Lessons from a century of conservation translocations

Translocation—moving individuals for release in new locations—is among the most important conservation interventions for increasing or re-establishing populations of threatened species. However, translocations often fail. To improve their effectiveness, we need to understand the features that distinguish successful from failed translocations.

Here, we assembled and analysed a global database of translocations of terrestrial vertebrates (n=514) to assess the effects of various design features and extrinsic factors on success. Unlike previous reviews, we analysed outcomes using standardized metrics i.e. a categorical success/failure classification, and population growth rate.

Probability of categorical success and population growth rate increased with the total number of individuals released but with diminishing returns above about 20-50 individuals. Releasing more animals may overcome stochastic variation in survival and reproduction, and could also indicate better overall resourcing of projects. There has been no increase in numbers released per translocation over time.

Positive outcomes—reported success and high population growth—were less likely for translocation in Oceania, possibly because invasive species are a major threat on the continent and are difficult to control at translocation sites. Increased rates of categorical reported success and population growth were found for Europe and North America, suggesting the key role of historical context in positive translocation outcomes. Releases of captive animals resulted in negative population growth rates, on average, while those of wild animals were positive. We also found evidence that success differed according to the nature of the threatening process and was higher in projects that released animals over longer periods.

Categorical success has increased throughout the 20th century, but that increase may have plateaued at about 75% since about 1990. No temporal trend in other variables explained this trend suggesting either additional unmeasured variable(s) were responsible for the improvement, or that authors have become more ready to attribute success to their efforts over time.