The role of pine plantations in source-sink dynamics of North Island robins

Managed pine plantations now constitute a large portion of mainland New Zealand. Despite many native birds inhabiting these exotic habitats, their value for biodiversity conservation is unclear. Although numerous studies have quantified densities of native bird species in pine plantations, it is unknown whether these individuals constitute self-sustaining populations. Here we address this question for North Island robins (Petroica longipes) in a Pinus radiata plantation in the central North Island. We compare survival and reproduction data of robins collected in three different compartments of the plantation from 2003-2005 to similar data collected from 20 unmanaged native forest fragments from 2002-2014. We used the data to derive estimates of finite rate of increase (lambda) using a Bayesian hierarchical modelling framework that accounted for site-to-site and temporal variation. The mean reproduction rate was much lower in the pine plantation, with females producing 0.34 (SE 0.15) independent juveniles per year, in comparison to 1.02 (SE 0.21) in fenced native fragments and 0.83 (SE 0.21) in grazed native fragments. These differences are attributable to lower nest survival, as nests in the pines had a 0.09 (SE 0.05) probability of surviving to fledging. compared to 0.33 (SE 0.04) for fenced native fragments and 0.28 (SE 0.06) for grazed native fragments. In contrast, the mean adult female survival probability was 0.64 (SE 0.13) in the pines in comparison to 0.55 (SE 0.04) in the native fragments. The lambda estimate for the pine plantation was 0.76 (SE 0.14), meaning lambda was unlikely to be >1 as is required for a self-sustaining population. The mean lambda was estimated to be 0.89 (SE 0.09) for fenced native fragments and 0.83 (SE 0.09) for unfenced native fragments, but varied among fragments with estimates close to 1 for some fenced fragments. Therefore, the pine plantation probably constituted sink habitat that retained robins due to immigration from the surrounding landscape, whereas at least some of the native fragments could potentially be self-sustaining.