Evidence for contemporary evolution of behavioural responses to introduced fish

Introduced predators are a major driver of worldwide biodiversity loss. However, if endemic prey are not rapidly extirpated by invaders, they may evolve antipredator traits that promote coexistence with invaders. We studied antipredator behaviours in fish-naïve and fish-exposed populations of Cascades frog tadpoles, Rana cascadae, to test the hypothesis that fish-exposed populations have evolved stronger defensive behaviours. We raised tadpoles from field-collected eggs in fish-free aquaria, and performed behaviour assays to quantify their behaviours with and without fish scent cues. Fish scent induced strong decreases in activity and increases in refuge use in both population types, potentially indicating a persistent ancestral response to fish. Populations co-occurring with fish were constitutively less active and in refuges more than naïve populations in the absence of cues, but had a smaller plastic decrease in activity and a smaller increase in refuge use when exposed to cues. Weakening of the antipredator response in fish-invaded populations may be a signal of contemporary evolution towards optimization of time spent foraging versus the fitness costs of time spent avoiding predation.