Too hot to fear? Temperature modulates metabolic response of ectothermic prey to predation risk.

Abstract

1. Rising temperature may affect individual performance, e.g. due to metabolism modification. Another stressor affecting metabolism is predation pressure, commonly present in freshwater environments, but rarely integrated into temperature-related studies. Non-native species that evolved under harsh climatic conditions are likely to perform better at elevated temperature than their native counterparts, saving more energy resources for antipredator responses compared to more temperature-sensitive native counterparts. However, this could be further modified by the organism size, as metabolic rate scales with body mass. 2. We investigated interactive effects of temperature, predation risk and body mass on metabolic rate of two amphipod species with different evolutionary history: Gammarus pulex (Gammaridae), a common native taxon in European freshwaters, and Dikerogammarus villosus (Gammaridae), a non-native species in Western and Central Europe, originating from the Ponto-Caspian region. Using intermittent flow-through respirometry, we measured their oxygen consumption rate (a proxy for metabolic rate) along body mass gradient at different temperatures (15 versus 24 oC) and in the presence or absence of fish kairomones (Eurasian perch, a common predator for both amphipod species). 3. We discovered species-specific effects of temperature on body-mass scaling of metabolic rate. The metabolic rate of D. villosus was unaffected by temperature. In contrast, metabolic rate increased with temperature in small, but not in large individuals of G. pulex. Furthermore, metabolic rate of both species increased in the presence of predation risk at 15 oC, but not at 24 oC. 4. Our results showed that both amphipod species have a limited ability to adapt their metabolism to rising temperatures, except small G. pulex. Assuming that the higher metabolic rate enhances prey defence ability, the lack of metabolic response to predator kairomones of both species at the elevated temperature may lead to an imbalance in predator-prey interactions with ongoing global temperature increase. 5. We showed that increased temperature is challenging to both native and non-native amphipods, and temperature effect on the native species depends on its size. Moreover, our study demonstrates that temperature modifies the effect of other common environmental factors, such as predation risk, on amphipod metabolism. Thus, taking these additional factors into account, as well as considering full species size ranges due to size-depended thermal sensitivity, will improve our understanding of the true ecological consequences of global warming in freshwater ecosystems.