Title: Can increased temperatures strengthen the enemy release of non-native species?
Samuel B. Fey, Department of Biological Sciences, Dartmouth College, Hanover, NH
Background
Increased temperatures resulting from anthropogenic climate change will likely restructure the composition of biological communities by allowing new species the opportunity to establish. However, predicting the ability of an organism to establish within an existing biological community remains challenging, especially in the face of climatic warming, and as such, climate change and the successful non-native species are often considered as independent forces that act on systems. Non-native species often succeed in establishing because pathogens, parasitoids, herbivores, or predators detrimentally impact these species to a lesser extent than native species. Here, we develop a theoretical model exploring whether the "enemy release" (ER) experienced by potential invaders is altered by increasing temperatures. We parameterize our model for a general system (enemy, native resource, invading resource) and specific system (fish, native zooplankton, invading zooplankton). We conducted a field mesocosm warming experiment manipulating water temperature (ambient, heated) and the presence of pumpkinseed sunfish (present, absent) with the non-native crustacean Daphnia lumholtzi and native D. pulex to test model predictions that D. lumholtzi would benefit to a greater extent from warmer temperatures in the presence of fish, than from the additive effects of temperature and the presence of fish alone.
Results and Conclusions
Our mathematical model and mesocosm experiment indicate that ER experienced by potential invaders is non-independent of temperature, and in certain instances, increases in temperature have the potential to strengthen the benefits of ER for the invading species. When the consumer and both resources exhibit identical responses to temperature, the invading resource experiences identical levels of ER across all temperatures; however, ER increases with increasing temperature when the enemy (consumer) has a higher thermal optimum or higher thermal sensitivity than both native and invading resources. Our fish-zooplankton model generated the hypothesis that increases in temperature would strengthen ER for non-native D. lumholtzi over intermediate (18-25OC) temperatures, and that its equilibrium abundance would increase with warmer water temperatures. In our mesocosm study, D. lumholtzi relative abundance increased in both mesocosms receiving fish predation and in heated mesocosms, but disproportionately increased in heated mesocosms receiving fish predation. These results suggest a direct connection between climate change and the establishment of non-native species that exists through altered ER.
