In the absence of gene flow (i.e., in fragmented landscapes), populations will become increasingly dependent on standing genetic variation within them and the generation of de novo mutations, both of which are greatly affected by small population size (Frankham et al., 2001, 2019 Weeks et al., 2011 Willi et al., 2006). These can be provided through gene flow between locally adapted populations distributed across environmental gradients (Kremer et al., 2012 Miller, et al., 2020). The fate of many populations will depend on the availability of genotypes that are pre-adapted to future climates rather than de novo mutation (Carlson et al., 2014). Standing genetic variation of small populations will be often insufficient for overcoming the requirements of keeping up with climate change (Aitken & Whitlock, 2013 Hoffmann & Sgrò, 2011 Willi et al., 2006). The genetic integrity of many animal and plant populations is further compromised by rapid climate change which can decrease the adaptedness of local animal and plant populations (Aitken & Whitlock, 2013 Anderson & Wadgymar, 2020 Derry et al., 2019 Hoffmann & Sgrò, 2011). It has been estimated that gene flow is currently inadequate for overcoming these risks in 26% of invertebrate, 29% of vertebrate, and 55% of plant species persisting in fragmented landscapes (Frankham, 2015 Frankham et al., 2019). The harmful effects of inbreeding depression and loss of genetic diversity are recognized as a major contributor to increasing global extirpation rates of small populations of sexually reproducing organisms (Markert et al., 2010 Ørsted et al., 2019 Speilman et al., 2004 Weeks et al., 2011). These processes can lead to the expression of deleterious alleles (known as genetic load, see Willi et al., 2013) and reductions in overall population fitness (known as inbreeding depression, see Frankham, 1995), and elevate risks of maladaptation by compromising locally adapted traits (Lopez et al., 2009) and erosion of standing genetic diversity (Frankham, 2015 Weeks et al., 2011). As a result, remnant populations are often small and isolated, rendering them vulnerable to the negative effects of genetic drift and inbreeding (Frankham, 2015 Weeks et al., 2011 Willi et al., 2013). Many native animal and plant species have highly fragmented distributions as a result of widespread habitat destruction and the ongoing onslaught of invasive species (Frankham et al., 2019). We advocate an approach where short- and long-term strategies are integrated into a decision framework that also considers nongenetic aspects of management.

This overview highlights where strategies are likely to have potential at population, species, and ecosystem scales, but also where they should probably not be attempted depending on the overall aims of the intervention. We emphasize the need to consider the nature of source and recipient populations, as well as associated risks and trade-offs for the various strategies.

We argue that understanding mechanisms leading to species decline and community impact is a key to successful implementation of these strategies. We draw on case studies from the literature and outline mechanisms that underlie how the various strategies aim to increase species fitness and impact the wider community.

Our aim here is to provide some clarification around terminology and to how these strategies are connected and linked to underlying genetic processes. These diverse strategies go by various names ranging from genetic or evolutionary rescue to provenancing and genetic resurrection. Strategies aimed at initiating, reinstating, or enhancing patterns of gene flow via the deliberate movement of genotypes around the environment are generating growing interest with broad applications in conservation and environmental management. In many cases, interventions will likely be needed to safeguard populations and species and to maintain functioning ecosystems. Animal and plant species around the world are being challenged by the deleterious effects of inbreeding, loss of genetic diversity, and maladaptation due to widespread habitat destruction and rapid climate change.

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