Light and energetics at seasonal extremes limit poleward range shifts

Seasonality in light becomes increasingly extreme at high latitudes, both in terms of the diel light–dark cycle and the duration of light summers and dark winters. In contrast to temperature, this latitudinal gradient in light seasonality is not affected by climate change. A key question is therefore whether light may act as a fixed constraint on warming-driven redistributions of organisms at high latitudes. One answer is provided by studying mechanistic models of visual foraging and temperature-driven physiology along latitudinal gradients to project where populations survive and acquire resources to reproduce, and where they demise. Here we contrast such models for two widespread planktivorous fish types. We identify two processes through which seasonality in light can act as a barrier to poleward range expansions at high latitudes: (1) longer dark winters lead to greater depletion of overwinter energy stores and (2) a longer duration of midnight sun entails higher foraging-related predation mortality. Using mechanistic models that incorporate visual foraging and temperature-driven physiology for two fish types, the authors reveal how latitudinal light gradients, which are not affected by climate change, can constrain warming-related shifts to high latitudes.