Evolution of predator-prey interactions during range expansion in an aquatic insect predator

1. Many ectotherms are shifting their distributions polewards, which has been associated with the evolution of phenotypic traits and their thermal plasticity. Trophic interactions may determine the dynamics and ecological impact of range expansions. However, it is largely unknown how trait evolution in edge populations shapes trophic interactions. 2. We studied evolutionary changes in the short-term (functional response) and long-term predator–prey interactions between an aquatic insect predator (the damselfly Ischnura elegans) and its prey (the water flea Daphnia magna) during the predator's ongoing poleward range expansion in northern Europe. 3. Using a common-garden warming experiment at 20 and 24°C we tested for differentiation between predator populations from edge and core regions in metabolic rate and functional response parameters, and used these empirical data to estimate the effects of range expansion on the short- and long-term predator– prey interaction strengths. 4. Metabolic rates did not differ between populations from edge and core regions nor between rearing temperatures. Functional response parameters and their thermal plasticity showed signals of evolution during the range expansion. Attack rates did not differ between predators from edge and core regions, but only decreased under warming in predators from the edge region. Handling times decreased under warming in predators from the edge region but increased under warming in predators from the core region. While handling times were shorter in predators from the core region at 20°C, these did not differ between regions at 24°C. As a result, the short-term interaction strength was higher for predators from the core region at 20°C, but not different between regions at 24°C. The predator–prey system from the edge region showed lower long-term system stability at 20°C, but this region difference disappeared under warming because the edge region stability then increased. 5. Our results suggest that rapid evolution of functional response parameters during a predator's range expansion reduced the direct feeding impact on its prey and made the predator–prey system from the edge region more unstable, but not under warming. This provides rare evidence that functional responses can rapidly evolve during range expansions, potentially destabilizing food web dynamics.