Restoring Ririwha (Stephenson Island)
Introduced rat species have significant impacts on seabird populations around the world through their predation on eggs and chicks. This is particularly so for breeding colonies on islands. One example of such predation occurs on Ririwha, an island owned by Ngā Uri o Whakakii (a hapū of Ngāpuhi) off Whangaroa Harbour, Northland, which has remnant populations of burrowing petrels and also kiore (Polynesian rat) and Norway (brown) rats, the former in large numbers. The hapū, via the Ririwha Restoration Trust, have asked Phil Lyver and Chris Jones to determine the effect of rats on egg survival and breeding success of oi (grey-faced petrel) populations and to develop population models to predict the recovery of oi in the absence of rats. This work is in line with Ngā Uri o Whakakii’s plan to eradicate rats from Ririwha to allow the restoration of its native plants and animals.
The island has, until recently, been used for farming and is still stocked with sheep, which are maintained to help manage the introduced grasses, kikuyu in particular, that cover much of the island. Small remnants of native vegetation persist in gullies and around the coastal margins.
Oi are of cultural significance to northern iwi, and once formed the basis of widespread customary harvest. However, their present numbers make such harvest unsustainable. Restoration of the small remaining oi populations on Ririwha is likely to be achieved only by removing the rats. It is hoped that the knowledge gained by Chris and Phil from this work (and from the eventual rodent eradication programme) will complement the team’s other research on sustainable harvesting of oi on the Aldermen Islands and Moutohora (Whale Island), and will guide the management of the birds’ recovery on Ririwha.
The team is looking at how egg survival and breeding success of oi vary in both time and space relative to the presence of rats. Prior to the proposed island-wide eradication of rats, the researchers have set up trapping grids, including 120-m-wide buffer zones, over two remnant breeding colonies to remove all rats while the birds are laying and incubating. Breeding success of oi is being monitored using fibre-optic cameras, or ‘burrowscopes’, which allow observers to detect breeding birds, eggs or chicks deep inside the birds’ burrows. Results from the trapped areas will be compared with those obtained from similar grids in other colonies where the rats are monitored but not trapped.
As well as being used to compare breeding success of oi in the presence and absence of rats in the same season, the untrapped areas will eventually be used to compare the breeding success of oi over time as the rat eradication programme (under the guidance of John Parkes) gets underway. The breeding performance of oi will be monitored for 3 years and the resulting data will be used in simple stage-based population growth models to predict the trajectory of the birds’ recovery.
The project is in its early stages, but the team has already carried out rat trapping at the two removal sites (c. 2 ha) for the 6 weeks of the oi breeding season when eggs and young chicks are most vulnerable to predation by rats. In just over 12,000 trap-nights, over 400 kiore were removed from the two grids. Both the researchers and the hapū are keen to find out how much this has contributed to the breeding success of oi in those small colonies as a forerunner to future island-wide eradication of rats, and both groups have high hopes for the ongoing restoration of Ririwha and its native biodiversity.
The Trust’s long-term vision is one of co-existence of people and native species to maintain historical and traditional practices in and around the island. Mike Sheehan of the Trust believes that such restorative initiatives provide a continuity of past and future practices that will, in turn, allow for new and developing technologies to stimulate growth and new opportunities.
This work is funded by the Foundation for Research, Science and Technology (Programme C09X0908: Te Hiringa Tangata-Bicultural Restoration of Coastal Forests using Sea Birds).