There is a need for predictive information on emerging invasive species threats and their impacts under rapid climate warming. Comparative functional response experiments have been shown to forecast ecological impacts of invasive species by quantifying their consumption of key resources. Such experiments can be designed to give predictive information under climate warming scenarios by comparing feeding efficiencies of trophically similar invaders and natives under different thermal conditions. I applied this approach to the Tench (Tinca tinca), an invasive Eurasian cyprinid fish currently spreading upstream in the St Lawrence River toward Lake Ontario, and the white sucker (Catostomus commersonii), a native fish with declining populations in Lake Ontario. Prior to these experiments, individuals of both species were acclimated to water temperatures of 18°C and 25°C; the latter temperature is within the range of mean summer maxima projected for the surface waters of the lower Great Lakes in the near future. At both temperatures, I measured the functional response of individuals to larval chironomids, which were offered to the fish in 3-hour trials at randomized prey densities ranging from 2 to 500.
White suckers were more efficient at prey consumption than tench at both experimental temperatures. The prey consumption of white suckers was not affected by temperature, whereas tench had a lower maximum feeding rate at 25°C. Despite their higher feeding efficiency during the short-term trials, white suckers exhibited a diminished body condition after a total of 90 days of being acclimated and tested under the elevated temperature. Therefore, higher per capita feeding efficiency of white suckers might not translate to greater competitive ability in the field, as such an advantage could be eroded if the tench population expands under climate warming. I hypothesize that the tench has a greater thermal compensation capacity than the white sucker, explaining both the stable condition factors in tench at elevated temperatures and a lower maximum feeding rate, contrary to an expected metabolic increase with temperature.