[meteorite-list] Mammoth extinction-- a wimper, not a bang

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Fri, 20 Nov 2009 00:41:15 -0500
Message-ID: <aqacg5pqcu7ja58uc96uk69sknkcgq6jku_at_4ax.com>

http://www.msnbc.msn.com/id/34046350/ns/technology_and_science-science/

Extinction of giant mammals altered landscape
Different plant communities popped up, wildfires increased, study suggests

The last breaths of mammoths and mastodons some 13,000 years ago have garnered
plenty of research and just as much debate. What killed these large beasts in a
relative instant of geologic time?

A question asked less often: What happened when they disappeared?

A new study, based partly on dung fungus, provides some answers to both
questions. The upshot: The landscape changed dramatically.

"As soon as herbivores drop off the landscape, we see different plant
communities," said lead researcher Jacquelyn Gill of the University of
Wisconsin, Madison, adding the result was an "ecosystem upheaval."

Gill and her colleagues found that once emptied of a diversity of large animals
equaling or surpassing that of Africa's Serengeti, the landscape completely
changed. Trees once kept in check by the mammoth gang popped up and so did
wildfires sparked by the woody debris.

The results, which are detailed in the Nov. 20 issue of the journal Science,
could paint a picture of what's to come if today's giant plant-eaters, such as
elephants, disappear.

"We know some of these large animals are among the most threatened that we have
on the landscape today and they have a lot of large habitat requirements and
they eat a lot of food," Gill told LiveScience. "If these animals go extinct we
can expect the landscape will respond."

Dung fungus
Gill and her colleagues analyzed sediment samples collected from Appleman Lake
in Indiana as well as data from sites in New York.

They focused on a dung fungus called Sporormiella that must pass through a
mammal's gut to complete its life cycle and reproduce via spores. More of such
spores indicate more dung and more megafauna around to contribute to the fecal
contents. Within that same sediment, the team looked at pollen and charcoal as
proxies for vegetation and fires, respectively.

Sediment layers accumulate over time and can indicate when the stuff embedded in
it was around. By matching up the dung spores along with vegetation and fire
indicators in certain layers, the researchers figured the large herbivores were
already declining before the vegetation started changing or wildfires took off.

The changes in spore abundance suggest the megafauna began to decline some time
between 14,800 and 13,700 years ago. By 13,500 years ago, the decline was in
full force, Gill said.

Rather than getting vaporized in an instant, the results suggest the animals
gradually dwindled for about 1,000 years.

Here's how it may have gone down: The large herbivores started to decline.
Without such leafy eaters to keep broad-leaved species in check, trees such as
black ash and elm took over a landscape once dominated by conifers. Soon after,
the accumulation of woody debris sparked an increase in wildfires, another key
shaper of landscapes, the researchers say.

What killed the mammoths?
As for what drove the beasts into their graves, Gill says the findings don't put
the nail in the coffin, but do rule out some ideas. To explain the extinction,
scientists have put forth climate change, hunting by humans such as the Clovis
people (known for using advanced spear tips), and even impact by a comet. The
answer could be a combination of several factors, scientists say.

Gill says this new study is a strong one because all of the evidence comes from
one place, and so the researchers aren't making comparisons across different
regions whose sediments may be off in terms of timing.

If the timing is accurate, as Gill says it should be, the findings can rule out
the idea of a meteor or comet killing off the creatures some 13,000 years ago.

And since the plant community didn't change until after the big guys began to
decline, that's a mark against climate change. (A warming climate was considered
the cause of a revamping of vegetation, and thus animal habitat.)

"At this site, we can say that habitat loss didn't cause the decline, because
the major habitat shift happens after the collapse [of the megafauna]," Gill
said. "And habitat change is a big line of argument in the climate camp. If
climate change is causing these extinctions, you'll have to evoke another
process than habitat loss."

Hunting, at least that by the Clovis people, can also be ruled out at the site.

"It seems as though the animals were already in decline by the time [Clovis]
people adopted this tool kit," Gill said, referring to the advanced spear tips
thought to be more efficient at taking down large prey than hunting instruments
used by humans prior to the Clovis.

The new study was funded by the Wisconsin Alumni Research Foundation, the
UW-Madison Center for Climatic Research in the Nelson Institute for
Environmental Studies, and the National Science Foundation.
Received on Fri 20 Nov 2009 12:41:15 AM PST