Behavioural and growth differences between experienced and naïve populations of a native crayfish in the presence of invasive rusty crayfish

1. Species invasions are a leading threat to native species and ecosystems. How populations of native species respond to the presence of invasive species will ultimately determine their long-term persistence. 2. In this study, we capitalise on a unique opportunity to compare the behaviour and growth of naïve and experienced virile crayfish (Orconectes virilis Hagen) populations in the presence of invasive rusty crayfish (O. rusticus Girard). In behavioural trials, experienced O. virilis (coexisted with O. rusticus for >30 years) showed more aggressive behaviour than their naïve counterparts. Naïve O. virilis retreated from aggressive interactions with O. rusticus 38% more often than experienced O. virilis. Experienced O. virilis spent 39% more time occupying shelter than naïve O. virilis. 3. There were also differences in O. rusticus behaviour: O. rusticus spent 24% more time occupying shelter with naïve O. virilis relative to experienced O. virilis. 4. In field mesocosm experiments with O. rusticus, naïve O. virilis declined in body mass by 1% while experienced O. virilis' body mass increased by 6%, thus highlighting the potential population-level implications of the previously observed behavioural effects. 5. Our work demonstrates significant behaviour and growth differences between naïve and experienced O. virilis in the presence of invasive O. rusticus. Whether this difference is the result of phenotypic plasticity or evolution by natural selection remains to be determined. Either way, this area of inquiry has implications for managing native populations and species in an increasingly invaded world.