Density Dependence During Evolutionary Rescue Increases Extinction Risk but Does Not Prevent Adaptation.

Evolutionary rescue allows populations to adapt and persist despite severe environmental change. While well studied under density-independent conditions, the role of density dependence, including competition, remains unclear. Theoretical models offer conflicting predictions, with density dependence either increasing extinction risk or enhancing adaptation. We empirically tested how density dependence influences evolutionary rescue by exposing experimental populations to a stressful environment for six generations under density-dependent or independent conditions, with populations where either evolution was possible or was prevented by replacing individuals each generation. Density dependence suppressed population size and increased extinction risk, whereas density independence enabled rapid growth, especially in genetically diverse populations where evolution was possible. Although density dependence raises extinction risk, it does not prevent populations from responding to selection, since surviving density-dependent populations still exhibited increased intrinsic and realised fitness. These findings reconcile theoretical discrepancies, showing density dependence can simultaneously increase extinction risk but may favour adaptation. Our results underscore the importance of considering density dependence in conservation strategies.