Aspects of Avian Island Biogeography in New Zealand

Abstract

Native land and freshwater birds, lizards and some groups of insects on New Zealand's offshore and outlying islands conform in their area/ species-number relationship to Preston's equation S=CAZ. A distance speciesnumber relationship also exists for birds but the correlation coefficient of the regression is not significant. As far as birds are concerned at least, islands are not at equilibrium in the MacArthur & Wilson sense (with an appreciable immigration rate balanced by a corresponding extinction rate and with a marked turnover of species). Instead, on unmodified islands, species complements are stable and turnover rates very low. Habitats and niches remain unfilled because of the limited number and range of propagules available from the mainland islands. Furthermore, many of the exotic species established by European man on the mainland islands have become self-established on offshore and outlying islands, including the least modified and the most remote. The three main reasons for some birds being absent from islands (aside from that of having perhaps once been present but now extinct or of not making a landfall under favourable circumstances) are: (i) some species are physically incapable of crossing the water gaps; (ii) others are psychologically incapable of committing themselves to such a crossing; (iii) islands are ecologically unsuitable for some species. Among endemics, the longer a form has been present in New Zealand (or the higher its taxonomic status), the more restricted is its distribution, and this applies to birds even when allowance is made for those which are flightless. The generalization may also apply at least to lizards, freshwater fish and some groups of insects. However, this tentative extension of it needs to be critically checked. Changes wrought by man, particularly European man, are sufficient to account for most, if not all, of the extinctions of native species which have occurred in human times.