Evolutionary rescue is a process where populations declining to extinction adapt evolutionarily to the point that they are capable of recovery and persistence. The probability of evolutionary rescue shrinks the longer a population spends at a small size where it is susceptible to demographic stochasticity and inbreeding depression. Contemporary models of evolutionary rescue acknowledge that time spent at small population size is an important determinant of extinction risk, but many fail to acknowledge a key influence on population growth in many systems: density dependence. Negative density dependence, e.g., due to competition for resources, may constrain population growth as population size increases. We modeled evolutionary rescue under density dependent growth, combining the classic population genetic model of Gomulkiewicz and Holt with demographic model incorporating density dependent growth and several sources of stochasticity. Using this model, we evaluated the effects of both evolution (stabilizing selection) and density dependence on the probability of extinction and time to extinction.
Results/Conclusions
We found that the probability of extinction could increase by orders of magnitude when growth was density dependent. As predicted, this increase in extinction risk was due in large part due to the amount of time populations experiencing density dependence spent at low population sizes, increasing their risk of extinction due to demographic stochasticity. In accordance with other theoretical results, we found that adaptation increased the likelihood of evolutionary rescue occurring, but even in evolving populations rescue was constrained by density dependent growth. These results demonstrate that contemporary models of evolutionary rescue may overestimate probability of evolutionary rescue in natural systems, where population growth is bound by finite space or resources. This insight suggests that additional precautions should be taken for management of small, threatened populations subject to poor environmental conditions.