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
