Climate warming, plant invasions and plant-enemy interactions

The climate is changing and temperatures are predicted to further increase in the future. Species respond to these changes by either adapting to the local warmer conditions and/or range shifting to higher latitudes. Some of these successful range shifting plants can become invasive in their new range. Therefore, there is a conceptual analogy of successful range shifts and biological invasions originating from other continents. Intra-continental plant species shift their ranges within the same contiguous land mass from which they originate. Inter-continental species originate from other continents from where they have been introduced before expanding in their new range. The aim of this thesis is to gain a better understanding of the plant-insect interactions that may contribute to the success of exotic plants that have expanded their ranges due to climate warming. More specifically I aimed to clarify whether climate warming-induced range expanding exotic plants are less suitable than native congener plants and whether these plants suffer less from aboveground enemies than native congener plants. In addition, I investigated if inter –and intra-continental exotic plant species differed in their suitability and if they responded differently to potential aboveground enemies. In the first greenhouse experiment, I tested the hypothesis that inter- and intra-continental exotic plants and phylogenetically related native plants from the same habitat do not respond differently to two aboveground polyphagous herbivores. Further I tested if intra- and inter-continental exotic plants experience less negative soil feedback than related native plants. I grew fifteen plant species with and without naive polyphagous locusts (Schistocerca gregaria) and cosmopolitan aphids (Myzus persicae) and exposed all plants to soils from their invaded range in order to test the feedback from the soil community to plant biomass production. My results show that that both inter –and intra-continental exotic plants on average were better defended against aboveground and belowground enemies than related native plant species. This suggests that successful range expanding plants may include species with invasive properties. Exotic plants have been shown to have neutral to positive soil feedbacks, while native plants experience negative effects from their soil biota. Belowground interactions can influence aboveground interactions and may change the relationships between exotic plants and their enemies. I examined how the performance of the two aboveground polyphagous herbivores S. gregaria and M. persicae species was influenced by feedback interactions between the plants and their soil biota and compared these responses in intra- and inter-continental exotic and related native plants. Locust mass was negatively affected by the plant specific soil community and larger on native than on exotic plants. Locust survival was also higher on native plants, but not affected by soil type. There were no differences between inter –and intra-continental plants. Aphid population size was not affected by soil type, but was highest on the intra-continental range expander. The body size of M. persicae was larger on control than on soils with specific plant communities and not affected by plant origin. One way of measuring the release of exotic plants from natural enemies is by comparing their herbivore loads with related plants that are native in the invaded range. These loads can be influenced by top down control of insect predators and parasitoids. In the field, I examined herbivore loads and predator pressure on two exotic (inter-continental and intra-continental) and two related native plant species. I found smaller herbivore loads on the exotic plant species than on the related native plants. Moreover, the herbivores on the exotic plants had a higher predator pressure than herbivores on the phylogenetically related native plants. These results imply that both types of exotic plants have a double advantage: enhanced bottom-up and top-down control of herbivores. Finally, I set up a field experiment to test the effect of herbivory on communities of exotic and native plants. I created ten communities with six exotic plant species and their phylogenetically related native species that co-occur in the same riverine habitat. Half of the communities were exposed to herbivory and the other half was grown in a herbivory-free environment. This study was done in order to test if exotic plants may dominate invaded plant communities exposed to aboveground herbivory and if this advantage of the exotic plants under herbivory would disappear when all plants were free of herbivores. Herbivory reduced aboveground plant biomass by almost half. However, exotic plants did not become the exclusive dominants in these communities, as some native species were well protected against aboveground herbivory as well. Plant species varied considerably in their responses to herbivory resulting in changes in community ranking. Interestingly, the proportional biomass contributions to the community were similar for exotic and native plant species and also not different between inter –and intra-continental plants. I conclude that release from aboveground enemies is not the only factor explaining the invasive success of intra- and inter-continental exotic plant species. In conclusion, climate warming-induced range expanding plant species originating from the same continent may possess invasive properties comparable to introduced inter-continental exotic plants. In the greenhouse and in the field, both inter- and intra-continental exotic plant species were more resistant against aboveground herbivores than native plants. In the greenhouse, the exotic plants suffered less from herbivory than related natives, although this did not result in their absolute dominance in the field when exposed to herbivory. Therefore, aboveground enemy exposure is not the only factor predicting the invasive success of intra- and inter-continental exotic plant species