Keystone Species on New Zealand Offshore Islands: Ecological Relationships of Seabirds, Rats, Reptiles and Invertebrates on Cook Strait Islands

Abstract

The influence of seabirds and rats on island ecosystems was measured to assess the applicability of the keystone species concept for ecology and conservation. Pitfall trapping for lizards and small invertebrates, soil sampling and stable isotope analysis was used to assess the roles played by seabirds and rats on six islands in the Marlborough Sounds. Both abundance and ordinal richness of invertebrates were found to be greater on islands with seabirds than on seabird-free islands. Although lizard distribution was strongly influenced by species-specific habitat requirements, the greatest numbers of lizards recorded in this study were found on seabird-inhabited islands. Although significant differences were not found, the C:N ratio of soils in seabird colonies in this study and at other sites was lower than that at seabird-free sites. Nitrogen stable isotope analysis showed that a proportion of the diet of animals at a range of different levels throughout the island foodweb was derived from seabirds. Rats were found to negate many of the positive effects of seabirds. As well as significantly lower numbers of seabirds, islands with rats had lower abundance of lizards and lower abundance and diversity of mall invertebrates than rat-free islands. Although both seabirds and rats play important roles in island ecosystems, neither conformed to a definition of a keystone species. Theoretical and practical problems were found with the calculation of keystone status for different taxa. Given that important species cannot be shown to be keystones, and that calculation is technically difficult (and maybe impossible) the classification of seabirds, rats, or any other species as keystone is not likely to advance theoretical ecology or assist with conservation management. Attempts to define keystone species were found to be unsuccessful and abandonment of the term was advised.