Friday, May 12, 2006

Update on the megafauna extinctions update The Guthrie paper mentioned earlier is up! Sub only though. New carbon dates link climatic change with human colonization and Pleistocene extinctions (Nature 441, 207-209 (11 May 2006))

Abstract:
Drastic ecological restructuring, species redistribution and extinctions mark the Pleistocene–Holocene transition, but an insufficiency of numbers of well-dated large mammal fossils from this transition have impeded progress in understanding the various causative links1. Here I add many new radiocarbon dates to those already published on late Pleistocene fossils from Alaska and the Yukon Territory (AK–YT) and show previously unrecognized patterns. Species that survived the Pleistocene, for example, bison (Bison priscus, which evolved into Bison bison), wapiti (Cervus canadensis) and, to a smaller degree, moose (Alces alces), began to increase in numbers and continued to do so before and during human colonization and before the regional extinction of horse (Equus ferus) and mammoth (Mammuthus primigenius). These patterns allow us to reject, at least in AK–YT, some hypotheses of late Pleistocene extinction: 'Blitzkrieg' version of simultaneous human overkill2, 'keystone' removal3, and 'palaeo-disease'4. Hypotheses of a subtler human impact and/or ecological replacement or displacement are more consistent with the data. The new patterns of dates indicate a radical ecological sorting during a uniquely forage-rich transitional period, affecting all large mammals, including humans.


There's one figure that would make all clear, but violating copyright laws ain't my bag, baby, so a description will have to suffice. He took several hundred C14 dates for five species (some new, some published) -- mammoth (Mammuthus), horse (Equus), bison (Bison), wapiti (Cervus), and moose (Alces) -- from the Alaska/Yukon Territory and compared them to dates for human materials in the same area. The results showed no wapiti or moose in the area until after 13k BP, very few bison until around 13.5k BP, whereas horse and mammoth were both common up until ca. 12.5k BP and 11.5k bp, respectively. The first human remains show up at a little before 12k BP.

Three hypotheses he examined were:
1) Disease: Ruled out because the extinctions were not synchronous.
2) Keystone:

Nor do these dates lend support to the 'keystone' hypothesis3, which proposes that the removal of mammoths by humans led to vegetational transformation, which in turn led to a secondary set of extinctions. In this AK–YT chronology, horse extinction seems to have preceded the demise of the proposed keystone species, woolly mammoth, by a calendar millennium.


This was also addressed in another paper (could've sworn I linked to it. . . .) on the dating of horses in Alaska that concluded (via statistical techniques anyway) that the likely latest horse remains were much closer to the latest mammoth remains.

3) Blitzkrieg:

The millennium-wide disjunction of first human dates and terminal mammoth dates indicates that humans could well have had a hand in the gradual extinction of mammoths, but not as in a century-scale 'Blitzkrieg' overkill2 in which a newly arriving wave of super-efficient human hunters broadly and abruptly devastated local megafaunas.


Basically, the dates for the earliest humans (ca. 12,300 BP) and the latest terminal date for mammoth (ca. 11,500 BP) overlap so much that the Blitzkrieg model -- positing a very rapid extinction shortly after the arrival of humans -- should not apply. Further, he notes that numerous other extinct species (camels, giant beaver, ground sloth) have terminal dates in this area well before initial human colonization (ca. 18k BP) which does not implicate them either. Also noted is that while there is abundant evidence for hunting of some non-extinct species in the area (bison and wapiti), there is not for the extinct horse; this odd pattern has been noted by Grayson and Meltzer as well for other species in North America.

The other question besides extinction is why the other species became so abundant at the same time:

Why were the two grazing specialists, bison and wapiti, absent or rare in AK–YT for five millennia before the transitional period, and how are we to understand their later remarkable boom? We know that conditions in this part of the mammoth steppe during the LGM were extreme. Dune fields, thick loess deposits and the virtual absence of lake sediments indicate arid and windy conditions with a largely treeless, short-grass–sedge–sage sward5, 16. These conditions of sparse forage and lack of riparian cover and alternate forage might account for the gap in bison and wapiti dates, but I suspect that the competitive advantage of the two caecalid grazing specialists, mammoth and horse, also had a role.


He goes on to argue that those species most adapted to the Pleistocene steppe environment -- the "caecalids" -- are the ones that went extinct and that the change in climate and vegetation that took place at the beginning of the Holocene strongly favored those species indicated as increasing. He also argues that many of the same environmental changes also favored human expansion into the area.

He's careful not to state that humans had no role in the observed extinctions, in AK/Yukon or elsewhere. Still, it does tend to put the kibosh on at least the Blitzkrieg model.