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Re: extinction



American chestnut is close to extinction because of a disease. If the tree 
species survives it will be because of human efforts to control the disease. 
So I'd say diseases can cause extinctions. 

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