Jim and others,
Your last sentence converges on the point I was trying to make: if you
compared native species, as a group, against exotic species, as a group, you
would find statistically significant ecological differences (ie, trends),
even though you would also find numerous exceptions to those
trends. A statistically significant trend is not negated by the existence
of outliers, any more than the tendency for men to be taller than women is
negated by the fact that many women are taller than many men.
Trends are important. Trends are objectively identifiable patterns, and
scientific conclusions can only be drawn based on objectively identifiable
patterns. If you focus on just one or a few data points, selected so as to
seem to either support or contradict a hypothesized trend, you aren't doing
science; you're just telling stories (and cherrypicking your data). Stories
can be OK for illustrating a point (ie, giving an example), as they can
sometimes convey an idea better than trendlines can, but you can't draw
scientific conclusions from a story or two.
Mark Dixon correctly predicts that there are recent meta-analyses that are
relevant to our discussion. I downloaded a few a couple weeks ago, but I've
spent too much time writing arguments to spare any for looking at the actual
evidence. Here's a relevant-sounding, recent review to look into, if
anyone's interested:
van Kleunen et al. (2010). A meta-analysis of trait differences
between invasive and non-invasive plant species. Ecology Letters 13(2):
235-245.
This and other useful-looking papers pop up if you search ISI Web of
Knowledge for "exotic native ecology" and refine the search to include only
reviews. Adding the word "community" refines things further, though I'm not
sure it doesn't exclued some relevant papers.
I should try to clarify again why I'm focused on the ecological differences
between exotic and native species. The idea that we should control invasive
species is a moral position (as is any position on a question of what people
"should" do). One attack against this position is as follows: (1) there is
no ecological difference between exotic and native species, so favoring one
over the other is completely arbitrary, and (2) even if there were a
difference, there is nothing morally good about favoring one over the
other. The second part of this attack is itself a moral position, and is
therefore not within the realm of things that can be settled by purely
rational debate. At any rate, I think it only becomes important if the
first point can be effectively countered. If ecological invasion by exotic
species has no ecological consequences, I have trouble seeing why it's so
important to control exotic invasive species. Fortunately, that first point
can be debated rationally, on the evidence. Most of what I've been trying
to say here, then, revolves around making the general case that we should
expect exotic species invasions to be ecologically consequential, and I've
been trying to provide a few examples of ecological differences I would
expect to find between exotic and native species, based on what I can
remember from various classes, papers, and conversations over many years.
Similarly, if human-mediated dispersal is ecologically indistinguishable
from dispersal by any other agent, that would seem to undermine the case for
trying to regulate human-mediated dispersal. If, on the other hand, we
differ substantially from other dispersal agents, and if we can control our
own efficiency as dispersal agents, AND if introducing exotic species to
ecosystems has ecological consequences, we can then ask the moral question
of whether we *should* control our efficiency as dispersal agents.
On the question of whether humans are separate from nature, I just want to
re-emphasize that there are positives and negatives to either view. If we
say we are separate from nature, we recognize that we are very different
from other species in important ways. The up-side is that we recognize our
power to change things and make predictions about what the outcomes of our
actions will be, so we don't have to go around blindly mucking things up
for everything else in our ecosystem if we don't want to. The down-side of
having the power to avoid blindly mucking things up is that we've often used
it to go around *deliberately* mucking things up. We have tended to
consider our actions only in terms of their short-term effects on our own
species, with nature divided into those things we can use and those things
that are in our way. The costs and benefits to considering ourselves part
of nature are just the reverse: it makes it more likely that we will
consider other parts of the ecosystem in our planning, but it allows us to
rationalize that anything we do is just as natural as anything any other
organism does, so the question of how we affect the rest of the ecosystem is
rendered morally neutral. However you look at it, humans can find a
reason to act purely in self-interest, and they can find a reason to factor
their impacts on the rest of nature into their decisions. I think we need
to hold both views in our heads, remembering both that we have the power of
taking consequential action with forethought and that we have the obligation
to consider the rest of the biosphere in our planning.
Finally, I want to say that I don't want to be made into a straw man here.
A few times, my positions have been depicted as absolutist on one front or
another. While I think it's worthwhile to thin the population of an exotic
species that is beginning to dominate and re-shape a natural system, I'm not
some cartoon xenophobe, stomping around, hating and killing every non-native
thing I see. While I think invasive exotic species cause ecological damage,
I realize that they are often symptoms of damage we have done in other ways
at least as much as they are mechanisms by which we do further damage. I
recognize that "damage" is not an objective term, but I am free to consider
a dramatic change from what I value "damage," just as everyone else does. I
know that the lines between exotic and native, human and non-human, and
invasive and non-invasive are fuzzy, and that many of these terms are loaded
with human values. I think we humans ought to stop expanding our population
and our economy, but I don't hate humanity for reproducing and pursuing a
higher standard of living. I could obviously go on (and on), but, in short,
if I have to debate something I've said, I'd rather debate what I actually
said than some ridiculously exaggerated version that happens to be easier to
tear down.
In any case, by now, I think I've made every argument I intend to make, and
I've made some arguments many times over. It takes a long time to write
these emails, and I doubt most of you want to read them anyway. I plan to
be quiet now and let others hash all this stuff out, if they are so
inclined.
Jim Crants
On Wed, May 12, 2010 at 6:00 AM, James J. Roper <[email protected]> wrote:
> James Crants wrote on 11-May-10 13:05:
>
> There's a difference between saying that two species are not ecologically
>> equivalent and saying that two categories of species are not ecologically
>> equivalent.
>>
>
> But, ecological "equivalents" are not really "equal" in such a way that
> they are substitutable in a community. I mean, you can't just say, take a
> Clay-colored Robin from Panama and replace the American Robin (even though
> they might be considered ecological equivalents) and then expect their roles
> to just fit right in in their new places.
>
>
> If exotic species (as a category) were ecologically equivalent to native
>> ones, you would still find that every species would differ from every other
>> species by at least a few measures. I'm saying that, as a category, exotic
>> species are ecologically different from native ones.
>>
>
> Now do you mean "until they are naturalized"? After all, take the House
> Sparrow, that has now crossed the continent and invaded many places in the
> Americas. Is it still ecologically different from natives?
>
> I would suggest that if you took both native and introduced species, and
> did a blind study, in which you looked at survival, interactions and so on,
> you would not get a clear cut difference in ecological characters that would
> identify (say, through a discriminant function analysis) introduced and
> native species. Take the persimmons I have here in my yard here in southern
> Brazil. Clearly introduced from Japan (I will eliminate them once I have a
> native fruit tree to replace them with), but they attract leaf-cutter ants
> to consume leaves, bees and other insects visit the flowers, all kinds of
> animals eat the fruits, and they seeds are quite viable and the plant could
> easily become invasive and probably is in many places. If you took a native
> plant here, like the Scheflera (Didymopanax) and checked it out, you would
> find that, as a sapling, it cannot handle our cold winters (frost burns
> every year), it gets hit by aphids so badly that it is often worse than the
> frost, and the leaf cutter ants also nail it. In the same time my one
> native sapling has remained at the same size (short, < 1 m tall), a
> persimmon has grown from a seed and is now producing fruit and is about 3 m
> tall. The Scheflera is at least 9 years old, while the persimmon is about
> 3. I would suggest that through any objective measurements by a naive
> observer, they would think that the Scheflera was NOT native and that the
> Persimmon was.
>
> So, my point is, that using objective measurements, I think we would not
> find that there are clear distrinctions between native and introduced
> organisms. We may find certain kinds of trends, but the errors associated
> with using those trends as guides to recognize native or introduced
> organisms will be large and so not very useful.
>
> Cheers,
>
> Jim
>
