I think the one distinction that has to be made (and I believe has been made by people like Davis and Gurevitch) is that there is empirical evidence of invasives causing extirpations across trophic levels (i.e. invasive predators and pathogens). It is within-trophic level (i.e. competitive exclusion) extirpations for which there is such little empirical evidence. And I am not at all certain, given the lack of evidence that it happens, that the spread of exotics will cause widespread extinction due to competition. Doesn't mean it won't happen, I'm just not certain it will. It would not be difficult to come up with a plausible model that included widespread dispersal of plants and no increase in the rate of extinctions.
Jeff Houlahan -----Original Message----- From: Dave Whitacre <[EMAIL PROTECTED]> To: [email protected] Date: Fri, 7 Apr 2006 14:34:37 -0600 Subject: invasive species--the broader, long-term perspective I realize that this interesting thread (which I have mostly just = skimmed) has been largely in search of hard data on = extinctions/extirpations. But for the longer-term view, I keep thinking of that paper (I'm sure = someone can supply the reference) that somehow used species-area curves = for different-sized portions of the globe in order to suggest what the = planet's ultimate species richness would be if species' distributions = were homogenized so that "everything occurs everywhere". The prediction = was, of course, shocking. A major reason that global species richness is = as high as it is, is that different species occur in different areas. While we can debate the quality of evidence, and it is certainly = worthwhile to study the mechanisms involved, does anyone doubt that the = truly long-term result of the continued spread of exotics will be many = extinctions? Dave Whitacre
