Optimizing release strategies: a stepping‐stone approach to reintroduction

Abstract

Evaluation of alternative management strategies enables informed decisions to accelerate species recovery. For reintroductions, post-release survival to reproductive age is a key parameter influencing population growth. Here, we trial a 'stepping-stone' method to maximize the success of captive-bred animals when the availability of more suitable wild-born release candidates is limited. Our approach makes use of relatively safe and established wild populations to prepare captive-bred individuals for eventual translocation to a final release destination, thus building resilience through establishment of multiple populations over time. We developed a novel multievent model integrating encounter history and biotelemetry data to evaluate reintroduction strategies for the critically endangered Vancouver Island marmot Marmota vancouverensis. We compared post-release survival of 176 individuals (52 wild-born, 47 captive-bred marmots released directly to destinations, and 77 captive-bred marmots released with a stepping-stone approach). Survival estimates to prime breeding-age (PBA), were then used to quantify expected success of potential release strategies. Our analysis indicates that post-release survival varies by source population and release method, as well as age, season, year, and years since release. Conditional on an objective of maximizing survival to PBA, our results suggest that using wild-born marmots for translocations as often as possible, and stepping-stone captive-bred marmots prior to final release, will result in the best outcomes. There was a 0.86 probability that survival to PBA was greater for captive-bred marmots released as yearlings using a stepping-stone approach (survival to PBA mode = 0.13, 95% CRI = 0.05-0.30) than for captive-bred animals that were directly released to destination sites as one-year-olds (survival to PBA mode = 0.04, 95% CRI = 0.01-0.24). Consequently, the stepping-stone approach yields much higher population establishment or growth potential than previous release strategies that used captive-bred marmots. Optimizing the combination of release candidates, sites and timing can thereby increase the effectiveness of reintroductions.

The stepping‐stone approach is promising but we need so much more

“…In their excellent introduction, Lloyd et al (2019) explain how release strategy is known to be a key factor in determining if a translocated population thrives in the wild or not, and has been the subject of much work. We know that the survival of translocated individuals straight after translocation is reduced, especially for captive-bred animals.…”

“…The Vancouver Island marmot is a large herbivorous rodent, which is social and colonial (Lloyd et al, 2019). The Vancouver Island marmot is a large herbivorous rodent, which is social and colonial (Lloyd et al, 2019).…”

“…Obviously, initial survival can have long-term repercussions for the new population, and we are yet to fully address this issue. Lloyd et al (2019) therefore put forward the idea of the 'stepping-stone' approach: release individuals into an existing wild population first, before moving them into historic or novel range to create a new population. Using the Vancouver Island marmot (Marmota vancouverensis), they set out to test their idea.…”

The importance of experimentation in translocation research

“…Lloyd et al (2019) find that translocating captive-bred marmots to sites where they coexist with wild-born marmots before founding new populations yields much higher population establishment and growth potential. Lloyd et al (2019) find that translocating captive-bred marmots to sites where they coexist with wild-born marmots before founding new populations yields much higher population establishment and growth potential.…”

“…Lloyd et al (2019) acknowledge this point, but clearly monitoring animals at the stepping-stone site seems an important element of future studies to ensure established and secure populations are not impacted by such conservation activities. Assuming marmots are limited by burrows, hibernacula and/or food, the population at Mt Washington may well have been at carrying capacity.…”