Geoscience Reference
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Human contribution to species extinction actually began before the Holocene, so
we must briefly return to the beginning of our current interglacial, and indeed earlier,
to look at this story.
Evidence from Alaska and the Yukon territory (Guthrie, 2006) suggests that there
woolly mammoths (
Mammuthus primigenius
) and a regional species of horse (
Equus
ferus
) died out around 12 000 years ago, around the time when humans arrived, with
very little overlap. Again the picture is more complex than first appears, for along
with the arrival of humans there is also the appearance of moose (
Alces alces
). Pollen
analysis shows that the area saw a transition through three principal types of biome.
The first was a cold, dry steppe up to around 13 500 years ago in which mammoths
thrived. The second was a transitional moister biome that existed from 12 000 to
13 500 years ago. Finally there was the rise after 12 000 years ago of mesic taiga
and tundra with dwarf birch (
Betula
). The suggestion is that plant species in this
ecosystem did not support many large herbivores. That humans arrived around that
time may have added critical pressure on top of the radical environmental change
taking place, both at the time of their arrival and the episode of ecological change
1500 years earlier. This environmental change was driven by changes in the global
climate as the planet moved out of the glacial. In this instance, humans themselves
were not (solely) responsible for the extinction.
In 2011 a large international team led by Eline Lorenzen, David Nogues-Bravo
and Ludovic Orlando reported their formidable study on late Quaternary megafaunal
extinctions. Beginning around 50 000 years ago, Eurasia and North America respect-
ively lost approximately 36 and 72% of their megafauna. In general, the proportion
of species that went extinct was greatest on continents that experienced the most
dramatic climatic changes, which implies the major role of climate change. However,
the pattern of megafaunal extinctions in North America and Australia approximately
coincides with the appearance of humans and this suggests a possible anthropogenic
contribution to these species' extinctions. To disentangle the processes underlying
population dynamics and extinction we investigate the demographic histories of
six megafauna herbivores of the Late Quaternary: woolly rhinoceros (
Coelodonta
antiquitatis
), woolly mammoth (
M. primigenius
), horse (wild
E. ferus
and domestic
Equus caballus
), reindeer/caribou (
Rangifer tarandus
), bison (
Bison priscus
and
B.
bison
) and musk ox (
Ovibos moschatus
). These taxa were characteristic of Late Qua-
ternary Eurasia and/or North America and represent both extinct and extant species.
Their analysis was based on 846 radiocarbon-dated mtDNA sequences, 1439 dir-
ectly dated megafauna remains, 1557 indirectly dated megafauna remains and 6291
radiocarbon determinations associated with Upper Palaeolithic human occupations in
Eurasia. They then reconstructed the demographic histories of the megafaunal herbi-
vores from the ancient DNA data, modelled past species distributions and determined
the geographical overlap between humans and megafauna over the past 50 000 years.
They found that whereas climate change alone largely explains the extinction of
Eurasian musk ox and woolly rhinoceros (both extinct just before the Holocene), for
many other now-extinct late megafauna Quaternary species changes in abundance
clearly depended on more than one factor. By themselves, neither climate change nor
humans alone can explain most megafaunal extinctions of this time. A combination
of the two is required. The balance of importance of these factors in such extinctions
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