Assessing conservation translocation outcomes with standardized metrics based on IUCN case studies

The use of conservation translocations to restore threatened species in the wild is dramatically increasing. Yet, failure of these conservation efforts is frequent and difficult to analyze because of the lack of a shared definition of translocation success, of standardized evaluation metrics, and of consistent durations of postrelease monitoring (PRM). This absence of consensuses hampers the analysis of temporal and taxonomic trends related to translocation planning. We reviewed 298 conservation translocation case studies published by the International Union for Conservation of Nature (IUCN) to compose a thorough overview of the objectives, success metrics, and PRM time frames adopted across translocation projects. We then used a decision analysis framework to establish recommendations on translocation evaluations. We found that 63.7% of case studies were rated successful by their authors, 36% of translocations were monitored over the long term (>5 years), and over time historical taxonomic bias to translocate charismatic species decreased. Translocation success was evaluated using more than 200 different proxies, which we summarized into 17 metrics falling under three categories related to behavior, demography, and health. Certain metrics were differentially used across taxa or PRM time frames or were associated with more favorable evaluations. Each incorporation of a success metric from a different category was associated with a 3.4-fold increase in the odds of being rated as more successful. We recommend evaluating the combination of at least one behavioral, two demographic, and one health objective to assess translocation outcomes in a standardized yet flexible manner across species and projects. Our results highlighted the need for further enabling efforts to support translocations, notably through habitat quality enhancement, threat mitigation, and disease risk assessment. Our review furthers progress toward a much-needed standardized framework to evaluate and improve the outcomes of conservation translocations.