Intra- and intercontinental variation in the functional responses of a high impact alien invasive fish

Recently, a body of literature has indicated the utility of comparisons among introduced and native species of their functional responses, that is, the relationship between resource use (e.g. predator consumption rate) and resource availability (e.g. prey density) to predict their impact. However, a key feature of this methodology, that has not yet been examined, is the degree to which the functional response curves of an introduced species differ within and between its native and introduced geographical ranges. Information on the variation in functional responses is key to make robust assessments on the ecological impact and to assess possible differences between native and invasive species. Here, we examine the predatory functional responses in multiple native and introduced populations of a globally high impact alien invasive fish, the topmouth gudgeon (Pseudorasbora parva). In standardised aquaria and with two prey species, significant differences in the functional responses among and between different populations occurring in the native and the invaded range were found. Among populations in the native range, the functional response indicated little variation, and fish always showed a Type II response, irrespective of the type of prey used. In the introduced range, populations showed a Type II response when fed chironomid larvae as prey, while a Type III response was observed when feeding Daphnia magna. Populations in the invaded range consumed overall more prey when fed D. magna compared to the populations in the native range. When feeding chironomid larvae, no consistent trend was observed. Context dependencies as well as species-specific traits and fish density most likely play an important role when comparing the functional response between populations occurring in their native and invaded ranges.