Populations that are adapted to a set of environmental conditions face potential fitness penalties if the environment changes. When these fitness penalties are strong enough to push mean fitness below the replacement rate, populations will decline and are at risk of extinction. A population that avoids extinction by adapting to new conditions is said to experience evolutionary rescue. Evolutionary rescue is of considerable conservation importance because of massive global environmental alteration and degradation. While theory has generated a useful framework for understanding evolutionary rescue, most current models lack common demographic or genetic features of conservation targets. We address one such feature by modifying the classic, deterministic quantitative trait model of Gomulkiewicz and Holt (1995) to feature negative density dependence in a stochastic population model. Using the Gomulkiewicz and Holt framework, we divide the rescue process into two phases: a descending phase (where mean population fitness is below replacement) and an ascending phase (where populations have adapted to the point that mean population fitness is above replacement). We then use the framework to generate predictions for how density dependent growth will influence the probability of extinction and rescue. We test these predictions using an individual-based simulation model.
Results/Conclusions
By depressing the population growth rate, negative dependence increased the risk of extinction and decreased the probability of evolutionary rescue. Density dependent growth increased extinction risk most for populations that were initially large, as small populations are less affected by density dependence. Density dependence primarily increased extinction risk during the descending phase. Thus, once populations had sufficiently adapted, they faced little additional risk of extinction from density dependent growth during the ascending phase. Density dependent growth did not influence the speed of adaptation, and thus did not directly influence the time until evolutionary rescue occurred. However, because negative density dependence increased the rate of extinction and therefore selected for higher fitness, rescued populations experiencing negative density dependence were more fit and experienced faster rescue than rescued populations with no density dependence. Our results suggest that density dependence constrains evolutionary rescue, increasing risk of extinction for populations which that are still adapting to novel conditions or environments. This has consequences for conservation biology and community ecology, influencing which species have adaptive potential to persist under global change. Negative density dependence and other realistic features of population regulation must be considered to create accurate models of evolutionary rescue.