As we all know, extinction or the threat of extinction is, in many cases, due to outright habitat loss. If the remaining habitat is invaded by a non-native plant that alters the function of the habitat, the endangered species may be pushed that much closer to extinction or, as another reader said, local extirpation. And highly endangered species can't handle too much local extirpation. I don't think we can look for the single event or cause that drives a species to extinction because more often than not it is probably due to the cumulative effect of many threats (invasive species, habitat loss, climate change, etc.)
A very interesting study came out last year showing the effects of removing grazing from vernal pools in the Central Valley of CA. These vernal pools provide habitat for a large number of endangered plants, invertebrates, and a few vertebrates. When grazing was excluded for three years from sites that had historical grazing, the cover of non-native grasses increased 88%, native plant species richness declined 25%, and aquatic invertebrate richness declined 28%. With regard to function, the hyrdoperiod of the pools decreased 50-80%. For a threatened species like the CA tiger salamander that needs 3-6 months to metamorphose from larva to adult, this functional change has very important ramifications. This whole scenario includes many causes that occurred over at least a 150 year period - widespread habitat loss, loss of native herbivores, introduction of cattle/sheep, and introduced plants. The point is that the exotic plant may not be the ultimate cause but rather the straw that broke the salamander's, fairy shrimp's, and endangered plant's back. The reference is: Marty, J.T. 2005. Effects of cattle grazing on diversity in ephemeral wetlands. Conservation Biology. 19:1626-1632. ----- Original Message ----- From: "Teresa Woods" <[EMAIL PROTECTED]> To: <ECOLOG-L@LISTSERV.UMD.EDU> Sent: Tuesday, April 04, 2006 11:49 AM Subject: invasive plants >A couple articles to consider: > > Gurevitch, J. and D.K. Padilla. 2004. Are invasive species a major cause > of extinctions? TREE 19:470-474. > > Davis, M. 2003. Biotic globalization: does competition from introduced > species threaten biodiversity? BioScience 53:481-489. > > I couldn't believe the conclusions of these articles when I read them > last year, but they did bring home the message that if indeed invasives > are causing extinctions, even community level ones, we need to be > documenting them in ways other than anecdotally. Hence, my question > about relevant publications. I'd love to see them. As I said, even > ones that show a correlation, as causation is justifiably hard to show. > > I am not as familiar with aquatic invasives except to know that some > like zebra mussels are extremely aggressive and destructive. But > regarding plants, invasives are often linked with disturbed habitats, > and it may be that habitat destruction is the primary cause of local > extinctions, and the invasives follow as the "final nail in the coffin > (Gurevitch and Padilla 2002)." It is also likely that the extinction > trajectories are definitely occuring but are longer-term, and just > haven't reached the end yet. > > The link to the NYTimes article was, as I said, meant to fan the flames > here -- not that I agree in any way with him, but to put on the table > what landscape designers and architects are surely also being > influenced by -- even if from, as has been correctly pointed out, a > very biased point of view. > > So my query still stands -- is there empirical evidence supporting the > sense that most of us have that invasives are causing native > extinctions? I'd love to have evidence to contradict the NYTimes > author's view. > > Teresa > > Teresa Woods > Graduate Assistant > Division of Biology > 232 Ackert Hall > Kansas State University > Manhattan, KS 66506 > 785-532-9834 > [EMAIL PROTECTED] >
