Assessing whether trait variations among individuals are consistent over time and among environmental conditions is crucial to understand evolutionary responses to new selective pressures such as climate change. According to the universal thermal dependence hypothesis, thermal sensitivity of metabolic rate should not vary strongly and consistently among organisms, implying limited evolutionary response for metabolic traits under climate change. However, this hypothesis has been rarely tested at an individual level, leaving a gap in our understanding of climate change impacts on metabolic responses and their potential evolution. Using the amphipod Gammarus fossarum, we investigated the variability and repeatability of individual metabolic thermal reaction norms over time. We found large variations in both the thermal sensitivity (i.e. slope) and expression level (i.e. intercept) of individual metabolic reaction norms. Moreover, differences among individuals were consistent over time, and therefore repeatable. Inter‐individual variations in body mass resulted in a high repeatability of metabolic expression level but had no significant effect on the repeatability of thermal sensitivity. Overall, our results highlight that inter‐individual variability and repeatability of thermal reaction norms can be substantial. We conclude that these consistent differences among individuals should not be overlooked when apprehending the ecological and evolutionary effects of climate change.

Predator-prey interactions play a central role in community dynamics and thus energy and matter transfers in food webs. Intraspecific variation in traits and particularly in trait combinations involved in trade-offs can alter predator-prey interactions but the underlying mechanisms governing these interactions are still unclear. Quantifying the relevant traits forming trade-off relationships and how these traits determine prey and predator fitness remains a major challenge, even for a single species. Here, we measured multiple traits related to defensive and competitive abilities to investigate the intraspecific trade-off between defence and competitiveness in 6 different strains of the green alga Chlamydomonas reinhardtii exposed to predation by the rotifer Brachionus calyciflorus and examine the consistency of the trait relationships and its consequence for the predator. We found significant differences in defence and competitiveness traits that were used to categorized prey strains as defended against predation and poor competitors, undefended against predation and good competitors, or intermediate in both traits. Furthermore, we found that the different morphological and trophic traits related to defence and competitiveness of prey strains were negatively correlated. The position of prey strains in trait space were consistent independent of the defence and competitiveness trait considered. As we compared trait differences between prey strains coming from environments where selection has favoured one trait or the other, these negative correlations strongly suggested the presence of a trade-off between defence and competitiveness. Our study represents the first empirical evidence of the consistency in the expression of a defence-competitiveness trade-off at the intraspecific level. Assessing the relation between relevant traits and trade-offs and understanding how it translates into fitness of prey and predator allows improving general theory on the outcomes of predator-prey interactions and ecosystem processes.

1. The balance of energetic losses and gains is of paramount importance for understanding and predicting the persistence of populations and ecosystem processes in a rapidly changing world. Previous studies suggested that metabolic rate often increases faster with warming than resource ingestion rate, leading to an energetic mismatch at high temperature. However, little is known about the ecological consequences of this energetic mismatch for population demography and ecosystem functions.2. Here, we combined laboratory experiments and modelling to investigate the energetic balance of a stream detritivore Gammarus fossarum along a temperature gradient and the consequences for detritivore populations and organic matter decomposition.3. We experimentally measured the energetic losses (metabolic rate) and supplies (ingestion rate) of Gammarus and we modelled the impact of rising temperatures and changes in Gammarus body size induced by warming on population dynamics and benthic organic matter dynamics in freshwater systems. 4. Our experimental results indicated an energetic mismatch in a Gammarus population where losses via metabolic rate increase faster than supplies via food ingestion with warming, which translated in a decrease in energetic efficiency with temperature rising from 5 to 20°C. Moreover, our consumer-resource model predicts a decrease in the biomass of Gammarus population with warming, associated with lower maximum abundances and steeper abundance decreases after biomass annual peaks. These changes resulted in a decrease in leaf litter decomposition rate and thus longer persistence of leaf litter standing stock over years in the simulations. In addition, Gammarus body size reductions led to shorter persistence for both leaf litter and Gammarus biomasses at low temperature and the opposite trend at high temperature, revealing that body size reduction was weakening the effect of temperature on resource and consumer persistence.