Future directions to escalate benefits of the stepping‐stone approach for conservation translocations

Through a reintroduction case study on the critically endangered Vancouver Island marmot Marmota vancouverensis, we introduced a ‘stepping-stone’ approach which utilizes the transition of released individuals among populations to maximize demographic growth potential (Lloyd et al., 2019). We greatly appreciate and hereby reflect on the thoughtful commentaries by Chauvenet (2019), Hayward (2019) and Thévenin (2019) to propose three key topics for future exploration. While the stepping-stone technique showed promise with a colonial, social mammal within a meta-population structure, Chauvenet (2019) wondered what parameters might explain these positive effects to better determine the applicability for other species. We agree such information would be valuable to further marmot recovery, and we also anticipate that the explanatory mechanisms may differ in type or magnitude among species. We encourage an adaptive management approach to iteratively address sources of uncertainty for new trials. As such, ongoing learning would optimize management decisions without precluding the opportunity to test this technique for other species. The stepping-stone approach is attempting to use highly suitable release areas to allow individuals to learn or adapt before transfer to additional sites that are demographically important but have conditions which make establishment difficult. Developing key survival behaviors such as foraging, finding shelter, and avoiding predators are critical for all species regardless of taxa or life history. Hayward (2019) highlighted the challenge of recapturing released individuals at the stepping-stone site before final release at the subsequent location. We agree that potential recapture rates should be predicted and monitored due to potential variation arising through behavior, release site conditions, and tracking technology. In our study design (Lloyd et al., 2019), we addressed this issue, quantified recapture rates, and reflected upon the results to address potential applicability for other species. Specific methods and metrics of stepping-stone trials should be adapted for each species and context. Overall, we believe the stepping-stone approach may be feasible across taxa if three core conditions are present: (1) at least one wild population exists and the program goals include additional population(s); (2) multiple release site populations are available that differ in establishment suitability; (3) individuals can be recaptured at initial release sites and transferred with minimal stress to subsequent release populations. The stepping-stone approach likely has primary applicability for improving the survival of naïve captive-bred individuals in the wild, but it could also be useful for wild-to-wild translocations. Post-release effects afflict nearly all translocations (Armstrong et al., 2017) regardless of source. Even wild-translocated individuals experience higher mortality post-release than resident individuals due to novel environments (Frair et al., 2007). As with captive-bred animals, survival would likely be lower shortly after transfer to final release destinations, but the stepping-stone approach would still be worthwhile if the cumulative survival for wild-to-wild translocations was higher than direct release to the final destinations. The stepping-stone approach could potentially be applied to all conservation translocation types, namely for reinforcements, reintroductions, assisted colonizations, and ecological replacements. The potential benefits of this approach needs to be considered in relation to the potential risks, including species-specific stress associated with capture and transport. Within reintroductions, the stepping-stone approach may be most valuable in the early establishment and growth phases (Thévenin, 2019) where intensive management may be most needed, but applications within the persistence phase could include improvements to supplementary releases aimed at boosting genetic diversity for example. We acknowledge that density dependence at stepping-stone site and final release sites should be monitored closely to ensure that additional releases do not exceed respective carrying capacity. We also suggest that transfers to final release destinations precede the additional supplementation of the stepping-stone site from captive-bred or wild-source populations. The stepping-stone approach can be viewed as part of a continuum of acclimatization tools, as an alternative to ‘soft’ release pens or hardening of individuals before full release. Hayward (2019) gives a good example of combining approaches innovatively as with African wild dogs, where naïve captive individuals are combined with savvy wild individuals in a soft-release pen to help social bonding and survival post-release. Creative application of techniques tailored to the various species and circumstances can help to achieve program objectives. Experimental approaches are increasingly called upon (Seddon, Armstrong & Maloney, 2007) to aid the scientific understanding of conservation translocation techniques and the effectiveness of management actions. Translocation study designs should test specific hypotheses with experimental approaches that enable and strengthen mathematical models to inform outcomes. The case of Vancouver Island marmots, where wild populations once numbered fewer than 40 in the wild, illustrates that small samples sizes and difficult detection challenges need not be barriers to applying an experimental stepping-stone approach. Defining and determining ‘success’ is a topic of constant interest and debate in conservation translocations. ‘Success’ should be evaluated at several stages and scales (Thévenin, 2019), with iterative assessments over time that utilize the monitoring information to optimize subsequent actions. As conservation translocations are normally long-term endeavors with multiple sources of uncertainty, different evaluation indicators that reflect the ecological conditions and align with the long-term objectives should be assessed with appropriate metrics over time. For example, evaluating survival to prime breeding age (PBA) at the final Vancouver Island marmot release sites is a short-term success indicator that iteratively informs release decisions but does not necessarily predict population persistence. Thévenin (2019) pointed out that an advantage of evaluating survival to PBA was that it connected an individual parameter to a demographic process influencing population establishment. We agree with Chauvenet's (2019) point that such short-term success indicators should eventually be evaluated in the context of long-term population persistence. Decreasing extinction risk is a global priority for the current 2020 Aichi targets of the Convention on Biological Diversity, and the role of conservation translocations is sure to escalate even further in the future (Swan, Lloyd & Moehrenschlager, 2018). We appreciate the insightful comments of A. Chauvenet, M. Hayward, and C. Thevenin to optimize the effectiveness of the stepping-stone approach to restore even more imperilled species and ecosystems.

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