Temperature‐Driven Differences in Feeding Performance of Native and Non‐Native Fish of Patagonia: A Functional Response Analysis

ABSTRACT Climate change and species introductions are currently the main threats to freshwater systems. The combination of global warming and salmonid introductions could have an enormous impact on native fishes: particularly affected will be interspecific interactions, key to the structure and functioning of ecological communities. The aim of the present study was to assess the combined effects of species interaction and temperature on the feeding performance, using multiple predator functional response (FR) analysis of a native and a non‐native fish from Patagonia, and the overall effect on the native amphipod used as prey. We evaluated intra‐ and interspecific interactions between the native puyen grande Galaxias platei and the non‐native rainbow trout Oncorhynchus mykiss across four experimental temperatures using the functional response (FR) approach, which describes how prey consumption varies with prey density. FR curves were first estimated for individual fish and then for conspecific and heterospecific pairings. From the individual trials, we predicted ‘expected’ FR curves under the assumption of no interaction between predators. These were compared with the ‘observed’ FR curves in combined trials to quantify intra‐ and interspecific predator interactions. In addition, we calculated the functional response ratio (FRR) as a complementary index to assess prey impact under different experimental conditions. Both species showed hyperbolic‐shaped (type II, destabilizing for prey population) FR curves for all treatments. The single FR curves of O. mykiss were higher than those of G. platei at all temperatures except 21°C, at which prey consumption dropped off abruptly. The strongest effects on prey were observed at high temperatures for G. platei (19°C and 21°C) and at lower temperatures for O. mykiss (13°C and 16°C). The presence of a conspecific had no negative effect on G. platei . Temperature had no effect on the conspecific FR curves. Under heterospecific conditions, prey consumption of G. platei was not negatively affected by the presence of O. mykiss . In contrast, O. mykiss reduced its feeding performance in the presence of G. platei . The overall prey consumption of both species in the heterospecific treatment did not vary with temperature. There was no evidence of a negative intra‐ or interspecific effect on the feeding performance of G. platei . Increasing temperature had a strong negative effect on O. mykiss prey consumption, but not on G. platei , suggesting that under future warming conditions, this non‐native salmonid will be negatively affected first. From the perspective of the prey population, the predation effect is likely to be greatest in the habitats occupied by O. mykiss alone. Our study contributes to the understanding of complex interspecific interactions between native and non‐native species in relation to temperature, and highlights that non‐native species are not always competitively superior to native species. In addition, under global warming scenarios, the non‐native species may be more negatively affected than the native species. This could have implications not only for the southern hemisphere, where galaxiids occur, but also globally in regions where salmonids have established self‐sustaining wild populations and interact with other native fishes.