Patterns of range contractions and extinctions in the New Zealand herpetofauna following human colonisation

Evidence from subfossils and from present distributions confirming range contractions and extinctions of New Zealand amphibians and reptiles is consistent with that from New Zealand landbirds, in which 40% of the fauna, including the largest species, has become extinct in the 1000 years since human arrival. The largest extant species of all higher taxa of herpetofauna—leiopelmatid frogs, tuatara, skinks, and geckos—are extinct on the mainland; 41 % of the extant fauna (27 of 65 species) survive largely or entirely on rat-free offshore islands; and many species are now restricted to a few isolated locations, remnants of once wider distributions, a pattern called "secondary endemism". Habitat alterations and occasional human predation may have contributed to range contractions, but the primary factor in extinctions is almost certainly introduced mammals, especially rats. At least three lines of evidence support this view: (1) species diversities and population densities are both far higher on rat-free islands than on mainland sites and rat-inhabited islands; (2) nocturnal species have suffered far more than diurnal ones—all populations of tuatara, two of four* INTRODUCTION Present concerns about catastrophic species declines the "global biodiversity crisis" are largely centred on continental areas, especially in the tropics. These declines appear particularly tragic because they are the direct effect of human activities that cause habitat loss, and which, with sufficient political will and foresight, could be stopped. However, they have been foreshadowed by similar biodiversity crises centred on island archipelagos, crises which in some areas began several thousand years ago and went unrecorded until they were revealed by recent archeological or paleontological evidence. Species declines on archipelagos can be attributed either to direct human effects, such as the destruction of forest habitats on Easter Island, and the overexploitation that extinguished birds such as the great auk (Alca impennis) and dodo (Raphus cucullatus), or to indirect effects of human actions, such as the introduction of alien species that eliminate resident taxa. As we show in the following review, the Received 4 July 1994; accepted 18 July 1994 ""Currently, three extant species of Leiopelma are recognised, but the Maud Island frog, regarded as L. hamiltoni, is likely to be described as a separate species, based on allozyme electrophoresis (B. D. Bell, C. H. Daugherty, J. M. Hay unpubl.) Editor. 326 New Zealand Journal of Zoology, 1994, Vol. 21 problem of indirect human effects has become the primary concern on many islands, especially in the New Zealand archipelago. Several extinctions and many range contractions were recorded in the New Zealand avifauna during the first 200 years of European colonisation. These declines were well known amongst Victorian ornithologists, who regarded the New Zealand avifauna as "one of the most interesting and instructive in the world..." and whose "'inevitable doom of surviving members" will be viewed with regret (Sibson in Hayman 1984). However, these declines were not unusual amongst the birds of island archipelagos. Over 90% of the birds that have become extinct since A.D. 1600 were island species (King 1984). This total includes 40% of the New Zealand terrestrial avifauna extinguished since the arrival of humans 1000 years ago (Atkinson & Millener 1991). Losses from New Zealand were exceeded in Hawaii, however, where at least 68% of the avifauna became extinct through the direct and indirect effects of prehistoric human cultures (Freed et al. 1987). Unlike many other island archipelagos, New Zealand is distinctive because it is a continental fragment. Many vertebrate taxa were widely dispersed through the archipelago, so, unlike true oceanic islands, there is little site-specific island endemism, except among offshore island groups such as the Kermadec, Three Kings, Chatham, and subantarctic islands (Towns & Ballantine 1993). Also, unlike other temperate oceanic archipelagos, the amphibians and reptiles (especially lizards) have diversified into numerous species occupying a wide variety of habitats (Towns et al. 1985). As a result, New Zealand probably has the most diverse lizard fauna of any temperate archipelago on Earth (Daugherty et al. 1990a). In this paper we review evidence for changes in the distribution of elements of the herpetofauna since the arrival of Polynesians and Europeans, describe known extinctions since human occupation, and examine the causes of both the extinctions and the population declines. We also compare range reductions and extinctions recorded in the herpetofauna with those in the avifauna. NEW ZEALAND HERPETOFAUNA Extinct fossil terrestrial groups and species The first fossil remains of an ancient terrestrial New Zealand herpetofauna were unearthed in the early 1980s, with the discovery of a bone fragment of a theropod dinosaur. Subsequently, fossil bones of pterosaurs, allosaurs, ankylosaurs, and hypsilophodonts have been discovered by J. Wiffen (Cox 1991), all from the Late Cretaceous Haumurian Formation (66.5 m.y. B.P.) of the eastern North Island. The presence of these fragmentary dinosaur fossils provides further evidence of the Gondwanan links of the early New Zealand vertebrate fauna. However, although the extant sphenodontids seem universally regarded as an archaic Gondwanan element in the fauna (e.g., Bell et al. 1985; Stevens et al. 1988), early fossils of this group have yet to be found in New Zealand. The known recent terrestrial herpetofauna of New Zealand comprises 71 species, six of which (three species each of frog and lizard) are presumed extinct (Table 1). However, of the lizards presumed extinct, only one, Cyclodina northlandi, is at present known in subfossil deposits (Worthy 1991). The skink Leiolopisma gracilicorpus was described as a presumed juvenile attributed to the "kawekaweau", a large lizard reported by Maori and early European explorers (Hardy 1977). The single known specimen was subsequently found to be a sexually mature male (D. Towns pers. obs.) and could therefore be of an extant species. A New Zealand origin for the gecko Hoplodactylus delcourti was assumed by Bauer & Russell (1986) because of its morphological affinities with other members of the genus and its Table 1 Species diversity estimates of the recent extant and extinct New Zealand terrestrial herpetofauna (includes species for which descriptions are still to be published). Data on species numbers are from Daugherty et al. (1994).