Non-indigenous Freshwater Plants

Larson, D. 2007. Non-indigenous Freshwater Plants – Patterns, Processes and Risk Evaluation. Doctoral dissertation. ISSN 1652-6880, ISBN 91-576-7314-5. Non-indigenous species (NIS) are species that are moved by man outside their native range. NIS that successfully pass through a number of invasion phases (i.e. introduction, establishment and invasion) are referred to as invasive species. Invasive species potentially cause severe environmental and economic impacts. This thesis highlights the invasion process of non-indigenous aquatic plant species. Patterns and processes in the establishment phase were assessed with a general focus. The importance of recipient area biodiversity for habitat invasibility was assessed by comparing native species richness in lakes where non-indigenous plant species had become established with non-invaded lakes. It was shown that habitats with established NIS on average had higher native species richness. Ecological niche modelling was used to test how the continental-scale distributions of non-indigenous freshwater plants in Europe might change during global warming. These models showed that species presently invasive in southern Europe can potentially establish also in northern Europe if the climate gets warmer. At the invasion phase, focus was more specific to a single species – the floating-leaved freshwater plant Nymphoides peltata. Microsatellite analyses of introduced N. peltata populations showed that six of seven sampled water systems contained a single genotype. Thus, where the species is introduced, vegetative reproduction constitutes the most important spreading mechanism. Impact of N. peltata on submerged species was assessed by using both a modelling approach and using traditional competition experiments. Both approaches showed that submerged plants were negatively affected by low to moderate N. peltata densities. This thesis contributes to a wider understanding of the invasion processes of nonindigenous freshwater plants in general, but of N. peltata in particular. Knowledge retrieved from more NIS case studies are needed before general patterns can be assessed.