BIOLOGICAL POLLUTION - Marine Pollution: What Everyone Needs to Know - page 200

West Coast, it can become invasive, as documented by Curtis
Daehler and Donald Strong of the University of California,
Davis. Its invasion of Willapa Bay, Washington is transform-
ing vast areas of tidal mudflats into dense vegetation, affecting
migratory waterfowl, shorebirds, and wading birds that forage
in the open mudflats. In San Francisco Bay, it has hybridized
with native Spartina species, threatening the native flora in
marsh areas. The hybrids are tougher than their parent spe-
cies and become even better invaders. Cordgrass was intro-
duced to China in 1979 from the United States for reducing
coastal erosion. It grows vigorously in China and has spread
over much of the coastline, where it is competing with native
Phragmites (some irony here?). It has also invaded mangrove
areas in which the canopy was opened by human disturbance.
Some fear that it could gradually replace these mangroves in
midsalinity regions of Chinese estuaries.
Can an alien species do some good?
Some alien species have been found to benefit native species
and the local environment. In Chesapeake Bay, for example,
the exotic red alga Gracilaria vermiculophylla has flourished
and dispersed widely. It turns out to provide nursery habi-
tat for juvenile blue crabs at places where native eelgrass has
declined (largely due to eutrophication). Phragmites is an effec-
tive barrier providing storm and flood protection and is very
effective at sequestering pollutants. Furthermore, it produces
more detritus and litter than Spartina, so it increases the marsh
elevation and may enable tidal marshes to keep ahead of ris-
ing sea levels.
In degraded New England salt marshes, green crabs can do
some good. Because of overfishing, natural predators of the
marsh crab, Sesarma reticulatum , have been depleted so that
marsh crab populations exploded. This native crab eats large
amounts of cordgrass, so their dense populations destroyed
large areas of salt marsh. Mark Bertness and Tyler Coverdale