Nutrient enrichment and climate warming effects on plant and herbivore invasions in wetland communitie

Ecosystems invaded by one invader often contain abundant populations of other invaders. This may reflect facilitative invader interactions, common response to anthropogenic factors (nutrient enrichment and/or climate warming), or result from common paths of introduction. My dissertation asked 1) do multitrophic invaders reciprocally facilitate each other's invasion, 2) does nutrient enrichment or climate warming contribute to plant invasions in native plant wetland communities and/or in exotic herbivore invaded communities, and 3) do nutrient enrichment, warming, and plant invasions individually or interactively affect exotic herbivore invasions in native plant communities. Results from a factorial field mesocosm experiment manipulating the order of exotic plant and herbivore invasions showed no evidence of reciprocal invader facilitation that would lead to an invasional meltdown in wetlands invaded by Pomacea maculata and Alternanthera philoxeroides. Pomacea maculata facilitated A. philoxeroides by preferentially consuming native plants. Alternanthera philoxeroides did not facilitate P. maculata invasions, suggesting that P. maculata invasions are independent of A. philoxeroides invasions. A factorial greenhouse mesocosm experiments showed that increasing nutrients directly increase P. maculata size, that exotic plants are not an ideal food source for P. maculata. Another greenhouse study showed that P. maculata size increased with increasing nutrients independent of plant origin (native vs. exotic). I conducted a field mesocosm experiments that showed that nutrient enrichment contributed to plant invasions independent of warming and that warming tended to reduce exotic plant performance. In another field factorial mesocosm experiment, I found that increasing nutrients increased plant growth, but favored exotic plants and that warming increased P. maculata's reproductive performance four-fold. Together these results suggest that exotic herbivores with great impacts on native plants will facilitate plant invasions, that nutrient enrichment and warming will increase herbivore invasions, and that nutrient enrichment will increase plant invasions. Furthermore, it is critical to assess species interactions and interactive anthropogenic factor effects on native and exotic species because they cannot be predicted based on single anthropogenic factors.