Characterising the trophic interactions of invasive species is critical to understanding and predicting their ecological impacts. The comparative functional response metric, which quantifies predator-prey interaction strength, is increasingly used as a tool to predict impacts and prioritize the management of high-impact invasive species. However, functional responses are not consistent across populations nor individuals of a species and, like other physiological metrics, functional responses exhibit intra-individual variation (e.g., plasticity), in response to external conditions. Here, we experimentally examine the potential for plasticity in functional responses in response to conspecific population density, an environmental variable that exerts effects on both individual-level physiology and population-level numeric processes. Using wild populations of two high-impact globally invasive freshwater fish species (Goldfish, Carassius auratus, and Common Carp, Cyprinus carpio), we experimentally applied a 10-week long acclimation to low vs. high conspecific density, then examined comparative functional responses. We found that both species had significantly lower maximum feeding rates after acclimation to high density, although species-level variation persisted, with Goldfish consistently outperforming Carp in feeding trials. Our results suggest that variation in conspecific density can cause considerable variation in per-capita functional responses, consequently affecting impact predictions across gradients of invasion intensity.