An experimental test of alternative population augmentation scenarios

Abstract

Human land use is fragmenting habitats worldwide and inhibiting dispersal among previously connected populations of organisms, often leading to inbreeding depression and reduced evolutionary potential in the face of rapid environmental change. To combat this augmentation of isolated populations with immigrants is sometimes used to facilitate demographic and genetic rescue. Augmentation with immigrants that are genetically and adaptively similar to the target population effectively increases population fitness, but if immigrants are very genetically or adaptively divergent, augmentation can lead to outbreeding depression. Despite well-cited guidelines for the best practice selection of immigrant sources, often only highly divergent populations remain, and experimental tests of these riskier augmentation scenarios are essentially nonexistent. We conducted a mesocosm experiment with Trinidadian guppies (Poecilia reticulata) to test the multigenerational demographic and genetic effects of augmenting 2 target populations with 3 types of divergent immigrants. We found no evidence of demographic rescue, but we did observe genetic rescue in one population. Divergent immigrant treatments tended to maintain greater genetic diversity, abundance, and hybrid fitness than controls that received immigrants from the source used to seed the mesocosms. In the second population, divergent immigrants had a slightly negative effect in one treatment, and the benefits of augmentation were less apparent overall, likely because this population started with higher genetic diversity and a lower reproductive rate that limited genetic admixture. Our results add to a growing consensus that gene flow can increase population fitness even when immigrants are more highly divergent and may help reduce uncertainty about the use of augmentation in conservation.

Recent evolutionary history predicts population but not ecosystem‐level patterns

“…Regardless of the mechanism, the finding of significantly higher individual growth in both Quare and Marianne gene flow treatments is surprising in light of the results from Kronenberger et al (), which documented consistent genetic rescue in Quare but not Marianne. The lack of consistent benefits of gene flow in Marianne treatments was previously interpreted as resulting from (a) higher overall initial levels of genetic variation in the recipient population and (b) relatively few generations of admixture over the course of the study.…”

“…Following conclusion of the full 32‐month gene flow study, subsets of these populations were moved to the MSU Experimental Pond Facility for our experiment in July 2017 (Figure ). Our experimental populations were comprised of Kronenberger et al () fish from three evolutionary history treatments across two populations: Quare with no prior gene flow, Marianne with no previous gene flow, Quare with previous gene flow from a HP source population, Marianne with previous gene flow from a HP source population, Quare with previous gene flow from a LP source population (designated “LPF” in Kronenberger et al, ), and tanks with no fish to provide a control for ecosystem measurements. Fish were not available for a Marianne LP gene flow treatment.…”

“…This study builds on a previous experiment that tested fitness effects of gene flow in replicated populations housed in glass aquaria in a laboratory environment (Kronenberger et al, ). Kronenberger et al () documented strong and consistent evidence for genetic rescue in a recipient population (Quare) that had initially lower levels of genetic variation, but inconsistent effects of gene flow in a second recipient population (Marianne) that started with higher levels of genetic variation.…”

“…This study builds on a previous experiment that tested fitness effects of gene flow in replicated populations housed in glass aquaria in a laboratory environment (Kronenberger et al, ). Kronenberger et al () documented strong and consistent evidence for genetic rescue in a recipient population (Quare) that had initially lower levels of genetic variation, but inconsistent effects of gene flow in a second recipient population (Marianne) that started with higher levels of genetic variation. Initial differences between the two recipient populations in heterozygosity (Quare H e = 0.71; Marianne H e = 0.87) and effective population size (Quare N e = 247; Marianne N e = 949) suggested that Quare guppies experienced stronger genetic drift than Marianne, and therefore, stood to benefit more from gene flow initially.…”

“…Guppies in the present study were sampled from subset of gene flow treatments from Kronenberger et al () (a) where recipient populations received either no augmentation (None), gene flow from a high predation (HP) guppy population, or gene flow from a low predation (LP) guppy population. Populations were censused bimonthly for a total of 32 months with a 6‐month pre–gene flow period, 12‐month gene flow augmentation period, and 14‐month post–gene flow period (b).…”