An experimental test of alternative population augmentation scenarios

Human land use is fragmenting habitats worldwide and inhibiting dispersal among previously connectedpopulations 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 sim ilar to the target population effectively increases population fitness but if immigrants are very genetically or adaptively divergent, augmentation can lead to outbreeding de pression. 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 im m igrant treatments tended to m aintain greater genetic diversity, abundance, and hybridfitness 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 lim ited 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. El uso humano del suelo está fragmentando los hábitats en todo el mundo y está inhibiendo la dispersión entre poblaciones de organismos previamente conectadas, lo que generalmente resulta en depresión endogámica y un potencial evolutivo reducido de cara al cambio ambiental repentino. Para combatir esto, a veces se utiliza el aumento de poblaciones aisladas con inmigrantes para facilitar el rescate demográfico y genético. El aumento con inmigrantes que son similares genética y adaptativamente a las poblaciones receptoras incrementa efectivamente la aptitud poblacional, pero si los inmigrantes son muy divergentes genética y adaptativam