Biology Reference
In-Depth Information
2001). In Lake St. Clair, between Lakes Huron and Erie, it appears that all
native freshwater mussels were extirpated by 1997 (Nalepa et al. 2001).
Because of the enormous densities reached by
Dreissena
species, they
also reduce the abundance of particulate foods required by both unionids
and other smaller clams (Strayer 1999).The Asian clam (
Corbicula fluminea
)
may also be a food competitor for native mussels in some situations.
Among aquatic vertebrates, competition has figured in the extinction
of several North American fish (Miller et al. 1989). Impacts of alien
species, often a combination of predation, competition, and hybridization,
are implicated in 68% of the 40 species or subspecies of fish that have dis-
appeared.
Among terrestrial vertebrates in continental areas, displacement of the
red squirrel (
Sciurus vulgaris
) by the gray squirrel (
Sciurus carolinensis
) in
Britain is one of the few examples that may be due to competition (Usher
et al. 1992). Beginning in 1876, gray squirrels were introduced to various
parks and estates on many occasions. Grays have replaced reds, especially
in broad-leaved forests, in much of England and Wales and are still spread-
ing in Scotland.The range of the gray squirrel is now about twice that of
the red squirrel. Recently, the possibility was suggested that a viral disease
introduced with the gray squirrel has contributed to decline of red squir-
rel populations (Tompkins et al. 2003).The barred owl (
Strix varia
) of east-
ern North America has expanded its range into the Pacific Northwest,
perhaps as a result of human impacts on the landscapes of the upper Great
Plains.This species may be displacing the ecologically similar and endan-
gered spotted owl (
Strix occidentalis
) in some areas (Kelly et al. 2003).
Extinctions and Extirpations due to Disease Agents
Alien disease agents have had a major impact on tree species in North
America. Perhaps the most destructive forest disease on record is the
chestnut blight (
Cryphonectria parasitica
), which caused the virtual elimina-
tion of the American chestnut (
Castanea dentata
) as an ecologically impor-
tant tree of the eastern deciduous forests. Early in the 1900s, this disease,
a fungal canker, was introduced to NewYork State on Asian chestnut trees
planted as ornamentals. The fungal hyphae spread through the phloem
and xylem tissues, girdling and eventually killing the tree (Liebhold et al.
1996). From New York, the fungus spread rapidly and in about 40 yr had
killed mature chestnut trees through the species' range, which extended
from Maine to Georgia and west to the Mississippi River. Although the
blight kills the aboveground parts of the tree, the roots often survive and
