Distinct impacts of food restriction and warming on growth, reproduction and survival

The reduction of body size with warming has been proposed as the third universal response to global warming, besides geographical and phenological shifts. Observed body size shifts in ectotherms are mostly attributed to the temperature size rule (TSR) stating that warming speeds up initial growth rate but leads to smaller adult size when food availability does not limit growth. Nevertheless, climate warming can decrease food availability by modifying biochemical cycles and primary production. The interactive effects of temperature and food availability on life history traits have been studied in small invertebrate species, but we have limited information on how temperature and food availability jointly influence life history traits in vertebrate predators, despite the observation that TS responses are amplified in larger species. Food availability can also influence growth, fecundity and survival and thus potentially modulate the effect of temperature on life history strategies. In this paper, we filled this gap by investigating under laboratory conditions the independent and interactive effects of temperature (20 or 30 °C) and food availability (restricted or ad libitum) on the growth, fecundity and survival of the medaka fish Oryzias latipes. Our results confirm that warming leads to a higher initial growth rate and lower adult size leading to crossed growth curves between the two temperatures. Food-restricted fish were smaller than ad libitum fed fish throughout the experiment, leading to nested growth curves. In addition, food restriction appears to amplify TSR by decreasing initial growth rate in the cold treatment. Fish reared at 30 °C matured younger, had smaller size at maturity, had a higher fecundity but had a shorter life span than fish reared at 20 °C, suggesting a "live fast die young" strategy under warming. Food restriction increased the survival probability under both temperature conditions corresponding to a "eat little die old" strategy. Finally, food restriction appeared to be advantageous as food restriction largely increased survival while have a weaker negative effect on growth and fecundity and no impact on age and size at maturity. Our results highlight the importance of accounting for the interaction between temperature and food availability to understand body size shifts. This is of importance in the context of global warming as resources (e.g., phytoplankton and zooplankton communities in aquatic ecosystems) are predicted to change in size structure and total abundance with increasing temperatures. Furthermore, we highlight the importance of considering ontogeny when investigating the effects of temperature-induced body size shifts on trophic interactions and community dynamics since thermal effects depend on the life stage of the organisms