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Title: Three types of rescue can avert extinction in a changing environment

item HUFBAUER, RUTH - Colorado State University
item SZUCS, MARIANNA - Colorado State University
item KASYON, EMILY - Colorado State University
item YOUNGBERG, COURTNEY - Colorado State University
item KOONTZ, MICHAEL - Colorado State University
item Richards, Christopher
item TUFF, TY - University Of Colorado
item MELBOURNE, BRETT - University Of Colorado

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2015
Publication Date: 8/15/2015
Citation: Hufbauer, R.A., Szucs, M., Kasyon, E., Youngberg, C., Koontz, M.J., Richards, C.M., Tuff, T., Melbourne, B.A. 2015. Three types of rescue can avert extinction in a changing environment. Proceedings of the National Academy of Sciences. 112(33):10557-10562.

Interpretive Summary: Preventing extinction of small populations in rapidly changing environments is crucial to long-term preservation of diversity, because the creation of large reserves is often not feasible. An option immediately available to managers is bringing migrants in to increase size or improve genetic composition of populations at risk. We experimentally manipulate different types and combinations of migrants to evaluate which will be most effective in rescuing populations from extinction. We find that migration of numerous individuals can reduce the probability of extinction. Migration of just a few genetically distinct individuals both reduces probability of extinction and dramatically increases fitness and population size. We suggest managers with limited conservation resources should prioritize genetic rescue over increasing demographic size for small populations.

Technical Abstract: Setting aside high quality, large areas of habitat to protect threatened populations is becoming increasingly difficult as humans fragment and degrade the environment. Biologists and managers therefore must determine the best way to shepherd small populations through the dual challenges of reductions in both number of individuals and genetic variability. By bringing in additional individuals, threatened populations can be increased in size (demographic rescue) or provided with variation to facilitate adaptation and reduce inbreeding (genetic rescue). The relative strengths of demographic and genetic rescue for reducing extinction and increasing growth of threatened populations are untested, and which type of rescue is effective may vary with population size. Using the flour beetle (Tribolium castaneum) in a microcosm experiment, we disentangled the genetic and demographic components of rescue, and compare them to adaptation from standing genetic variation (evolutionary rescue in the strictest sense) using 244 experimental populations founded at either small (50 individuals) or large (150 individuals) size. Both types of rescue reduced extinction, and those effects were additive. Over the course of 6 generations, genetic rescue increased population sizes and intrinsic fitness substantially. Demographic rescue facilitated an increase in intrinsic fitness of large but not small populations. Both large and small populations showed evidence of being able to adapt without migrants, from standing genetic variation. Our results support the practice of genetic rescue in promoting adaptation and reducing inbreeding depression, and suggest that in larger populations demographic rescue alone may suffice even if only moderately inbred individuals are available for addition.