Assessing local adaptation in Swedish crayfish : using common-garden experiments on life history traits

Sammanfattning

Local adaptation is a central concept in evolutionary ecology, and comparative studies of ecologically similar native and invasive species can be used to examine the temporal dynamics of adaptive processes. This thesis investigates these dynamics across native noble crayfish (Astacus astacus) and invasive signal crayfish (Pacifastacus leniusculus) populations in Sweden. Noble crayfish have occupied diverse climatic regions across Sweden since the last glaciation, which suggests population-specific adaptations in growth and hatching phenology as observed in other ectotherms studied along latitudinal clines. In contrast, signal crayfish are characterised by a shorter colonization history, and certain populations now exhibit declining body sizes, raising questions about whether these trends result from local adaptation or environmentally induced plasticity. Using common-garden experiments and a PST-FST framework, this thesis examines whether population differentiation in both species have a genetic basis. In noble crayfish, population-level differences in growth, hatching date and survival were detected. Local adaptation in growth represents the most probable inference, though PST estimates were not fully robust, and therefore neutral divergence through genetic drift cannot be ruled out as an alternative explanation. Hatching date showed significant evidence of local adaptation (PST > FST), although environmental carry-over effects cannot be entirely excluded. Signal crayfish exhibited very low PST values and no significant growth variation under common-garden conditions, indicating no genetic divergence in growth and suggesting that observed body size declines reflect environmentally induced plasticity or anthropogenic factors such as harvest pressure rather than local adaptation. Together, these findings show that local adaptation is a temporally dynamic process, shaped by colonization history and environmental variation. The contrasting patterns between noble crayfish and signal crayfish demonstrate how the relative importance of local adaptation and phenotypic plasticity in life-history traits differs between long-established native populations and recently introduced invasive populations, consistent with theoretical expectations. These findings have implications for management, supporting the use of local populations in noble crayfish restocking and emphasising the need for further research on the drivers of body size declines in signal crayfish populations.