Climate warming and nutrient enrichment increases invasive plant competitive ability and invasive apple snail reproduction (Pomacea maculata) in native wetland communities

Background/Question/Methods

Habitats are often invaded by multiple invasive species which may result from interactions among invaders or common responses to environmental conditions. Climate change has been predicted to expand species distributions and enhance invasion success. Current knowledge on warming effects on invasions is based primarily on bioclimatic models that often neglect other environmental factors. Investigating multiple environmental factors effects on invasive species is critical due interactions among multiple factors at the community level. In this study we asked: Does herbivore growth and reproduction depend on plant origin? How does nutrient enrichment, warming, or their interaction influence plant and herbivore invasions? We addressed these questions using a 2x2x2 factorial mesocosm field experiment replicated 5 times for 16-weeks. Forty freshwater native wetland communities were established and randomly assigned to: 1) warming (ambient or increased 2 deg C), 2) nutrient addition (control or 6 ppt N & 1.7 ppt PO4), and 3) plant origin (2^nd round of planting was exotic plants or additional native plants). All mesocosms received apple snails (Pomacea maculata).

Results/Conclusions

Apple snail mass was higher with nutrient addition only with warming. This suggests that nutrient addition with warming will enhance snail invasion success but that such increases would be limited in current climate conditions. Apple snail reproductive output increased 4 fold with warming versus controls and was independent of other factors, suggesting that warming alone will increase apple snail population growth rate and contribute to range expansion. Exotic plant biomass increased with nutrients. Native plant biomass increased with nutrients only in absence of exotic plants. This suggests that nutrient enrichment will accelerate invasions. Exotic plants were unaffected by warming while warming reduced native plant fitness, suggesting that warming may enhance plant invasions in these wetland ecosystems. This study indicated that multiple invasions do not reflect interactions among exotic species. Rather, habitats invaded by multiple invasive species are a result of common responses to environmental factors including interactions among anthropogenic factors. Significant interactions among factors emphasized that investigating multiple factors in invaded habitats is critical, therefore we cannot rely on bioclimatic models that do not consider other environmental factors to predict composition patterns, invasion success, and impacts on native species.