Constant versus seasonal photoperiod and temperature yield divergent life-history responses to climate warming

A key goal of predictive ecology is to understand how global warming affects ectotherm growth and development. Researchers often rear organisms under constant temperature and photoperiod regimes, raising the question of whether responses under these simplified conditions reflect those in realistic, seasonally varying environments. We examined key life-history traits of the damselfly Lestes sponsa across six combinations of constant and seasonal temperatures and photoperiod regimes, simulating present-day and future climates with a projected 4°C warming. Trait responses differed qualitatively and quantitatively among treatments. Mass and development responses to warming shifted direction depending on regime. For example, the response in mass to warming was reversed between constant (decrease) and seasonal varying (increase) conditions. In addition, developmental synchrony was stronger under seasonal photoperiods, and the temperature-size rule held only under constant conditions. Our results highlight that incorporating seasonal variation is crucial for developing a realistic framework for predicting species' responses to climate change.