Environmental Engineering Reference
In-Depth Information
interlopers. Cohen says that the Chinese clam, like many an immigrant, was probably lurking in the bay
all along and simply took advantage.
The key point about the bay is that it has never been a stable ecosystem with a fixed set of species.
Most Californians have not been around there long enough to witness it, but the bay is subject to occa-
sional dramatic spring floods that course down the rivers and flush the bay with fresh water. When that
happens, says Cohen, the entire ecosystem is rebooted. Jay Stachowicz, a marine biologist at UC Davis,
agrees, telling me when we met in his lab: “The bay is dynamic, constantly being purged by freshwater
floods. It is after those events that introduced species make their move, but if they didn't something else
would.”
It is this dynamism that makes the bay what it is. Those conservationists who bemoan every change
in the bay, every arriving alien species, and every disappearance are missing the point. The constant
change is the reason why, despite more than a century of bombardment by alien species, San Francisco
Bay is in such robust health. Don't take my word for it. In early 2013, the international Ramsar Con-
vention on wetlands accepted the US government's designation of the bay as being of “international
importance.” It called the bay “a natural wonder and a critical ecological resource,” one of North Amer-
ica's most ecologically important estuaries. Yes, three hundred of the thousand or more species of fish,
mammals, birds, invertebrates, and plants in and around the bay are recent arrivals, but that is something
to celebrate, a sign of health rather than sickness. The bay may well be, as Cohen and Carlton claimed
almost twenty years ago, the most invaded estuary in the world. But that is the key to its other defining
feature, what the Ramsar secretariat agreed was its “superlative biodiversity.”
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“Change is the norm
here,” Cohen tells me. “It's the story of California, for species as well as humans,” says Stachowicz.
Another dynamic American ecosystem is the Florida Everglades. It too has been overrun by alien spe-
cies in the past century, rather like San Francisco Bay. Today a quarter of all the fish, reptiles, birds, and
mammals in the Everglades are exotics. But it is hard to work out what we should regard as native or
why anyone should imagine the area would show any kind of natural balance and ecological stability.
The wetland is recent, only about five thousand years old, and has experienced constant changes in sea
level. Even if humans had stayed away, it would still be in a perpetual state of flux and a refuge for
migrant species.
I agree that is not an easy message to digest when the big-daddy newcomer in the Everglades is
something as big, aggressive, hungry, and foreign as the Burmese python (
Python molurus bivittatus
),
one of the world's five largest snakes. There are now an estimated thirty thousand of them lurking in its
wetlands. The largest of them are able to catch and eat the wetland's former top predators, alligators.
How did they get there? Florida is a long way from Burma. An obvious explanation is the craze for
pet snakes in the 1990s, when their numbers in the Everglades took off. Maybe, as they began to reach
their full size of up to sixteen feet, scared owners got rid of them—into the nearest creek or after a car
ride to the swamp. But even though the Everglades are a nice warm and wet habitat, not so different
from back home in the marshes of the Irrawaddy, sporadic pet releases seem unlikely to be to blame for
such a large population. The most likely source is a mass escape that happened when Hurricane Andrew
ripped through southern Florida in August 1992. In Homestead, a Miami suburb right on the edge of the
Everglades, it wrecked a warehouse where they bred pythons for pet shops. The walls fell in and tanks
were smashed. Reportedly, nine hundred pythons of varying sizes were living there. The baby ones may
have been literally blown into the Everglades. Big ones perhaps just slithered away into the swamp.
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