Jim and others,
In the discussion off-forum, we were unable to come to any conclusions
because we could not agree on answer to even the most fundamental question:
is the distinction between exotic and native species ecologically
meaningful? If you can't agree on that, there's no point in going on to ask
whether there's such a thing as an invasive exotic species, whethere
invasive exotics are a problem, and what, if anything, we should do about
it.
In our conversation, Matthew Chew argued that the distinction between native
and exotic species is ecologically meaningless. A species does not have
higher fitness because it is dispersed by humans instead of other agents.
Most species dispersed by humans fail utterly in the new environment to
which they were dispersed. Very few species are evolutionarily specialized
for human-mediated dispersal (I think exceptions would be some of those
species we use as crops, pets, and livestock, and some agricultural weeds
that have evolved such that their seeds are difficult to separate from crop
seeds). An "invasive" exotic species shows the population dynamics you
would expect for any species that is rapidly expanding its range, regardless
of its origin. If exotic and native species are not biologically
distinguishable, then the distinction is merely "historically incidental."
The categories are not ecologically meaningful, and they are only useful for
marshalling support for one group of plants (natives) and opposition to
another (exotics).
Actually, Dr. Chew adhered strictly to the term "alien." Many people write
and talk about "alien" species, and this term, as well as the term
"invasive," provoke hostility. They do not serve us well if we want to
discuss these things rationally. On the other hand, since Dr. Chew
considers these terms to be ecologically meaningless, he is not obliged to
suggest alternatives, and he does not. I use "exotic" instead of "alien"
because it seems less inflammatory, but Dr. Chew and I agree (I think) that
there is no way to discuss exotic and native species without ending up
favoring one category over the other, regardless of what labels we put on
them.
That's my summary of Dr. Chew's arguments, as I understand them. As he
amply demonstrated in the off-forum discussion, he can make his case much
better than I can. I have CC'ed him because I don't think he's on this
forum, and he might want to make his point in his own words.
Initially, my argument was on moral grounds: whatever negative effects
invasive species have on native species are the fault of our species (unless
a non-human disperser was responsible for the intial long-distance-dispersal
event, which very rarely happens), and, as moral agents, we are obligated to
try to undo or mitigate the harm we cause to others. That's my Catholic
upbringing speaking, I guess, and it's apparently not a compelling argument
to someone who hasn't already reached the same moral conclusion on exotic
invasives.
I was working on a factual argument against the assertion that exotic
species are not ecologically different from native species, but I have not
had time to check what I believe to be true against the evidence. Maybe
others can help on the evidence, but I'll keep working on it. For now,
here's what I think is true:
(1) Exotic species, on average, interact with fewer species than native
species, and their interactions are weaker, on average. In particular,
they have fewer parasites, pathogens, and predators, counted in either
individuals or species. This is especially true of plants, and especially
non-crop plants. I suspect, but have not heard, that exotic plants also
have fewer mycorrhizal associates than native ones, but I doubt that they
have significantly fewer pollinators or dispersers. Meanwhile, back in
their native ranges, the same species have the same number of associations
as any other native species.
(2) Very-long-distance dispersal by humans confers a fitness advantage over
very-long-distance dispersal by other agents, on average, for two reasons.
First, humans often disperse organisms in groups, such as containers of
seeds, shipments of mature plants and animals, or large populations
contained in ballast water, allowing them to overcome the Allee effects
(lack of mates, inbreeding depression) their populations would face if
introduced as one or a few individuals. We also often take pains to
maximize the establishment success of organisms we disperse, by shipping
healthy, mature plants and animals and propogating them when they arrive,
while non-human dispersal agents usually introduce small numbers of
organisms, often nowhere near their peak fitness potential (e.g., seeds,
spores, starving and dehydrated animals).
(3) Although the population dynamics of invasive species do not differ by
what agent introduced them (whether humans brought them, some other agent
did, or they evolved in situ), it is ecologically consequential that human
activities are generating so many more invasive species than natural
processes usually do. Aside from maybe continents or oceans merging through
plate tectonics, nothing non-human introduces such a flood of new species to
new environments as we humans have in the last several centuries.
(4) To arrive at the conclusion that the terms "native" and "exotic" (or
"alien") are ecologically meaningless, you must approach the issue this
way: if there is no set of criteria by which one can reliably categorize an
organism as native or exotic in the absence of historical evidence, the
distinction is meaningless. I think the valid approach is this: if there
is no set of criteria by which one can reliably distinguish the category
"native species" from the category "exotic species" (*after* the
categorization is done based on geographic history), the distinction is
meaningless. By analogy, the first approach is like saying that there is no
difference in height between men and women because one cannot reliably
identify the height of a person by their sex, while the second approach is
like saying that there is a difference in height between men and women
because men are, on average, significantly taller than women.
That's all. If you've read this far, I salute you.
Jim Crants
On Sun, May 9, 2010 at 8:38 AM, James J. Roper<[email protected]>
<mailto:[email protected]> wrote:
Wayne, and others,
This email was nebulous enough to where it appears to me that several
concepts are being bantered around to the detriment of resolving any.
Of course all terms are relative - we humans made up language to put names
on things to help us.
The problem of invasive species is important or not, depending on your
particular philosophy, so you would have to come to some common grounds
first to resolve what invasive is second.
The problem of invasives is just like the problem of endangered species.
99% of all species that ever lived are extinct, so we know that it is a
consistent evolutionary process. Probably 99% of all species that exist
started out somewhere else. However, the glitch is that in our generation,
we are causing the extinction of many species at a much more rapid rate than
nature ever did, and we are causing the introduction of species in new
places at a rate much more rapid than nature ever did.
As Elton in his classic book on introduced species stated (here
paraphrased), Because of introductions and their consequences, we will be
left with a world much simpler, much less diverse, and much less
interesting.
Sincerely,
Jim
Wayne Tyson wrote on 07-May-10 16:47:
Ecolog:
Back on April 12, 2010, I posted an enquiry along these lines that
resulted in an off-list discussion between three Ecolog-l subscribers and
three others. A lot of interesting points were made, but this side
discussion did not, in my view, settle the matter of what terminology, if
any, should be used to describe the ecological phenomena associated with
plants (and other organisms) that "colonize" or "invade" parts of the earth
upon which they did not appear/evolve before dispersal by human culture
(including various artifacts and impacts and domesticated plants and animals
and their cohorts).
Since the off-line discussion did not seem to resolve the issue beyond
opinions, I am submitting my version of the results for consideration by the
Ecolog community.
Among the points (you can ignore these, but they give SOME idea of where
the discussion wandered) made by various correspondents were:
1. Persistence is an interesting problem, since it requires an arbitrary
stipulation. Fitness is demonstrated (or not) generation by generation.
2. . . .why ARE so-called "natives" of a higher value than so-called
"exotics"? How far back are we supposed to go before something is
considered "native?"
3. . . . humans should learn how the land works, make minimal changes and
only necessary ones, and try to adapt to the landscape as best as possible,
using history's lessons to create our future. Trying to make zero
"footprint" or impact or change as we live our lives is like trying to swim
without getting wet or making ripples.
4. Eventually Albert Thellung split 'aliens' into 7 distinct categories in
1912: ergasiophytes, ergasiolipophytes, ergasiophygophytes, archaeophytes,
neophytes, epecophytes, and ephemerophytes; plus two more denoting 'wild'
plants growing in modified habitats. Search any of them and they'll pop up
in recent central European literature, but they're dead letters in the
Anglophone world.
5. Alien and invasive are both relative. The labels are relevant only in
areas where new populations have (respectively) appeared, and spread in some
discomfiting manner. They provide no information about any biological
essence of any species . . .
6. What matters is fitness under prevailing conditions.
7. . . . the whole question of what response to invasive species is
morally best is beside the point.
8. For now, I still believe that each of these terms reflects an objective
reality, but that each has nebulous boundaries.
9. The danger of separating natural from artificial mentally might be that
we think we have to exclude nature wherever we go. The danger of not
separating them is that it can help us rationalize an anything-goes approach
to natural systems.
10. Have we decided on any definitions, or are there still differences
about terminology? Are we ready to list them yet, even if with a
multiplicity of definitions? Either way, it looks like we're making
entertaining progress in the realm of associated phenomena. Maybe that's the
first, if indirect, hurdle in gaining a workable set of terms?
11. My question is, what belongs there, and why?
12. . . . the important thing is to keep the lines of communication
open--ESPECIALLY with those who have "alien" ideas.
13. Once an idea catches on, it's next to impossible to replace it with
another one--something like the tenacity of an alien species--or, one might
also say with equal "validity" or "spin," that, like the popular pastime of
reasoning by analogy, that it is an example of resistance to invasion.
14. I am interested in the question of whether we ought to "subsidize the
unfit, and suppress the fit."
My own summary interpretation of some of the various conclusions are:
1. All organisms move from place to place by some means.
2. Some don't survive in some places.
3. Some survive and reproduce in "new" places better than some of the
organisms that apparently evolved adaptations in accordance with site
conditions.
4. Because of various semantic alliances, word meanings and etymology, and
interpretations thereof, terms like "colonizer," "invader," and "alien" are
deemed unsatisfatory to some for the purposes of disciplined enquiry into
ecological phenomena.
5. Testable hypotheses seem to be lacking.
This is all very incomplete; I hope that contributions from Ecolog
subscribers will help to make it more so, if not resolve the issue(s).
WT
--
James J. Roper, Ph.D.
Ecologia, Evolução e Dinâmicas Populacionais
de Vertebrados Terrestres
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