Get a move on: the value of rescue

A painful, recurring problem in conservation biology is a lack of replication, randomization and experimental controls needed to ascribe the effects of applied treatments to observed outcomes. Imperiled populations are frequently in trouble because they are small and isolated, making it practically and ethically problematic or impossible to test the predictions suggested by elegant theories of population biology. Furthermore, many animal populations that gain conservation attention are large, long-lived and difficult to study on a human timescale. Consequently, there exists a void between studies of shortlived, model organisms in the simplified lab environment and studies of long-lived, charismatic megafauna in the wild. In this issue, Kronenberger et al. (2017) help fill this void with an experiment to test theories of the value of immigrants into small populations using Trinidadian guppies Poecilia reticulata of evolutionary biology fame. The implications of their study are useful and important for conservation biology. Predicting the effects of gene flow, or immigration, is a longstanding problem in conservation genetics and management (Edmands, 2007). A few immigrants can provide much needed adaptive genetic variation and result in recipient population growth (or rescue) because the immigrants relieve inbreeding depression (Frankham, 2015) or inverse density dependence (Brown & Kodric-Brown, 1977). However, population rescue is not always as simple of an outcome as it may first appear (Mills & Allendorf, 1996; Tallmon, Luikart & Waples, 2004). There are risks in adding immigrants to imperiled populations because individuals from adaptively or genetically divergent populations could lead to outbreeding depression, social breakdown, disease spread or other undesirable effects that hasten population extinction (Hedrick & Fredrickson, 2010; Frankham, 2015). The risks of gene flow are heightened by the fact that some populations of conservation concern are cherished precisely because they represent highly divergent evolutionary lineages, and this uniqueness could be lost forever if immigrant genes swamp what has taken aeons to evolve. In this context, Kronenberger et al. (2017), took advantage of knowledge gained from decades of outstanding evolutionary research on Trinidadian guppies to create replicated, small populations of guppies in lab mesocosms. To these replicate populations, they added immigrants either from an adaptively divergent, but moderately genetically divergent population (Fst = 0.12) from the same drainage, or from an adaptively similar, but highly genetically divergent population (Fst = 0.35) from another drainage. Importantly, they also maintained no immigration, control populations. They report benefits of immigration treatments, in particular from the adaptively similar and genetically divergent population. The effects are present across a suite of demographic measures, including population size, which is the gold standard for evidence of rescue. Importantly, there are no observed negative impacts of immigration treatments. Kronenberger et al. (2017) support the small population paradigm and bring experimental rigor to support previous, high profile studies of single, isolated animal populations that precluded replication but still suggested beneficial fitness effects of immigration (Madsen, Ujvari & Olsson, 2004; Hogg et al., 2006; Johnson et al., 2010; Adams et al., 2011). As Kronenberger et al. (2017) discuss, there are a number of reasons to interpret these results conservatively. Reasons to be circumspect include the use of migrants from a single source, the short duration of the experiment (<2 full generations), uncontrolled selection imposed by the artificial mesocosm environment, and the inability to tease apart confounded benefits of immigrants to demography versus genetics. Of course, as long as experimental research is limited by funding there will be trade-offs between external and internal validity in every empirical study. It is important to point out that the findings of Kronenberger et al. (2017) are boosted by similar population rescue effects seen as a result of immigration in a companion, multigeneration study of wild Trinidadian guppy populations (Fitzpatrick et al., 2016). Kronenberger et al. (2017) provide clear evidence of short-term population rescue from moving a few individuals into small populations. They also support suggestions from theory of the value of using adaptively similar source populations for immigrants wherever possible (Frankham, 2015). It is important to point out, however, that while a few immigrants can provide fitness benefits to isolated, small populations and are a powerful, underused weapon in the conservation arsenal, they do not add habitat or halt whatever ecological problems made recipient populations small in the first place (Hedrick et al., 2014). Nevertheless, rescue of wild populations by adding a few immigrants is much