Beyond barriers: ecosystem functions of alien aquatic plants

Human impacts have become a dominant force in shaping Earth’s biosphere. One major consequence is the global reshuffling of species, both through intentional introductions, but also unintentional because human infrastructure benefits the movement of certain species. Consequently, ecosystems are increasingly a mix of native and introduced species and can be termed novel communities. Native species and communities are judged on, and valued for, their provisioning of ecosystem functions, which are weighed against any negative impacts. Of introduced species, mainly negative impacts are studied, because of their evolutionary novelty they are assumed threaten native ecosystems, habitats and species. Yet, although introduced species are increasingly abundant, little is known about whether they provide desirable ecosystem functions. The Netherlands contains many valuable freshwater ecosystems. In these systems, freshwater plants provide food to fish, waterfowl and macroinvertebrates, but they also increase habitat structure, inhibit phytoplankton blooms and regulate greenhouse emissions. Many freshwater plants have locally disappear in recent decades due to human impacts on lakes, rivers and ponds, and expensive measures were taken to restore these systems. Yet, in many cases these restored freshwaters are colonised by introduced freshwater plants, which are subsequently eradicated or heavily managed. Dutch freshwaters contain dozens of introduced freshwater plants, yet little is known on the functions that these species provide. The main question of this thesis is whether introduced freshwater species provide ecosystem functions, despite lacking evolutionary history. At the heart is that a judgement of introduced species needs to consider both costs and benefits. In a series of mesocosm and laboratory experiments the functioning of large groups of native and introduced species were compared. Besides testing the role of plant origin in ecosystem functioning, also the role of traits was studied. In conclusion, native and introduced freshwater plants were similar in their provisioning of ecosystem functions. Ecosystem functioning varied strongly with plant identity, and depended on multiple plant traits. Because plant traits show biogeographic patterns, the biogeographic origin of plants is more informative than the traditional native versus introduced dichotomy. In addition, the growth form and tolerance of plants both explained variance in ecosystem functioning. Many introduced plants are well adapted to the human-impacted ecosystems, so combined with the beneficial functions that they provide, introduced plants can help maintain ecosystem functioning in these disturbed ecosystems. Therefore, these results indicate that evolutionary novelty is no persistent barrier in the ecosystem functioning of novel communities. These findings do not imply that introduced species cause no negative impacts. Certain introduced species have negative impacts just like native weeds or pests. Critically, the findings point out that humans may not want to maintain double standards in judging native and introduced species