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The Role of Climate Change
Versus Human Impacts—Avian Extinction
on South Island, New Zealand
ABSTRACT
The Late Quaternary avifaunas of South Island, New Zealand,
reveal discrete faunal assemblages for the contrasting environments
offered by wet, closed forest and open, grassland-shrubland-forest
mosaics. These faunal associations are recognizable
in deposits dating from the last glacial (Otiran) and the Holocene
periods. Sites in western regions of South Island exhibit significant
differences in the fauna's species composition between deposits
formed in the last glacial period and those from the present interglacial
period, but sites in the east do not. Several species became
regionally extinct at the end of the glacial period, but all survived
in the east until the present millennium. Although climate change
caused the redistribution of species, all Late Quaternary extinctions
in New Zealand were ultimately caused by humans during
the last 1000 years.
Introduction
New Zealand has three main islands and numerous smaller
ones, and it occupies the southernmost comer of Polynesia in
the South Pacific Ocean. It is of continental origin but has been
separated from other land masses for the last 80 million years
and is now 1500 km from Australia. Its long isolation has resulted
in a unique avifauna with a high degree of endemism
and many flightless species (Fleming, 1979; Millener, 1990;
Bell, 1991).
Fossil deposits have been known from New Zealand since
the early nineteenth century; they are rich in material and are
widely distributed (Atkinson and Millener, 1991; Worthy and
Holdaway, 1993). Most early work sought to describe the
unique elements of the fossil fauna, notably the various species
of moa (Aves: Dinornithiformes; see references in Anderson,
Trevor H. Worthy, Palaeofaunal Surveys, 43 The Ridgeway, Nelson,
New Zealand.
Trevor H. Worthy
111
1989), whereas paleoecological studies were lacking. Although
fossil deposits in caves, swamps, and dunes provided extremely
abundant remains, as recently as 1979 fossil avifaunas older
than the Holocene in New Zealand were considered rare and
limited in size (Fleming, 1979). Since then, extensive investigations
of cave deposits combined with the intensive use of radiocarbon
dating have shown that faunas of the last glacial age
are common (Worthy, 1993a; Worthy and Holdaway, 1993,
1994a, 1995). Analysis of moa faunas throughout New Zealand
showed that there was a pattern to the distribution of species
that was related to habitat (Worthy, 1990).
This paper summarizes some of the important new information
arising out of these and other recent studies of the Quaternary
avifauna of New Zealand by the author and R.N. Holdaway.
The primary purpose of the research has been to
document fossil avifaunas and to describe the faunal changes
brought about by climate during the last glacial-interglacial cycle,
mainly during oxygen isotope stages 1 and 2. Study areas
around South Island, New Zealand, were chosen for the range
of climatic conditions each now has. Each area was kept small,
usually 10-20 km across, to minimize geographic and present
climate variation. These factors are assumed to have been instrumental
in the control of vegetation physiognomies, so a relatively
homogenous vegetation structure within each area is assumed
and is related to the faunal composition. The
distribution of birds was most affected by whether the vegetation
was a closed-canopy forest or a mosaic of shrubland and
grassland. The floral composition of the forest seems to have
been of secondary importance to its structure, because closed
forests, whether dominated by beech or by one of several
podocarps, all had the same moa assemblage in the late Holocene.
Also, grassland-shrubland associations in the subalpine
zone have markedly different floras from those in lowland environments,
but the same birds characterize both areas.
METHODS.—For each study area, all available fossil faunas
of Holocene or Pleistocene age were examined, and extensive
new collections were made. Efforts were made to obtain faunas