The impacts of climate change and biotic interactions on non-native plants in Norway

Invasive species are considered one of the greatest threats to biodiversity today, causing huge ecological and economic impacts globally. Understanding the factors which may contribute to their success is therefore of paramount importance, particularly in a changing climate. Global warming is predicted to benefit invasive plants by increasing their potential range sizes, causing expansion into higher latitudes, and may result in increased plant performance due to the phenotypic plasticity traits shared by many invasive plants which facilitate their rapid response to change. In addition, biotic interactions, particularly in the form of herbivory, can largely impact the performance of invasive species. The enemy release hypothesis is a leading hypothesis to explain why invasive plants become successful in their invaded ranges, positing that their success is in part due to release from natural enemies. Other biotic interactions which might influence invasive plants include competition with native plants, which may limit the performance of invasive plants or even completely suppress their growth. Research into factors which may limit or facilitate invasive plants is even more important in high-latitude regions such as Norway where climate change is occurring at a faster rate. Norway is already home to over 2,000 non-native species and research into the effects of climate change or biotic interactions on non-native plants in Norway is lacking. In this thesis, I address climate change in terms of future temperature and precipitation changes, and I use latitude as a proxy to represent future climatic warming. I investigate biotic interactions mostly in terms of insect herbivory, but with some focus on competition with native plants. The majority of this work focuses on a variety of different non-native plant species in Norway, but I also include a study across Central and Northern Europe with a Norwegian focus. More specifically, I identified a set of high-risk species which are not yet present in Norway but could become naturalised or invasive if introduced, and investigated the areas of Norway most suitable to these species. I then investigated the enemy release hypothesis and how this changes across latitude, to predict changes in biotic interactions under climate change, using herbarium specimens and botanical garden survey data. Finally, I carried out a common-garden experiment in Norway to determine the effects of artificial warming, herbivore exclusion and native plant competition on the performance of a set of non-native species. From this research, I first found that Norway is already suitable to a large number of non-native species and that its suitability increases under climate change. I then found evidence both contradicting and supporting the enemy release hypothesis, and found that in some contexts herbivory levels are reduced at higher latitudes but in others herbivory does not differ across a latitudinal gradient. Finally, I found that artificial warming increases the performance of non-native species and that presence of herbivores and competition with native plants can limit non-native species performance. These results suggest that in Norway, climate iv change is likely to benefit non-native species either by causing Norway’s climate to be more hospitable and easier to invade, or by facilitating faster growth of non-native plants. Not all non-native plants benefit from enemy release and climate change may result in greater levels of herbivory in some contexts. Finally, non-native plants in Norway may be limited by native biota. These results, particularly in terms of enemy release, show how ecological trends can vary, and therefore highlight how generalisations within invasion science must be avoided and the importance of system-specific research. This research has implications for invasive species management in Norway, by demonstrating which factors may limit non-native plants in Norway and how they might respond to climate change, and should inform biosecurity decisions about which species’ import should be regulated