explain these patterns. Our findings contribute to a growing appreciation of drivers and impacts of biological invasions.

The impact of plants on soil enzymatic activity and their potential for remediation of oil-contaminated soil has been widely studied but information about tree species is scarce. Here, we used seedlings of four tree species (Ailanthus altissima Mill, Fraxinus rotundifolia Mill, Melia azedarach L. and Robinia pseudoacacia L.) to investigate rhizosphere effects on key soil enzymes (dehydrogenase, urease, alkaline and acid phosphatase) and to evaluate oxidative stress in response to oil sludge-contaminated soil (0, 100, 200, 400 g kg^-1). We observed a large species-specific stimulation of soil enzyme activity often far exceeding the figures in unplanted soil with peaks mostly occurring under low oil contamination (100 g kg^-1). The strongest stimulatory effect on dehydrogenase and urease activity produced R. pseudoacacia at the low and intermediate and F. rotundifolia at the highest contamination level. No clear pattern emerged for acid phosphatase activity, which was equally stimulated by all species at the highest contamination level. Alkaline phosphatase stimulation was dominated by F. rotundifolia at the low and by R. pseudoacacia at the higher oil contamination levels. Foliar H₂O₂ rose significantly in a species-specific manner in response to oil contamination, triggering an upregulation of the antioxidant defence, which began to show signs of exhaustion at the highest pollution level and revealed that oxidative stress was highest in A. altissima. Our results imply that phytoremediation efforts can be optimized through carefully designed plant species assemblages aligning stimulatory effects on soil enzyme activity and oxidative stress tolerance with the severity of the oil pollution.