Assessing the value of follow-up translocations: a case study using New Zealand robins

Following a reintroduction, one or more additional translocations may be conducted to supplement the initial population. Such follow-up translocations are common, but the logic behind them is usually unclear. We used population viability analysis to assess the benefit of supplementing a population of New Zealand robins 14 months after reintroduction, at which time the population had 6 females and 22 males. Parameter estimates from the first 13 months' data suggested marginally positive population growth, hence addition of females was predicted to substantially increase population viability by reducing demographic stochasticity. We therefore proceeded with the follow-up translocation. We re-evaluated this decision 5 years later, by which time it was clear that the normal fecundity rate was much higher than that observed in the first year after reintroduction. Fecundity was also low in the year of translocation for females released the second year, hence the best explanation for this low fecundity was an effect of the translocation itself. Our post hoe analysis predicted that the initial population had had a 100% chance of surviving for at least 30 years, hence the follow-up translocation was unnecessary. We further predicted that even if our initial parameter estimates had been correct, the follow-up translocation could have been delayed by up to 9 years without reducing the benefit derived. The best strategy would therefore have been to wait for additional data, and to reallocate the resources used for the follow-up translocation to research on the reintroduced population. Our simulations only included benefits of follow-up translocations due to reducing demographic stochasticity. However, we also discuss the possible benefits due to reduction of Allee effects, addition of genetic variation, and increased sample sizes, and compare these to the potential costs. (C) 2001 Elsevier Science Ltd. All rights reserved.