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 the 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 compared them with adaptation from standing genetic variation (evolutionary rescue in the strictest sense) using 244 experimental populations founded at either a smaller (50 individuals) or larger (150 individuals) size. Both types of rescue reduced extinction, and those effects were additive. Over the course of six generations, genetic rescue increased population sizes and intrinsic fitness substantially. Both large and small populations showed evidence of being able to adapt from standing genetic variation. Our results support the practice of genetic rescue in facilitating adaptation and reducing inbreeding depression, and suggest that demographic rescue alone may suffice in larger populations even if only moderately inbred individuals are available for addition.genetic rescue | extinction | migration | evolutionary rescue | adaptation H uman activities, climate change, and habitat loss are putting thousands of species at risk for extinction (1). Traditional conservation approaches that concentrate on improving habitat quality and size are becoming challenging to implement as human populations expand and degrade natural resources at an ever increasing rate. Thus, conservation efforts that are constrained by the availability of habitat may instead need to focus on improving a species' prospect of survival by maintaining sufficient population sizes and supporting the ability of populations to adapt to changing conditions. The most successful approaches are likely to be eco-evolutionary in focus, and thus aim to manipulate evolutionary processes such as inbreeding and the potential for adaptation, to influence ecological dynamics and, ultimately, population persistence.Eco-evolutionary approaches often rely on facilitating movement of individuals among small, threatened populations (2). Brown and Kodric-Brown (3) showed theoretically that immigration in natural systems can save small populations from extinction and referred to this process as the "rescue effect." This phenomenon has since been well documented (4-8), and human-facilitated immigration has been used to rescue populations threatened by degraded habitat o...

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The recent Californian hot drought (2012–2016) precipitated unprecedented ponderosa pine (Pinus ponderosa) mortality, largely attributable to the western pine beetle (Dendroctonus brevicomis; WPB). Broad-scale climate conditions can directly shape tree mortality patterns, but mortality rates respond non-linearly to climate when local-scale forest characteristics influence the behavior of tree-killing bark beetles (e.g., WPB). To test for these cross-scale interactions, we conduct aerial drone surveys at 32 sites along a gradient of climatic water deficit (CWD) spanning 350 km of latitude and 1000 m of elevation in WPB-impacted Sierra Nevada forests. We map, measure, and classify over 450,000 trees within 9 km2, validating measurements with coincident field plots. We find greater size, proportion, and density of ponderosa pine (the WPB host) increase host mortality rates, as does greater CWD. Critically, we find a CWD/host size interaction such that larger trees amplify host mortality rates in hot/dry sites. Management strategies for climate change adaptation should consider how bark beetle disturbances can depend on cross-scale interactions, which challenge our ability to predict and understand patterns of tree mortality.