All excellent points. But does anyone know of any disciplined effort to sort out all the details that would enable anyone to answer Nielsen's question? I have only nibbled at the edges of this issue.
Yes, it's always silly to be arbitrary, but even coming close, perhaps saying something like, "based on these criteria [everyone please now give your criteria!] these organisms have had these effects upon undisturbed ecosystems, and are therefore considered to be AMONG the "worst" invasive species . . ." is a worthy challenge. I will be so bold as to try to initially sort the candidates into two (well, three) piles--just for starters: 1. Alien, but not invasive. Species dependent upon direct or indirect human agency for continued persistence or existence and will decline or disappear under ecosystem "pressure" (such as when an indigenous ecosystem is restored, with or without the help of human action) when that factor is withdrawn or stopped. It may take years or decades, but the important concept here is the TREND, not whether or not the phenomenon satisfies some anthropocentric, arbitrary "standard." 2. Alien and invasive. Species not at dependent upon direct or indirect human agency beyond being a simple vector of propagules, and which clearly invade undisturbed ecosystems to their detriment, e.g., loss of species or degradation of pre-existing organisms or populations, perhaps to the eventual point of extirpation or extinction. Of these sub-categories, of course, extinction might be considered the most important. However, the mitigating contributions of invaders to ecosystems (especially those under other human influences*) should perhaps be considered. 3. Alien, but not introduced by humans. Species which invade by means not even remotely connected to human activity. (This is a difficult standard to meet on the basis of linear observation; for example, an organism might migrate from one continent or ecosystem to another and be a vector for another organism that was present at the site of origin as a result of human activity. I'm sure y'all can improve on this feeble start. But until more definitive evidence comes along, I suggest that category 2 is where most of the five will be found. WT * For example, a non-persistent, rather benign alien, "tree tobacco" Nicotiana glauca was introduced to North America (particularly the Southwest) from South America and "invades" both naturally disturbed (e.g. riparian areas) and human-disturbed sites. "Technically," these plants are "invasive," and as such sometimes appear on the "hit lists" of various well-intentioned agencies. However, this plant can be useful in restoration projects as perching sites and food sources for hummingbirds. Allocating "weed-whacker" resources to the extirpation of this species might not only be needlessly costly, but detrimental to hummingbird populations and the dispersal of indigenous propagules as well. Admittedly, they are not very pretty, and since I am allergic to nicotine I dare not touch them, but I do not whack them outside of my "yard." At 02:18 AM 11/16/2007, 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.
