Costs are key when reintroducing threatened species to multiple release sites

Threatened species with reduced and fragmented habitats can be reintroduced into their historical ranges to establish new populations. Multiple sites might be an option for reintroductions; therefore, managers must determine when to open sites (e.g. establish infrastructure and improve conditions), release individuals into those sites, and eventually cease releases. Careful planning of this schedule, incorporating the cost of actions, is imperative at the outset of a program. To address this challenge, we consider a reintroduction plan under different cost scenarios for three potential reintroduction sites. In particular, we investigate the implications of having either no ongoing site-management cost, a financial ongoing site-management cost, or a demographic cost of continuous releases. We couple population and management models to find a schedule that maximizes total abundance over time of bridled nail-tail wallaby Onychogalea fraenata (released in fixed numbers each breeding season from a stable source population) using stochastic dynamic programming. We find that the type of ongoing cost influences the structure of the optimal schedule. If active release sites cost nothing to maintain, there is no incentive to cease releases. In that case, the optimal schedule is to open sites sequentially, then release individuals to the smallest active population for the entire length of the program. A financial cost for managing active sites alters this result; once all sites are open and have populations of a critical threshold size, sites should be closed sequentially. A higher mortality rate (demographic cost) at active compared to inactive sites completely changes the structure of the optimal strategy. Instead of opening all sites in the first few management stages, only one site should be active any time to reduce the demographic impact of releases. Our general results provide a guide for planning future reintroduction programs and illustrate the importance of categorizing and understanding ongoing costs for reintroduction planning.