Another point I'd like to pick up on is that a distinction should be made between introduced species (those outside of their native range) and invaders (introduced species that aggressively expand their range and cause problems). While I agree that it may be counterproductive to be hyper-vigilant and crusade against every introduced species, I think that increasing awareness of the problems posed by invaders is valuable. Furthermore, because a fraction of species introductions inevitably become invasive, minimizing introductions is a way of averting future invasions. One very important example of this is the ongoing introduction of species to San Francisco Bay in ballast water -- at a rate of 1 every 14 weeks! (Cohen & Carlton 1998, Science) I think that directing the public's attention to the importance of concerns like this is the only way that we'll convince our leaders to muster the political will to address these problems.
Additionally, the hypothesis that healthy, intact ecosystems are necessarily resistant to invasion, is at best context dependent. While in general, anthropogenic disturbance often facilitates invasion, there is variation in the aggressiveness of invaders -- and there is also variation in the susceptibility of different ecosystems and habitat types. I strongly disagree with the implication that without other human impacts, invasions would not be a problem. -Joe Tyburczy P.S. In keeping with my marine theme, the invader I love to hate is Caulerpa taxifolia, the introduced green alga that has few natural enemies and has invaded marine environments across the globe, including large areas of the Mediterranean. William Silvert wrote: > I'll pick up on two of Wayne's points. One is that "some aliens that > do little harm" -- this is true, and some aliens are introduced > deliberately. Mustangs are alien to N. America, and are widely > appreciated. Many ornamental plants are deliberately introduced. My > mother was a member of the Florida Native Plants Society, and felt > that they were fighting a losing battle against the imports. An > interesting downside is that often introduced plants in dry areas > require lots of water and this creates problems. > > As for the comment that healthy ecosystems resist invasion, this > depends on whether they have had a chance to immunise themselves by > past experience. Because mammals were unknown in Australia, their > introduction was impossible to resist. The same is often true when > snakes or mosquitos arrive in regions where nothing similar has every > been present. Often the best defence against an invading species is a > predator that can control it, but if such predators are not already > present, it may take a few million years for them to evolve. > > Sometimes man has tried to counter one alien invasion by introducing > another alien species to control it -- which brings into action the > Law of Unintended Consequences. It's a tricky game to play. > > Bill Silvert > > > ----- Original Message ----- From: "Wayne Tyson" <[EMAIL PROTECTED]> > To: <[email protected]> > Sent: Friday, November 16, 2007 1:08 AM > Subject: ECOSYSTEM Health Alien invasions persistence decline limits > control Re: semi-silly question from John Nielsen > > >> There are some aliens that do little harm; some even provide >> benefits. This statement is anathema, heresy, fighting words, to >> many, many very caring people. But so many of those caring people >> have their egos inextricably wrapped up in this very laudable >> mission--it is often their reason for living, often it is a filler of >> a hole in a person's life. One can't argue with that. >> >> Here's the heart of my "rant." Healthy ecosystems tend to resist >> invasion. (However, the introduction of an alien species can, in >> some cases, but not all, truly invade healthy ecosystems. -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Joe A. Tyburczy Oregon State University Department of Zoology 3029 Cordley Hall Corvallis, OR 97331-2914 541-737-5359 [EMAIL PROTECTED] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
