Assessing the population impacts and cost-effectiveness of a conservation translocation.
Abstract
Managers often move, or translocate, organisms into habitats that are assumed to be suitable; however, the consequences of these translocations are usually not rigorously assessed. Robust assessment of these management experiments should consider impacts to both donor and recipient populations and compare the cost-effectiveness of translocations to other actions. Here we evaluate the translocations of a federally listed fish species, humpback chub within a tributary to the Colorado River in its Grand Canyon reach (Arizona, USA). We analyse mark-recapture data with multistate models to estimate vital rates (growth, survival and movement) for the donor and recipient populations while accounting for substantial temporal variation in vital rates. We then use stochastic matrix projections to quantify the impact of translocations on adult population size. Lastly, we compare the costs of translocations to another, legally required management action, non-native fish removal, by modifying an existing bioeconomic model. We estimate that six of eight translocations during the study period positively impacted adult abundance and that the overall population impact was positive. Population projections suggest that each chub translocated per year increases the equilibrium adult population size by 1.2 (95% CI: 0.4-2.2) adults, lessening the need for non-native fish removal. The continuation of translocations at the current rate is expected to save managers ~$50,000 per year by decreasing the annual probability of removals from 0.26 to 0.15. Further savings and decreases in removals could be attained by avoiding translocations in years when there has been no winter/spring runoff and modifying the number of translocated individuals based on the estimates of juvenile production in the lower Little Colorado River. Synthesis and applications. Translocations that increase the abundance of a rare species can sometimes be viewed as a hedge against future declines that might necessitate more costly interventions. Quantifying population benefits and economic costs of management actions like translocations and comparing alternative actions can lead to cost-effective conservation that is more easily sustained.