Abstract
Threatened species management in New Zealand has been successful largely through the transfer of threatened animals to predator-free offshore islands and predator-free, fenced mainland sanctuaries. These conservation approaches are not always feasible and more recently, conservation programmes have involved the release of animals into unfenced mainland islands where introduced predators are intensively controlled but remain in low numbers. This project involved the reintroduction of buff weka (Gallirallus australis hectori) to Motatapu Station, an unfenced mainland island on New Zealand’s South Island. Past reintroductions of buff weka to their natural range on the mainland have all failed. A lack of post-release monitoring has meant the exact cause and timing of these failures is unknown. This research investigates the ability of buff weka to establish a self-sustaining mainland population in the presence of low predator abundances. Nineteen buff weka (15 males, 4 females) were transferred from predator-free islands in Lake Wakatipu, South Island, to Motatapu Station. Buff weka were held in a soft-release enclosure for six weeks prior to release to allow for acclimatisation to the release site. Using a combination of very high frequency (VHF) and Global Positioning System (GPS) telemetry, the reintroduced population of buff weka was monitored for four months post-release by homing in and sighting each bird approximately every two days.
After their release no buff weka dispersed off Motatapu Station, with the greatest dispersal distance being 2.19 km from the release site. The majority of buff weka survived an initial 30% mortality limit during the first three weeks. However, by the end of the study 15 (79%) buff weka had died due to predation by introduced mustelid species, ferrets (Mustela furo) and stoats (M. erminea). Buff weka took 19.6 days (± 14.4 SD) to settle down and based on adaptive local convex hull (a-LoCoH) home ranges, the average home range of buff weka was 39.23 ha (± 61.90 SD) and found within the 4500 ha predator-trapping area. Using resource selection functions (RSF), buff weka resource selection patterns on Motatapu Station were examined at two spatial scales: 1) home range establishment within the study area; and 2) resource selection within individual buff weka home ranges. At each spatial scale buff weka selected for areas that contained dense ground cover provided by bracken and shrub habitats and also areas that were close to water.
The lack of dispersal by buff weka suggests the presence of favourable resources on Motatapu Station needed for buff weka to establish a population. However, the low survival rate indicates that the existing predator-trapping network is not extensive or dense enough to maintain predator numbers at a level low enough for buff weka to coexist. Predation of buff weka was responsible for the overall failure of this reintroduction. These findings emphasize the challenges faced by New Zealand conservation managers in protecting threatened species in mainland areas. Unfenced mainland islands may be a viable alternative to the more expensive mainland approach of creating predator-proof sanctuaries, however; refinement of this technique will be necessary if threatened species recovery programmes are to meet the goal of establishing mainland populations in New Zealand.