Theoretical models have shown that for populations in decline, evolutionary rescue via natural selection can lead to renewed population growth, but if population size decreases to a small enough level, the population is at risk of extinction due to demographic stochasticity. After more than seventy generations of exposure, prairie dogs have not developed significant resistance to Yersinia pestis infections, suggesting that when prairie dog populations are infected, they succumb to stochastic effects before evolutionary rescue can occur. We use an ecologically-validated model of this scenario to explore the potential for adaptive resistance to prevent extinction following Y. pestis exposure. We extend the model to evaluate the potential effects of vaccination strategies on the ability to evolve increased resistance.
We find that when maladapted populations, such as prairie dogs, are exposed to a highly pathogenic and environmentally persistent pathogen, such as Y. pestis, there is no opportunity for heightened resistance to evolve, and extinction occurs as a result of mortality from infection and demographic stochasticity. In contrast, species starting with higher resistance factors are capable of evolving further increased resistance, and extinction is rare. Vaccination added to the model maintains sustained populations, but it also allows for an increased infectious reservoir populated by susceptible offspring. These findings can be used to influence management decisions regarding susceptible populations.