What is better for animal conservation translocation programmes: soft- or hard-release? A phylogenetic meta-analytical approach.
Animal conservation translocation is an important tool available to conservation biologists to address problems of isolated, declining or endangered populations. This approach includes both captive-bred and free-ranging origin animals, which are used to rescue genetically limited populations and re-establish extirpated populations. Both soft- and hard-release protocols (the release of animals with or without acclimatization, respectively) are used in animal conservation translocation programmes; however, there is no consensus on whether one has better conservation outcomes than the other. Here, we analysed data from 17 studies to measure the efficiency of both techniques for fauna conservation. Using phylogenetic meta-analysis, we compared results from articles that used soft- and hard-release protocols to determine the overall effect size. In addition, we examined if the success metrics, type of environment, taxonomic group and animal's origin affected the outcomes of each type of translocation programmes. We calculated 61 effect sizes for 17 species. We found that the soft-release protocol is approximate 40% better than the hard-release protocol (Estimates = 0.44, CI 95: 0.11-0.76). Soft-release programme increased success by 77% (Estimates = 0.78, CI 95: 0.37-1.19) when movement metrics were used (as compared to hard-release) and were 41% more successful with terrestrial species. In general, soft-releases showed better outcomes by reducing movements away from the release site, but this was driven mostly by terrestrial reptile translocations (77% chance of success); when birds and mammals or the other success metrics were evaluated, both release techniques had similar effects. Lastly, the origin (i.e. captive or wild) of the released animals did not influence the success rate of soft- versus hard-releases. Synthesis and applications. We conducted a meta-analysis to evaluate which is the best release protocol for success in animal conservation: soft- or hard-release. Our results showed that soft-releases are in general better than hard-releases, especially for reptiles. Protocol outcomes were similar for birds and mammals and were not linked to the origin of the released animals. We recommend that the decision of which protocol to use needs also to consider the financial costs of the used protocol.