Evidence for contemporary evolution to optimize behavioral responses to introduced fish

Background/Question/Methods

Introduced predators are one of the leading causes of biodiversity loss worldwide. However, if native prey are not immediately extirpated by invaders, they may evolve in response this new selection pressure. We studied anti-predator behaviors in fish-naive and fish-invaded populations of Cascade frog tadpoles (Rana cascadae) from the Klamath Mountains of California to test the hypothesis that fish-invaded populations have evolved greater reductions in activity and increased refuge use. We raised tadpoles from eggs collected from the field in fish-free aquaria and performed behavioral assays with and without fish chemical cues.

Results/Conclusions

In both population types, fish chemical cues induced strong decreases in activity and increases in refuge use. However, contrary to expectations, the effect of fish chemical cues on behavior was less strong in fish-invaded than in fish-naïve populations. Data patterns suggest that populations co-occurring with fish were less active and in refuges more than naïve populations in the absence of cues, but maintained activity levels and refuge use better when exposed to cues. There may be selection pressure on anti-predator behaviors when fish are present because of high fitness consequences of activity reduction when cues are constantly present. Refuge habitats may cause realized predation risk to be lower than perceived predation risk. Additionally, the risk-allocation hypothesis predicts weakened anti-predator behavior when periods of high risk are frequent. Weakening of the anti-predator response in fish-invaded populations may be a signal of contemporary evolution toward optimization of time spent foraging versus time spent avoiding predation.