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.