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 realized fitness. These findings reconcile
theoretical discrepancies, showing density dependence can simultaneously
increase extinction risk but may favor adaptation. Our results underscore
the importance of considering density dependence in conservation
strategies.