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It therefore seems unlikely that the last of many phases of alternate
constriction and expansion of areas of particular vegetation
physiognomies caused by climatic shifts contributed to
megafaunal extinction in New Zealand. New Zealand differs
fundamentally from North America or Australia in that humans
were not present 10,000 years ago. The New Zealand data,
demonstrating a combination of regional extirpations at the
end of the Pleistocene and total extinction when humans arrived,
suggest that neither overkill nor changing environments
are wholly explanatory hypotheses. Both are contributing factors
in a complex interaction. Murray (1991) reviewed the evidence
on megafaunal extinction in Australia and concluded
that although many factors were involved, the megafauna
would have survived until European arrival without the influence
of aboriginal humans. The New Zealand data fit with
Murray's conclusion.
In summary, climatic change at the end of the Pleistocene led
to widespread habitat change in continental Australia and
North America, vastly reducing the available habitat for the
megafauna and their associates inhabiting Pleistocene vegetational
mosaics. These species were thus compressed into small
areas, where, in the absence of humans, they are likely to have
survived as they did in New Zealand. Humans entered both
North America and Australia several thousand years before the
end of the last ice age and its associated climatic and vegetational
changes. It seems probable that the continued exploitation
of megafaunal species as food resources, after environmental
changes had severely constricted the ranges of these
species, then led to their extinction.
The concept of the community in the evolving biota can be
examined by studying the pattern of changes in the range of
species. The individualistic model suggests communities are an
amalgam of species that respond to changes in their environment
in accordance with individual tolerances. As a result,
communities are continually evolving, and modern associations
do not necessarily represent analogs for previous time periods
(Graham, 1985, 1992; Graham and Grimm, 1990). North
American fossil faunas provide much support for this concept
(Graham, 1992). An alternative hypothesis is that individuals
are constrained by their ecological requirements and form recognizable
associations that move across the landscape following
available habitat. Alroy (MS) reanalyzes the North American
data and finds much support for this concept of ecological
tracking. The homogeneity of the Euryapteryx assemblage
throughout Pleistocene and Holocene landscapes of South Island
suggests that this suite of species was inextricably linked
by habitat requirements; hence, it supports the ecological tracking
model. The presence of Euryanas and Aptornis in Holocene
forests of Mt. Cookson is an apparent contradiction of the discreteness
of the Anomalopteryx and Euryapteryx assemblages,
but it is explained by this area being an ecotonal zone between
dry and wet areas and thus supporting a mix of species.
COMPARISON OF CLIMATIC VERSUS HUMAN EFFECTS.—The
small, relatively unmodified area of Takaka Hill invites comparison
of the relative importance of climatic and human effects
on the fauna. At the end of the last glacial period, the
moas Megalapteryx didinus, Pachyornis elephantopus, P. australis,
and Euryapteryx geranoides were displaced from Takaka
Hill. With them went their main predator, Harpagornis
moorei, and at least the associated species Euryanas finschi,
Aptornis defossor, Fulica prisca, and Gallinula hodgenorum.
Dendroscansor decurvirostris was probably lost from the area
at the same time. During the Holocene, the faunal composition
remained constant from the time of forest reestablishment,
about 9000 to 10,000 years ago, until humans arrived in New
Zealand. Major changes in the composition of the fauna resulted
from various human activities and from predation by newly
introduced mammals (Cassels, 1984; Anderson, 1989; Holdaway,
1989; Bell, 1991). Initially, only humans and the Pacific
Rat {Rattus exulans (Peale)) preyed on the native fauna, but
with the coming of Europeans another wave of predators swept
through the forest (King, 1984), and much of the land was
cleared for farming.
The result has been that in the last 1000 years, 10 species on
Takaka Hill have become locally extinct: Apteryx owenii, A.
haastii, A. australis, Anas chlorotis, Strigops habroptilus,
Porphyrio hochstetteri, Xenicus gilviventris, Mohoua ochrocephala,
Callaeas cinerea, and Philesturnus carunculatus. A
further 11 species became globally extinct: Anomalopteryx
didiformis, Dinornis struthoides, D. novaezealandiae, Sceloglaux
albifacies, Aegotheles novaezealandiae, Circus eylesi,
Coturnix novaezelandiae, Xenicus longipes, Traversia lyalli,
Pachyplichas yaldwyni, and Turnagra capensis. Therefore, at
least 21 bird species living on Takaka Hill about 1000 years
ago have been extirpated by the activities of humans and various
introduced mammals, with most losses being among the
terrestrial browser and nocturnal guilds (Holdaway and Worthy,
1996). Of the remaining birds, Nestor meridionalis and
Cyanoramphus spp. are in serious decline in the area. In the
first wave of extinctions associated with Polynesian arrival,
the species that became extinct were large and flightless
(moas, Aptornis, Cnemiornis), and thus susceptible to human
hunting, or were very small and flightless or ground nesting
(e.g., flightless acanthisittid wrens and procellariids), and thus
subject to predation by the Pacific Rat. With the arrival of
mustelids, other rats, and cats, other mainly flightless or weakflying
species became extinct or endangered {Sceloglaux, Strigops,
Nestor). The impact of humans caused the regional or
total extinction of more than double the number of birds that
were only displaced from Takaka Hill at the Otiran/Holocene
transition and so was considerably worse than the effect of
major climate change.
Note Added In Press: In 1998, the Weka Gallirallus australis
went extinct on Takaka Hill. The losses continue.