NATURE ECOLOGY & EVOLUTION 1, 0105 (2017) | DOI: 10.1038/s41559-017-0105 | www.nature.com/natecolevol 1 news & views PUBLISHED: 1 MARCH 2017 | VOLUME: 1 | ARTICLE NUMBER: 0105 D e-extinction refers to creating living organisms that are genetically very similar to members of an extinct species. Technical barriers to de-extinction are being steadily overcome, and researchers have begun to ask whether and when de-extinction ought to be done. Writing in Nature Ecology & Evolution, Joseph Bennett and colleagues take up the matter as it pertains to biodiversity conservation — that is, when, if ever, de-extinction should be used as a conservation tool. Teir approach is to try to determine “the relative cost versus beneft for biodiversity” of engaging in de-extinction with reintroduction. Tey do this by frst predicting what conservation eforts for reconstituted species would involve (by extrapolating from conservation eforts for proxy extant species; see pictured examples), and then predicting what the impacts of those eforts would be on extant threatened species 1 . In almost all the cases and scenarios considered, they fnd that the cost to conservation beneft ratio is worse for de-extinction than it is for extant threatened species — that is, funding reconstitution, reintroduction and maintenance of extinct species would either provide smaller conservation benefts than funding conservation of extant threatened species or limit the number of extant species that could be conserved. Because the conservation costs would outweigh the conservation gains, they conclude that “it is unlikely that de-extinction could be justifed on grounds of biodiversity conservation”. Given the scarcity of conservation resources, consideration of conservation costs and benefts is undoubtedly relevant to project decisions. But what role (or how large a role) ought cost–beneft analyses play in those decisions, particularly when they involve novel conservation strategies such as de-extinction? Should they be, as Bennett et al. claim, “a crucial consideration in deciding whether to invest in de-extinction or focus our eforts on extant species”? One concern about afording cost– beneft analyses of the sort conducted by Bennett et al. too large a role in conservation decision-making is that they may not be highly reliable. Among the challenges are: projecting possible outcomes and assigning probabilities to them (the ‘unknown outcomes problem’); assigning values to those outcomes (the ‘value assignment problem’); and converting diferent types of environmental values — for example, ecological, biodiversity, ecosystem services — into a common comparable metric (the ‘commensurability problem’). Many of these challenges are exacerbated when dealing with extinct species because of the greater uncertainties and unknowns involved. Tese arise from, for example, the novelty of the techniques, the less- robust information about the species, and potential confounding factors, such as whether de-extinction would constitute a moral hazard and the interaction efects between de-extinction and other types of conservation eforts and the funding for them. For these reasons, when assessing de-extinction as a conservation strategy in general, or candidate de-extinction projects specifcally, a cost–beneft analysis might be less informative than it is with respect to more traditional conservation strategies for well-studied extant species. Cost–beneft analysis must also not marginalize other important values and considerations. In the case of de-extinction, the most prominent arguments for it do not appeal to its being a cost-efective conservation strategy. Tey are instead based on restitutive justice, recreating lost values, and revising conservation paradigms 2–8 . Tese ethical considerations are thought by those who raise them to establish a positive responsibility to engage in de-extinction. Te question then becomes whether there are any compelling reasons not to do it. Te issue of whether there is a presumption for or against de-extinction is crucial to the cost–beneft issues that concern Bennett et al. Te reason for this is DE-EXTINCTION Costs, benefits and ethics Cost–beneft analysis suggests that the costs of de-extinction could imperil conservation of extant biodiversity in many cases. But there is also an ethical dimension to this debate that cannot be ignored. Ronald Sandler The extinct huia and the extant North Island kōkako. The huia (left, Heteralocha acutirostris) was the largest species of New Zealand wattlebird, and became extinct in the early twentieth century, probably due to a combination of over-hunting and deforestation. The North Island kōkako (right, Callaeas wilsoni) is an extant, but near-threatened, species of New Zealand wattlebird. These and the other species used by Bennett et al. are listed in their Supplementary Information 1 . Image credits: The Natural History Museum / Alamy stock photo (left); David Wall / Alamy stock photo (right). ©2017MacmillanPublishersLimited,partofSpringerNature.Allrightsreserved.