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Re: extinction
On Apr 27, 15:07, Earth Magazine wrote:
} Subject: Re: extinction
>
> On the other hand, the biotic invasion of North America since Columbus has
> been a pretty big event, comparable to the opening of a land bridge. Yet it
> didn't cause a mass extinction that will show up all that prominently in the
> paleontological record. What future paleontologists will see (assuming the
> Endangered Species Act works reasonably well) will be a huge influx of new
> species, plus a sudden change in the range of many of the old species, with
> large animals in particular suffering drastic decreases in range. Dozens of
> species, maybe more, have gone extinct, including some important ones like the
> passenger pigeon. More will follow. But it's no K/T, to paraphrase Lloyd
> Bentsen.
>
> Tom Waters
> Earth magazine
> 72350.1764@compuserve.com
>-- End of excerpt from Earth Magazine
There is, of course, one really good example of a land bridge forming, and
the ecological chaos that followed it, about 5 million years ago, when the
Isthmus of Panama formed and North and South America became connected.
It's prehuman, too, so we don't have to worry about any special effects due
to humans.
Did it cause an extinction event? Yes. But how big? Native South American
genera declined by 13% and North American by 11%, but there was also a
significant subsequent radiation of the migrant forms.
That's big enough to notice, but it's nothing like the K/T. I don't really
see why that kind of effect would scale up to K/T levels, no matter how big
the areas being bridged.
Figures from:
Gould, S.J. 1983. "Oh Grave, Where Is Thy Victory", _Hen's Teeth and Horse's
Toes_, New York: W.W. Norton, pp 346-350.
He references:
Marshall, L.G.; S.D. Webb; J.J. Seposki, Jr.; and D.M. Raup. Mammalian
evolution and the great American interchange. _Science_ 215:1351-57.
--
Bob Myers Unocal Energy Resources Division
Internet: Bob.Myers@st.unocal.com P. O. Box 68076
Phone: [714] 693-6951 Anaheim, California 92817-8076