Mark Davis's article correctly points out that very few extinctions -- especially of plants -- can be attributed to competition, but note that he suggests two possible explanations. One of these is simply that competitive exclusion is slow in plants. This is consistent with a wide range of studies of island biogeography (when islands are isolated from larger land masses, plant communities have a very long 'relaxation time' to the new equilibrium), and a number of recent studies addressing what has come to be called 'extinction debt'.
This last may be a critically important concept for long-term thinking about conservation; populations can linger for a long time in a status where extinction is a virtual certainty over the long-term. This is likely to be particularly a problem in plants. Mature plants can be both extremely long-lived and tolerant of conditions (including competition) that severely limit recruitment -- a good formula for a 'walking dead' population that might not be readily recognized as doomed. In any case I suspect one could document from existing studies populations of native species have been substantially reduced in number/density by invasives (Davis cites a few relevant studies). Reduced population size is equivalent to increased risk of extinction, at least locally; that's fundamental population biology. Davis makes several plausible arguments why competition is unlikely to become important as the DIRECT cause of extinctions due to introduced species (these don't exclude the possibility that competition would make species more vulnerable to other direct causes). But it's also likely that is one of those situations, so common in ecology, where the lag between cause and effect (at least obvious or irreversible effect) makes it very difficult to assess what's happening empirically (and to demonstrate the risk to people without good grasp of the principles)... Kerry D. Woods Natural Sciences Bennington College Bennington VT 05201 [EMAIL PROTECTED] faculty.bennington.edu/~kwoods Teresa Woods wrote: >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] > > >
