Evolutionary rescue in source-sink systems: When does dispersal best facilitate adaptation in declining populations?

Lily F. Durkee and Ruth A. Hufbauer, Graduate Degree Program in Ecology, Colorado State University, Lily F. Durkee and Ruth A. Hufbauer, Agricultural Biology, Colorado State University

Background/Question/Methods
A critical goal of modern evolutionary ecology is to establish how to effectively manage populations in human-altered habitats. Source-sink dynamics can arise when a population with good-quality habitat and high fitness exists that is able to support declining populations in poor habitats through immigration. Dispersal can delay extinction and can also provide novel genetic material that may facilitate adaptation and demographic recovery in sink populations, a process called evolutionary rescue. However, dispersal among habitats that differ in quality can also slow or prevent adaptation to local conditions. Source-sink dispersal, therefore, can promote adaptation and evolutionary rescue, or it can prevent adaptation, limiting population fitness. This study evaluated source-sink dispersal using the red flour beetle, Tribolium castaneum, as a model system. Poor-quality sink habitats were treated with a pyrethroid insecticide. Three dispersal strategies were chosen that were guided by current conservation recommendations: 1 migrant per generation, 5 migrants per generation, a single large dispersal event, as well as a no migration control. Two types of migrants were also evaluated: individuals with high demonstrated dispersal ability (dispersing migrants) and non-dispersing migrants. Differentiating migrants by dispersal ability will shed light on whether observed behavior can be used as criteria to choose individuals for translocation.
Results/Conclusions
The impacts of one-way source to sink dispersal of dispersing vs. non-dispersing migrants were assessed by tracking population size and fitness over multiple generations. By generation 3, only populations receiving no migrants had regained a positive growth rate in the sink environment, illustrating how migration can constraint adaptation. Interestingly, among populations with dispersal, those receiving dispersing migrants exhibited higher fitness and lower extinction rates when compared to those receiving non-dispersing migrants. This may be occurring because dispersing migrants have depleted energy reserves and mate less with resident individuals, allowing adaptation to occur. In the longer term (generations 5-10), we expect populations receiving migrants to have higher fitness than controls due to the influx of novel genetic material and reduction in genetic load. Overall, this work seeks to determine the utility of established management dispersal strategies for both recovering long-term fitness and promoting adaptation to a novel environment.