Cultural Eutrophication Potentiates the Ecological Consequences of Fish Invasion and Invader Diversity

Species invasions often have strong effects on invaded habitats, potentially causing dynamic feedbacks between the receiving system and the invading organism.  One type of feedback can occur where invading populations diverge in ecologically relevant traits resulting from exposure to different environments, such changes in turn feed back to influence invader trait distributions.  Ultimately, such “eco-evolutionary dynamics” that couple changes in the invader and environment may in theory exacerbate or mediate changes in local ecosystem conditions.  We investigated possible interactions between ecological and evolutionary processes by examining recent divergence in a common invasive fish species, the white perch (Morone americana).  White perch populations in Maine have successfully invaded lakes spanning a productivity gradient, providing diverse selective pressures that may result in altered fish morphology, physiology and ecological role. Adaptation by these fish may, in turn, feed back to affect lake productivity and community structure through several ecological and chemical pathways.  By using mesocosms seeded with plankton and fish from different source ponds we were able to cross fish (oligotrophic, eutrophic, no fish) and lake community (oligotrophic, eutrophic) treatments to determine the relative importance of fish invasion and fish population divergence on ecosystem conditions, as well as how such effects might differ with background lake conditions.  Juvenile white perch captured from oligotrophic and eutrophic lakes were first reared for a short time in a common garden environment, whereupon, fish were stocked in tank mesocosm habitats.  We hypothesized white perch adapted to eutrophic environments would lead to greater stimulation of algal production because of changes in trophic morphology and ecological role observed during a survey of white perch across a trophic gradient of lakes.  Algal standing crop (chl-a) was monitored over the course of the 4 week experiment while gross primary production (GPP) was measured at the end of the experiment.  No significant differences in chl-a were observed in the oligotrophic environments: fish source and no fish treatments were statistically indistinguishable.  However, in the eutrophic environments, significant interactions were observed between fish and community source.  Similarly, the presence of fish or fish source did not influence GPP in oligotrophic environments.  However, in eutrophic environments, effects became visible.  These results suggest eutrophication may potentiate the emergent effects of invaders and invader diversity on ecosystem conditions.  Hence, cultural eutrophication and fish invasion may not only be problematic on their own, but interact through ecological and eco-evolutionary feedbacks that may compound ecosystem change and management challenges.