On Tue, 2006-04-04 at 20:22, Elizabeth Leger wrote: > I don't understand how the metric for measuring impacts of invasive > species became "extinction" or even "local extirpation" of native > species. > > If I measure 90-99% cover of exotic species in a system (totally the > norm in California's central valley grasslands), I'm not supposed to be > upset about the impacts of invasive species impacts because there > haven't been any local extinctions? That makes no sense. > > Beth
I think there is no question that extinctions have occurred. If you look at the Federally listed Endangered Species the Bay Checkerspot butterfly ( Euphydryas editha bayensis). It is now confined to areas of serpentine grassland south of San Francisco where the endemic plant species can resist the invasives. It is a showy species, I just wonder what else has been lost from the invertebrate community. I think just by saying that local extirpations occur you are ignoring the bigger long term effects. Species get lost from habitats even when quite a bit of it is still around due to metapopulation effects. Our UK relative of E. editha E. aurinia is extinct from areas where there still seems to be some habitat, we now know as a result of metapopulation studies that it just isn't enough. Neil Jones [EMAIL PROTECTED] http://www.butterflyguy.com/
