WELCOME TO AMERICA - The New Wild: Why Invasive Species will be Natures Salvation - page 41

The weed warriors are doing neighborhood gardening rather than eradicating alien species. Some
aliens must stay. A third of California's native butterflies depend on nonnative plants for their food. 34
The monarch butterflies would be in trouble without the state's alien eucalyptus trees, and the bees
need eucalyptus nectar during the winter. “We know we can never get back to nature as it was,” says
Schwartz. “There is no balance of nature, no climax ecosystem that nature wants to revert to. We just
want to stop takeovers. That's all.”
San Francisco Bay is the most invaded estuary in the world. Or so Andrew Cohen, then of the San
Francisco Bay Institute, and James Carlton of the Maritime Studies Program of Williams College, in
Mystic, Connecticut, said in Science in 1998, and nobody has contested it. 35 “Exotic species dominate
many of the ecosystems,” they said. The bay contains Amur clams from Russia, Atlantic green crabs,
Black Sea jellyfish, Chinese mitten crabs, Japanese gobies, and a New Zealand isopod called Sphaeroma
quoyanum that burrows into mud banks. The eastern mudsnail has replaced some native snails and car-
ries a flatworm that nibbles at the skin between your toes, causing “swimmers' itch.” Mediterranean
mussels cling onto rocks. There are foreign clams, barnacles, sea squirts, worms, sponges, hydroids, and
sea anemones.
If ambitious people from the Old World have tended to head west for the New World, then ambitious
Americans seem to have headed west for California. I guess it is not so odd that species have done the
same thing. After all, they may have hitched rides in cargo or attached to the wheels of a wagon train or
bolted themselves to the hulls of ships.
Since the Gold Rush in the late 1840s and 1850s there has been a biological rush into the bay, and
foreign sea life continues to pour in beneath the Golden Gate—stuck to ships' hulls and oil rigs, carried
in ballast tanks, and hidden in cargo unloaded at the ports of San Francisco, Oakland, and Sacramento.
More than half the invasions have happened since 1960, with new species arrivals since then averaging
one every fourteen weeks. “Drilling platforms and oil rigs can bring whole communities of organisms,”
Cohen told me when we met at his house in the hills overlooking the bay. And because the rigs stay, the
whole community may hop off and take up residence. Other sources of accidental releases include dis-
charges down rivers and drainage pipes. The Japanese anemone may have come from local aquariums.
The international trade in fish bait has brought marine worms from Europe, Japan, and even Vietnam.
Deliberate releases include striped bass, which arrived in rail cars from the East Coast in 1870 to estab-
lish a local game fishery, and Atlantic and Japanese oysters, which have been farmed in the bay since the
late nineteenth century. With the oysters came a range of parasites and pathogens that are still around.
Most fish and shore birds in the bay are still natives, says Cohen. But the food they eat has changed
dramatically. There are at least three hundred alien plant and animal species established in the bay. Some
invasions happen very fast. Cohen remembers that “in the fall of 1986, a college field class found three
new clams in a sample of mud on the bottom of the bay soon after a big spring flood.” One of them,
a Chinese clam called Potamocorbula amurensis , within two years became the most abundant clam in
the southern bay, carpeting large areas of mud and sand and making up 95 percent of the biomass. 36 “It
would have been filtering the entire volume of the South Bay every day,” says Cohen. It was eating all
the plankton, so the plankton blooms typical of the bay simply stopped. Meanwhile, native copepods
(crustaceans that drift in the water) disappeared from parts of the bay and were replaced by Asian ver-
sions. Fisheries also collapsed. 37
But was the arrival of the Chinese clam to blame? It may sound like a clear-cut case, but Cohen
is not sure. The 1986 spring flood that preceded the Chinese invasion dramatically freshened the bay
water. That wiped out most of the dominant Atlantic clams, which were themselves nineteenth-century