Comment: Novak et al. (2021) overestimated the successes of species translocations and minimized their risks

When deciding whether to translocate a species, it is important to know likely benefits and risks. Thus, Novak et al.'s (2021) analysis of past species translocations to assess their successes and unintended consequences is welcome. However, their conclusion that "The widespread benefits and paucity of negative impacts stemming from conservation translocations are a signal to regulators, decision-makers, and stakeholders that conservationists can be entrusted with the safe and timely use of translocations" is undermined by several problems that led them overstate the successes of translocations and minimize their negative impacts. Their conclusions about the overwhelming success of translocations ("Translocations have played and will play a vital and necessary role in conserving 70% of US endangered species," "conservation translocations routinely yielded their intended benefits") are not supported by their data. The figure of 70% includes the 41% of listed or recovered species for which translocations have been performed, 16% for which translocations are planned, and 12% for which possible future translocations are implied by existing or planned captive propagation. For the 28% of species for which no translocation has yet been performed, it's impossible to know whether translocations will benefit species conservation. Including possible future translocations in the 70% success figure is extremely optimistic because it is unsupported by actual successes. In addition, translocations played "a vital and necessary role" in species conservation for only a fraction of the 41% of species that were translocated. Table 1 summarizes the history and success of translocations for a sample of 20 species randomly drawn from Novak et al.'s data set that they identified as having been translocated. Three to five of these species appear not to have been translocated into nature at all. Ten to twelve of the species that were translocated failed to establish translocated populations, were translocated in such small numbers that conservation impacts up to this point must have been small, and/or were translocated so recently that it is impossible to assess their long-term success. Novak et al. (2021) did not present any quantitative criteria by which translocations could be judged as "vital and necessary," but a generous reading of Table 1 suggests that translocation has been vital to the conservation of perhaps 5 of the 20 species (the Kootenai River population of Acipenser transmontanus, Amorpha crenulata, Gila purpurea, Mustela nigripes, Rana sevosa). Consequently, the number of listed and recovered species for which translocation has been "vital and necessary" is probably closer to 10% than 70%. Novak et al. also minimized the negative impacts of translocations, writing that: "Of the 1014 total taxa we found with recorded conservation translocations spanning 125 years, we found only one restricted instance that caused a loss of biodiversity"; and (referring to biocontrol agents in the United States) "only 3% (21) resulted in negative unintended consequences." But the sources that Novak et al. searched for negative or unintended impacts (e.g., USFWS recovery plans and Schwarzländer et al. [2018]) do not typically contain such information. None of the recovery plans or other USFWS documents used to compile Table 1 contained data on unintended impacts of translocations, although some species in Table 1 are known to have the potential to cause problems (e.g., Gurney et al., 2015; Schiffman, 1994). Schwarzländer et al. (2018) did not address nontarget effects of biocontrols at all, and the compendium upon which it is based (Winston et al., 2014) mentioned only a few known nontarget effects in passing, rather than a complete list. Novak et al. found few negative impacts of translocations because they consulted sources that did not routinely report negative impacts, and so did not provide a reliable estimate of the severity or frequency of possible negative impacts of translocation. Novak et al. further claimed that routine monitoring of game species should detect negative impacts of species translocations. This assumes that monitoring data are strong enough to support analyses of impacts and that someone did such analyses. The paucity and ambiguity of analyses of the effects of zebra mussels (a species with enormous impacts) on game fish populations (Higgins & Vander Zanden, 2010; Strayer et al., 2004) illustrates how unwise it is to rely on detecting impacts using game species. Although unintended impacts of translocations have yet to be accurately assessed, I agree with Novak et al. that most past translocations probably had small unintended impacts. Rather than attributing this to the motivation of the translocation (conservation), I suggest that small unintended impacts have been a product of the kinds of translocations that have been most common: small to modest augmentations or reintroductions of specialized species whose populations are kept small by multiple factors. Based on the literature on non-native species (e.g., Ricciardi et al., 2013), the impacts of species translocations may be predictable from (i) species traits, (ii) how radical the translocation is (augmentation of an existing population vs. re-establishment at a recently occupied site vs. introduction to a new site), (iii) and the rigor of planning and translocation protocols (as Novak et al. emphasized for biocontrol). To the extent that future conservation translocations differ from past efforts in the traits of the species (e.g., potentially widespread keystones like woolly mammoth and passenger pigeon hybrids vs. narrowly endemic specialists), in moving species outside of recent ranges (assisted migration), or in protocols, past impacts may not reliably predict future risks. I agree with Novak et al that translocations have yielded important benefits for conservation and other purposes, and will be essential in confronting climate change and other threats to biodiversity and ecosystem services. But translocations have often failed to reach their goals, diverting resources from other activities, and some have had harmful consequences. If we are to maximize the benefits of future translocations, we must fairly assess their benefits and their risks. Despite their admirable goals, Novak et al. added little to our understanding of how often translocations succeed, or how often they have harmful impacts. This work was supported by the G. Evelyn Hutchinson Chair at the Cary Institute of Ecosystem Studies. None. David L. Strayer conceived and wrote the paper, and did all data analyses. All data collected in this study are presented in the paper or in Supporting Information S1. Appendix S1 Supporting information Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

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