The term "weed" is undeniably subjective, in almost every use I've heard
or
seen. The desirability of the species in question to humans is part of
most
definitions of the word. In my experience, ecologists avoid the term
"weed"
in formal contexts precisely because it is only meaningful in terms of
human
values.
In contrast, "invasive" is used rarely enough in common speech that
ecologists feel more comfortable using that term. I don't think the big
challenge of defining "invasive" for ecological purposes is any inherent
subjectivity, but merely the same challenge we face with every semantic
debate: finding a definition that is both precise and agreeable to all
users. The difficulty of finding such a definition has not been an
insurmountable obstacle to discussing invasive species intelligently.
There's a mountain of literature about the effects of invasive species on
native species, the invasibility of ecological communities, the traits
that
make an exotic species invasive, and the question of whether invasive
species are an ecological problem or only a symptom of ecological
problems.
We can define "invasive" objectively enough; the subjective part is
whether
we care if ecological communities are invaded by exotic species.
As for whether a pine grove would die of its own accord if left
undisturbed,
it depends on what other species are present. In northern Wisconsin and
the
Upper Peninsula of Michigan (the region I'm most familiar with,
ecologically), Jack pines, red pines, and white pines are all early- or
mid-successional trees. In the absence of large-scale disturbances, they
are eventually replaced by late-successional trees like sugar maple and
eastern hemlock. The pines get displaced because they create
environmental
conditions (shade) in which they cannot reproduce. The hemlocks (which
are
also evergreens) and maples are able to reproduce in the environmental
conditions that pines produce, as well as those they themselves produce,
so
they can persist as long as nothing else kills them. On the other hand,
if
every tree or large shrub species more shade-tolerant than white pine went
extinct in the region, totally undisturbed forests there would come to be
dominated by white pines. I have a hard time imagining what such forests
would look like, given that white pines can get over 200 feet tall and
live
up to 450 years, but can't reproduce in their own shade. But then, the
very
concept of a totally undisturbed forest seems absurd to me.
Incidentally, I would not agree that a weed is simply "a plant that
creates
environmental conditions in which it cannot reproduce." That might be a
definition of an "early- or mid-successional plant" (or maybe a "ruderal"
or
"pioneer" plant), but I think the term "weed" is just too laden with
subjective baggage to be given a definition that doesn't include human
values. That's with all due respect to Richard Lewontin, who's much
smarter
than I am.
Jim Crants
On Tue, Apr 13, 2010 at 10:27 AM, Kelly Stettner <
[email protected]> wrote:
Wayne, you bring up a "thorny" issue...one of my favorite conundrums. A
weed by any other name, eh? From the wiki entry you provided, I see many
references to weeds as "undesirable" or "nuisance" plants with little use
to
humans. I agree that "the term weed in its general sense is a subjective
one, without any classification value, since a 'weed' is not a weed when
growing where it belongs or is wanted." (Emphasis mine)
For instance, dandelions are unwanted nuisances when a person wants a
well-manicured, well-behaved lawn. To the wine-maker and the child,
however, dandelions are very desirable. Japanese knotweed is a scourge
to
many who own river-front property infested with the tall plant;
bee-keepers,
however, are very grateful for their late-season profusion of blooms
providing needed nutrition for their fuzzy little wards. Weediness, it
seems, is in the eye of the beholder.
I am fascinated by Prof. Lewontin's definition of a weed as a plant that
creates "environmental conditions in which it cannot reproduce." Pine
trees
as weeds...Interesting concept! By that definition, a pine grove, left
undisturbed by fire or man's axe, would die off of its own accord? Would
that apply to all evergreens, or just pines? I'll have to look him up
and
read some of his research, see if I can delve deeper.
I know there has been a concerted effort over the past few years to
define
"invasive" as well; not sure if that effort is making any headway or if
it,
too, is somewhat subjective. I, for one, consider poison ivy to be both
invasive and noxious, but it's only noxious to humans, apparently. Were
we
covered in fur or feathers, the urushiol oil could not contact our tender
skin and cause the burning rash so famous among hikers and Boy Scouts.
By
the same token, my daughter loves Creeping Charlie or Ground Ivy, and can
spend hours nibbling the delicate little purple flowers for their minty
essence.
There is also the question of abundance and "aggressiveness." What some
call "aggressive" growth could be seen as "successful adaptation to
environmental pressures," could it not? Japanese knotweed is a "first
responder," so to speak, in volcanic situations in Japan, being one of
the
first (if not THE first) plant to re-colonize after a lava flow. It was
introduced to the US as an ornamental at first and then as a stream bank
stabilizer to hold soil in place. It has since spread very successfully
all
over the country. There are critters that use it (bees, ants, other
insects) and it is edible when very young and relatively tender.
So I guess that, ultimately, we ought to pony up and admit that most
definitions of "weed" and "invasive" are going to distill down to what we
humans value and desire. And that our needs, values and desires are
going
to change over time.
Respectfully,
Kelly Stettner
Black River Action Team
www(dot)BlackRiverActionTeam.org
blackriverclea...@yahoo(dot)com
Date: Sun, 11 Apr 2010 22:49:58 -0700
From: Wayne Tyson <[email protected]>
Subject: Plants Colonizing Weedy or Ruderal or Invasive Arguments and
Distinctions?
Ecolog:=20
Fools rush in where the exalted fear to tread, but here goes:
A certain certainty seems to persist around the subject of colonizing
species or "weeds." I have visited a couple of sites (one highly
professional, but still confusing) which contain what appear to me to be
quaint statements, hidden amongst the valid phrases.
http://en.wikipedia.org/wiki/Weed and its associated pages, for example,
may be sowing the seeds of confusion far and wide.
I do not know how widespread these alien, ruderal, or feral ideas are,
or whether I am persistently misinformed. It seems that with respect to
"weed" in particular, authors prefer to "hedge" rather than clarify or
qualify--at least sufficiently, in my view. An ecological view seems to
be lacking, even for "balance."
I do not want to lead others down the garden path, but I think is it
high time this confusion was clarified and settled, at least to the
point where differences are made distinct.
Are you similarly disturbed or confused? Would you please participate in
getting to the root of the matter, right here on Ecolog?
Thank you all for your help.
WT
--
James Crants, PhD
Scientist, University of Minnesota
Agronomy and Plant Genetics
Cell: (734) 474-7478
