Variations in tolerance to climate change in a key littoral herbivore

Changes in global climate patterns are affecting marine ecosystems, challenging species’ environmental tolerances, and driving shifts in their distributions. In the Baltic Sea, a brackish water body with low biodiversity, the isopod Idotea balthica is a key herbivore species that has a strong top–down effect on habitat-forming macrophytes. Our aim is to understand how the predicted future combination of hyposalinity and warming will affect the survival of this mesograzer throughout the Baltic Sea. By conducting a manipulative aquarium experiment, we simulated future conditions and measured the survival, at different spatial scales, of replicated populations from the entrance, central, and marginal Baltic Sea regions. Overall, the survival rate was strongly affected by the predicted future combination of hyposalinity and warming, but the intensity of the impact varied both among and within regions. Populations from the marginal Baltic Sea responded negatively to climate change. Populations within the entrance varied in their survival responses, with the geographic variation suggesting the existence of spatially distributed genetic variation in tolerance to climate change. In summary, the future combination of hyposalinity and warming is likely to induce a southward shift in the distribution of I. balthica in the northeast marginal region of the Baltic Sea. However, the geographic variation in tolerance shown by the entrance populations indicates that, for this Baltic region, the species may contain the potential for future adaptive responses in tolerance to climate change.