All
three of these plants, along with a host of other species,
are now considered major threats, and organizations
like the U.S. Fish and Wildlife Service and The Nature
Conservancy are devoting considerable resources to trying
to get rid of them and their ilk.
One
characteristic that these plants have in common is that
they are alien: they did not grow on this continent
before the arrival of the Europeans. Another characteristic
they have in common is that they grow. Lavishly. And
they spread. Abundantly. They are more successful than
many of our natives.
Why?
Some of them have a simple physiological superiority.
Glenn Dreyer, director of the Connecticut College Arboretum
in New London, performed careful studies comparing the
reproductive potential of our native American bittersweet
(Celastrus scandens) and the invasive
oriental bittersweet (C. orbiculatus).
He found that the oriental species produced more viable
pollen, more seed per fruit and more fruits per plant
than the native, and that more of its seeds germinated.
No wonder the native species is considered by DEP to be
"of special concern" while the oriental engulfs
tall trees all around the state. Sometimes,
their competitive success might be due to the absence
of a predator found in their homelands; sometimes it
might be due to a subtle difference in climate and light
regime between here and their homeland English ivy,
for instance, is native to northern Europe, where its
growth might be limited by a shorter growing season.
But once planted throughout the eastern US, as it has
been, it perhaps has found the longer summers much to
its liking and "taken off."
Many
alien species leaf out earlier than our natives, and
thus get a headstart on the growing season. This phenomenon
struck me clearly one day in late April as I strolled
through a nearby nature preserve. The understory - which
was overrun with multiflora rose, winged eunoymous and
other aliens - bore the delicate soft green of new growth
, while the native oaks, hickories, viburnums and cherries
were still shackled in their winter gray.
Another
possible reason for the spectacular spread of some alien
species is that many of them thrive on disturbance,
of which we have created plenty. Sunny, open roadsides,
bare soils in vacant lots, compacted soil, forest trails:
these are not habitats that were common in pre-settlement
America.
Whatever
the reasons, many alien species have taken hold in this
country and become established to the point where they
are having serious impacts. In coastal Texas, the imported
Chinese tallow tree has caused the transformation -
in less than ten years - of entire ecosystems from prairie
to woodland. The Nature Conservancy has stated that
in some cases non-native species are the "single
greatest threat" to species or communities their
preserves are designed to protect.
The
problem is particularly acute in Florida and Hawaii,
but Connecticut is not immune. Oriental bittersweet,
multiflora rose, Japanese stilt grass, Japanese honeysuckle,
purple loosestrife, privet, Japanese barberry, black
swallowort, Norway maple, tree-of-heaven, are just a
few of the several hundred alien species that grow in
the state (about 20% or our state flora is alien).
Others include Queen Anne’s lace, day lily, rugosa
rose, butter-and-eggs, chicory, red clover, and peppermint.
Poison ivy, ragweed (the major cause of hay
fever)and Phragmites (the great invader
of salt marshes) are native.
How
do these aliens get here? Some of them sneak in as seeds
on nursery stock or other material. Thorough detective
work on the part of two Swarthmore College professors
determined this to be the route of entry for mile-a-minute
vine, a thorny menace from Japan which is heading our
way. Poring through accession records of the Scott Arboretum,
which is based on the Swarthmore campus, the two were
able to correlate the appearance of the pesky vine to
the purchase by the Arboretum of certain rhododendrons
from Japan.
Many
species, however, enter the country in the full light
of day, escorted with open arms and high hopes by government
agencies, horticultural interests and the general public.
Such was the case for kudzu, purple loosestrife, oriental
bitterwseet, multiflora rose, Japanese honeysuckle and
a host of others.
For
decades, these plants are often innocuous, staying decorously
in the front yard, the eroded slope, or wherever they
were planted. Then they produce seeds, the seeds sprout,
the new plants produce more seeds, and the species has
jumped the fence, crossed the line from useful, cherished
ornamental to alien invasive. And then it’s too
late to stop it.
What
can one do to prevent the spread of alien species? Once
a species like oriental bittersweet becomes established,
there is no easy way, indeed no known way, to eradicate
it on a large scale. The Massachusetts Native Plant
Advisory Committee has released a "hit list"
of aliens that shouldn’t be planted any more by
state agencies. (The list includes Norway maple, autumn
olive, Japanese honeysuckle and multiflora rose.)
Should
Connecticut have such a list? Some states have banned
purple loosestrife form the nursery trade. Should Connecticut
follow suit? What should we do? There are no easy answers!
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