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Invasive plants may be "weeds," says Mack, but the two terms
don’t mean quite the same thing. A native plant that thrives in
roadcuts, burned forests, or vacant lots might qualify as a weed
but is not an invader; it prepares the ground for other plants and
then recedes to low population levels when its job is done.
"When we talk about invasive species, we’re not concerned with
native species which have always played some colonizing role in
these ecosystems," says Mack. He’s also not concerned about
non-native plants that become naturalized without harming
their new neighbors. Most of our food crops are non-native, for
example, as are forsythia, lilac, and other garden standbys.
Mack estimates that about 100 of every 1,000 species that are
introduced will become naturalized or self-sustaining beyond the
garden gates. Of those, one or more will likely become a serious
problem. That sounds like the odds are on our side, but the
dizzying pace of global trade means that humans are no longer just
another "natural" dispersal mechanism for plants, akin to birds and
wind. People in every part of the world bring into their homelands
thousands of alien species every year. And as our current struggles
show, a single invader can do tremendous damage.
One of the most intriguing aspects of invasive plants is that
they rarely tip their hand early on. They muddle along in or near
the garden for a few years or decades, before they establish
self-sustaining populations. Then, after another lag, they burst
out in full assault mode. Most of the species we’re battling in the
Pacific Northwest were introduced in the region 100 to 130 years
ago.
"We’re dealing with a biological phenomenon, which means that it
doesn’t change linearly over time," says Mack. Their populations
grow very slowly at first, then explode in a logarithmic growth
curve. He compares this pattern to the rapid spread of an epidemic.
"Just as problems with human disease often go undetected, or at
least unattended to, until it’s too late, the same thing is true of
these organisms. . . And that actually leads to why these
problems tend to get out of hand: the public and the policymakers
don’t pick up on the danger until it’s virtually too late."
Whether a species will reach the population explosion stage
depends on whether it can reproduce well enough to nudge its
numbers up to the point when compound growth kicks in. Small
populations just can’t gain traction, even in a favorable habitat.
That’s because of stochasticity, a scientific term for chance
events. Stochasticity might be demographic—none of the members of a
population leave offspring—or environmental—a summer
hailstorm wipes out the whole population.
"A small immigrant population has almost no chance of
persistence, even if it can tolerate the basic parameters of the
environment, simply because of these random events that occur,"
says Mack. He contends that we aid and abet potential invaders by
protecting them from destruction by stochastic events. We water
them during dry spells, shelter them from cold and wind, and
nourish them with fertilizers. We even add more individuals if the
original population struggles. As we cushion the random blows of
nature, we enable a small, vulnerable population to put out more
and more offspring, expanding its numbers to the point that it
can survive those stochastic events.
Even so, there must be something about invasive plants that
makes them invasive. We coddle all of our garden plants, yet only a
few jump the fence. Some of the warning signs are obvious. A plant
that readily spreads and puts out a lot of seeds, with little
encouragement from the gardener, is probably bad news. Melissa
Smith, a graduate student in Mack’s lab, is trying to develop more
precise measures of invasive potential. She’s evaluating several
species of bamboo for their ability to thrive in inland northwest
forests and become invasive in our region. One, golden bamboo
(Phillostachys aurea), has already invaded parts of Florida,
Texas, and Oregon.
"Some of these species have piqued people’s interest with how
vigorously they grow in the introduced habitat," says Smith. "And
then some are completely benign. It would be nice to be able to
differentiate between the two."
She’s measuring traits like drought resistance and how
efficiently the plants photosynthesize under a forest canopy. Doing
multiple tests is important, says Smith, because she doesn’t want
to wrongly condemn a species as a likely troublemaker. If a plant
scores as potentially invasive on all five of her tests,
"then I could effectively say to some ruling body, ‘Look, this
plant does all these things under all these conditions, which other
plants do that have proven to be invasive, so we probably shouldn’t
let this in, or we should do so judiciously.’"
Over the years, research by other scientists has pointed to
predictive features such as the number of seeds produced per
year, tolerance of a wide range of conditions, and close kinship
with another species that has already become invasive. All of those
measures are cause for suspicion, says Mack, but unfortunately,
none of them is a sure sign that a species will jump the fence—and
lack of such features is no guarantee that it won’t.
"This is one of the perplexing things about biology," he says.
"It’s not mechanics or physics. . . the behavior of one [species]
doesn’t necessarily predict the behavior of another."
Despite the difficulties, he thinks it’s critically important to
find some way to evaluate invasive potential. If we don’t,
our only option is to wait for an invasion to happen, and then
scramble to fight it and remedy the damage it’s done. That’s been
our usual approach so far, resulting in what Mack calls a huge
"hidden tax" we don’t even realize we’re paying.
"We don’t know how much of a bill we’re actually paying for
having shrugged our shoulders over the last hundred years or so,"
he says, "and we’re dealing with a lot of pests that, had they been
dealt with appropriately early on, wouldn’t be problems now."
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Turn your yard into a beachhead! Try one of these
beauties, all proven invaders in the Pacific Northwest.
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