An a priori assessment of the impact of harvesting from five wild populations for conservation translocations

Abstract

Given the frequency with which translocation is implemented as a conservation tool, remarkably little research has assessed the sustainable management of translocation source populations. We sought to make an a priori estimate of the impact of multiple alternative harvesting scenarios on five passerine species endemic to Norfolk Island which may benefit from future translocation. Population parameters for our five focal taxa were quantified using distance sampling at 298 point surveys conducted in 2019. Intensive nest monitoring between 2018 and 2020 was used to estimate reproductive rates. We modelled population trajectories for all five taxa under alternative harvesting scenarios in forward projections over a 25-year period to assess the likelihood that focal populations could recover from a harvesting event. We used sensitivity analysis to assess the robustness of models to uncertainty around some population parameters. We estimate that Norfolk Island National Park supported 1486 Norfolk Robins Petroica multicolor (95% confidence interval (CI) 1017–1954), 7184 Slender-billed White-eyes Zosterops tenuirostris (95% CI 5817–8551), 2970 Norfolk Grey Fantails Rhipidura albiscapa pelzini (95% CI 2094–3846), 3676 Norfolk Gerygones Gerygone modesta (95% CI 2869–4482) and 1671 Norfolk Golden Whistlers Pachycephala pectoralis xanthoprocta (95% CI 1084–2259) in 2019. All five species were predicted to recover from the harvest of 50, 100 or 150 individuals within 10 years. Despite considerable variation in population parameters, we demonstrate that all five focal taxa have the potential to sustain harvesting at rates required for future conservation translocations. We provide a clear comparison of differing intensity harvesting strategies for on-ground managers. More broadly, we provide a rare example of an a priori assessment of the impact of harvesting for translocation.