Environmental Engineering Reference
In-Depth Information
there were no cheers for the demise of an alien invader. Rather, the media pointed out the honeybee's
$20-billion-a-year value to the US economy. (Curiously, such are the cultural nuances involved, at this
point it became known as an American bee rather than a European one.)
Invading pests and diseases are a special problem in the United States. Tree pests in particular find
that American forests often provide easy pickings. The gypsy moth was brought to the country in 1869
by Étienne Léopold Trouvelot, a French republican and amateur entomologist. He wanted to cross it
with silk worms to create a profitable textile. Instead, some larvae escaped into the woods from behind
his home in Medford, outside Boston. They have been eating their way through the hardwood trees of
North America ever since.
The boll weevil moved north from Mexico to ravage the US cotton crop in the 1890s. Around the
same time chestnut blight, a fungus, hitched a ride with some Asian chestnuts brought in for nursery
stock by one of the US government's early botanical explorers, Frank Meyer, most famous for a Chinese
lemon introduced to the United States and named after him. Chestnut blight has since virtually erad-
icated indigenous chestnuts. In recent times a green beetle called the emerald ash borer showed up in
packing material on a ship from Asia that docked near Detroit. It has since been laying larvae in the bark
of ash trees. Once they hatch, the larvae bore into the tree and kill it.
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Nobody would make a case for wanting to embrace such troublesome arrivals—though stopping
them at the border is fiendishly hard. But amazingly to some, there may be an argument for giving at
least a tepid welcome to another bête noir of American conservation, the zebra mussel (
Dreissena poly-
morpha
).
As a general rule, aliens take hold first and best in disturbed places, whether the result of hurricanes
or floods or human habitation. Few places in the mid-twentieth century were as disturbed by human
presence as Lake Erie, the smallest and most industrialized of the Great Lakes. Erie's ecology—or
rather, its absence—was headline news. The lake had been declared “dead” after a heavily polluted river
entering it, the Cuyahoga, caught fire several times in the 1960s. It was never quite dead, but pollution
had killed off most of Erie's once-remarkable collection of three hundred native species of mussels.
Then, in the mid-1980s, a foreign mussel showed up. The zebra mussel was stuck to the hull of a ship
from the Caspian Sea. It was arguably a just, if accidental, retribution for the US export to the Soviet
empire of
Mnemiopsis
, the “blob that ate the Black Sea.” The ship came up the St. Lawrence Seaway,
and the mussels loosed their grip in Lake St. Clair. By the summer of 1989, they had slipped next door
and were all over Lake Erie, showing what Soviet mussels could achieve in an environment that had
wiped out the natives.
26
They didn't look obviously tough. Zebra mussels—so named because of their distinctive
stripes—are tiny and largely immobile. But they spread easily, attach to any surface, and can attain
extraordinary concentrations. This invasion was a very collective endeavor from the onetime land of
collectivism. Most importantly, they didn't mind the lake's pollution. As filter feeders, they ran vast
amounts of the lake's vile water through their bodies in order to extract the plankton, their main food.
This was good news for the lake. Zebra mussels turned out to be the best janitors Erie ever had. They
were almost the only filter feeders able to swallow the gunk and survive. Each of the tiny animals was
gulping down, filtering, and excreting as much as a quart of water a day. Billions of the creatures can
filter a lot of water that way. During the 1990s, there were probably enough of them to process the lake's
entire volume in a week. Along with the plankton, they ate up most of the pollutants suspended in the
water, excreting it onto the lake floor. Along with human cleanup efforts inspired by the Clean Water
Act, the effect on lake waters has been transformative.
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