Posted 01 April 2010 - 12:40 PM
I hope you don't think I'm defending the introduction of exotics- certainly I'm not! What I am defending is rigorous investigation, for reasons of principle (I'm against sloppy thinking) and pragmatism. If we use "just-so" stories about invasives to argue for policy change, our argument is weak and subject to easy refutation. "Where's the proof?"
A lot of the info indicating that decline of one species is "caused" by the presence of another species is anecdotal or correlational in nature. One species' population goes up, another's goes down; this doesn't mean that one is driving the other out, but could instead be related to some third trend, such as a change in habitat availability. Any number of bird hunters are convinced that turkey move into an area and destroy the quail or grouse living there, but the truth is that the smaller birds prefer habitat in an earlier stage of succession than the turkeys do. As an old field gets brushier, the small birds head out for more preferred haunts and the turkey move in.
Similarly with bluebirds and house sparrows- the conflict between them only arises when nest sites are in limited supply. This was not the case when field rows, woodlines, and windbreaks with standing dead timber were plentiful, but changes in land use (such as modern, "efficient" cropping) have eliminated many of these. Where bluebird box programs have been instituted or where succesional farmsteads are again providing lots of bluebird-useable nest sites, as here in Tennessee, both species are common. So, it's not a case of an invasive species causing a native to decline- it's more complex than that.
There is also precious little evidence to support the wolf/coyote exclusion hypothesis. Wolves were vanishingly scarce in the east for a century before coyotes made their way here, and coyotes were sympatric with both red and gray wolves in the west long before that. Wolves and smaller canids are not direct competitors in most environments.
In all of these cases, the common thread is a change in land use that sparked both the increase of invasives and decrease of natives. However, land use changes are a very complex issue. For one thing, there's no clear line between "natural" and "artificial" land use. The modern North American landscape developed with people in it. Many eastern populations of grassland-loving birds, flowers, insects, etc. relied on the vast "barrens" created or maintained by Native American hunters' fires; when the hunters were driven out and the fires suppressed, they switched to pastures and fallow fields. If we were to take a "hands off" approach and let the landscape "heal" itself, these pseudoprairies would be reclaimed by the forest and the grassland-lovers would disappear.
Sorry for the diatribe. More later if I remember.