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Brichieri-Colombi et al. data set and 39% (94) of articles with zoo involvement. The proportion of publications with zoo staff as coauthors increased over time (r = 0.480, p < 0. 01) (Figs. 1c & d) from 0% in 1974 to 42% in 2013.

 Discussion

Our analyses provide a first insight into the significance of captive breeding for conservation translocations in North America. Over half of published North American animal conservation translocations included releases from captive-bred populations. Yet only one-quarter of such captive-bred species releases come from zoos. This is lower than the 32% contribution of zoos to captive breeding for releases published in the GRP studies (Gilbert et al. 2017) and the 59% reported by Beck et al. (1994). Taxon-specific results, in contrast, closely matched findings in Gilbert et al. (2017), with zoo-sourced releases more likely for amphibians and terrestrial invertebrates than mammals, birds, and reptiles. The negligible proportion of zoo-bred fish (2%) and marine invertebrate (0%) translocations identifies areas where zoos could improve their conservation contribution.

The low overall contribution by zoos to captive breeding for release may in part reflect the limited real estate available for captive breeding in zoos. Moreover, zoos individually and even jointly often hold relatively few individual animals per species (Lees & Wilcken 2009; Conway 2011) and may not house the species in greatest conservation need. Conde et al. (2011) noted that zoos housed only roughly 15% of globally threatened species, and only 6.2% of globally threatened amphibians were held in zoos in 2014 (Dawson et al. 2016). Organizations, other than zoos, that breed animals for release in greater number are often taxon-specific facilities at governmental (e.g., Patuxant Wildlife Research Centre, Maryland), academic (e.g., University of Florida), and private organizations (e.g., San Rafael Aviaries breeding center, British Columbia). State and federal wildlife agencies in particular are heavily involved in North American conservation translocations (Beck et al. 1994; Brichieri-Colombi & Moehrenschlager 2016; Harding et al. 2016). Zoos may be able to increase their contribution of suitable release candidates through collaborations, such as the Conservation Centres for Species Survival (C2S2 2018) and Amphibian Ark (Amphibian Ark 2017), and further study of major contributors, other than zoos, to conservation translocation is warranted to guide alternative collaboration models.

We considered only published research identified using 2 search engines, and search terms may have missed some zoos. Possibly, captive breeding by zoos focuses more on species non-native to North America and thus makes greater contributions to translocations elsewhere. Such decisions may reflect either (or both) strategic decisions around global conservation priorities or the potentially greater public appeal of exotic species (Skibins et al. 2017). Although we suspect that releases of North American zoo-bred animals would be less numerous on other continents than in North America, important conservation contributions have certainly been made in other regions. For example, scimitar-horned oryx (Oryx dammah) (Pauling et al. 2017) were extinct in the wild, but reintroduction releases to Chad have recently been sourced in part from North American zoos. In Europe, EAZA members were involved in some capacity including funding or professional expertise in 42% of non-European conservation translocations (Gilbert et al. 2017), but the proportion including actual animal releases is likely low. We encourage similar analyses in other regions, both those where conservation translocations are common (e.g., Oceania) and those where they are rare (e.g., South America), to help elucidate commitment to native versus non-native species. Similarly, parallel analyses examining involvement of botanical gardens in plant conservation translocations could yield insight on patterns and approaches with potential for mutual learning opportunities.

Although zoo contributions to captive source populations used for release have grown over time (Fig. 1b), the observed trend seems too gradual to establish zoos as significant players in release-targeted breeding any time soon. We encourage zoos to not only increase the proportion of threatened species in respective collections, but also to create network-wide prioritization processes to identify species that would imminently benefit from conservation translocations involving captive breeding. Additionally, greater involvement likely requires an increase in the number of zoos participating in captive breeding linked to foreseeable releases. Stronger engagement should occur with government and non-government organizations in line with the recently adopted One Plan Approach, which encourages zoo collaborations to produce holistic conservation strategies (Barongi et al. 2015). Increased engagement should not only be instigated by zoos; other institutions would likely be surprised by the untapped value zoos could bring to collaborations focused on conservation outcomes in the wild.

Of course, zoos are involved in conservation translocations in many ways other than captive breeding, such as assurance populations, research (Harding et al. 2016), and education, and overall levels of zoo involvement in our analyses matched findings by Gilbert et al. (2017). Such contributions are important and valuable to conservation translocations (Beck et al. 1994) and conservation in general. Especially promising is our finding that zoos have substantially increased their contributions to the science of conservation translocations through authorship. Increased participation in the development and documentation of scientific advances (in addition to strength in hands-on implementation), is in line with the Conservation Biology Volume 33, No. 1, 2019