Differences between threespine stickleback populations hides any signs of adaptation in response to an invasive predator, northern pike

Dale Stevens II and Kaitlyn Mathis, Department of Biology, Clark University, Worcester, MA, Matthew Wund, Department of Biology, The College of New Jersey, Ewing Township, NJ

Background/Question/Methods
Studying evolutionary responses to invasive species is critical to understanding how populations adapt to this form of human-mediated environmental change. However, distinct populations of the same native species may be locally-adapted to their own unique environments, which may influence how they respond to the arrival of an invasive species. As such, it is important to examine the effects of invasive species on multiple native populations in order to account for potential variation in their evolutionary responses. To address this point, we asked how the antipredator behavior of threespine stickleback fish (Gasterosteus aculeatus) populations in Southcentral Alaska have evolved following the repeated introduction of the predatory apex predator, northern pike (Esox lucius). We used F1 offspring from six stickleback populations (three with and three without northern pike), and measured their behavioral plasticity in response to a simulated pike predation event. We predicted that stickleback from populations with northern pike would show stronger antipredator responses, and be less behaviorally plastic than those without northern pike. However, given the highly locally adapted nature of stickleback populations, any effects of pike invasion may be overshadowed by population differences.
Results/Conclusions
In general, stickleback from pike-invaded populations were more active, and on average more behaviorally plastic than pike-free stickleback; however, our statistical results suggest that the general effects of pike invasion are overshadowed by local adaptation to stickleback populations. This population effect is driven by variation within both the pike-free and pike-invaded groups, as one population in each group more closely resembles most populations in the other group. Thus, our results suggest that the more active and more plastic nature of fish from northern pike predation already exists in at least some stickleback populations without northern pike, and we cannot assume that it is a result of pike invasion. Future work comparing invaded and un-invaded populations needs to strongly consider that the differences observed between populations may be the result of pre-existing local adaptation, and that the only true way to determine the general effects of invasive species is to measure their effects across replicate populations.