Are introduced takahe populations on offshore islands at carrying capacity? Implications for genetic management

Translocation to island reserves is a common strategy in New Zealand and elsewhere for safeguarding species against introduced predators. When successful, however, the closed nature and relatively small size of many island sanctuaries can result in populations quickly reaching their carrying capacity, which in itself can present further challenges such as reduced productivity and population growth rates associated with density-dependent effects as well as increased rates of inbreeding. As part of its management strategy, small numbers of the highly endangered takahe (Porphyrio hochstetteri) were translocated during the 1980s and 90s from the last remaining natural population on the mainland of New Zealand to four offshore islands where introduced predators had been eradicated. We used logistic regression and generalised linear models to assess trends in population growth and recruitment and to evaluate whether the island metapopulation displays density-dependent effects on productivity. Our results indicate that the island metapopulation appears to have reached its carrying capacity, as reflected in an increasing ratio of non-breeding to breeding adults, and recent declines in juvenile production. These density-dependent effects are likely to constrain management strategies aimed at maintaining genetic diversity and minimising inbreeding. A recommendation to increase the immigration rate of takahe onto islands via translocations of birds from the source population on the mainland may be ineffective unless surplus birds are removed.