Biology Reference
In-Depth Information
from Lake Ladoga, Russia (Therriault et al. 2002). The Neva River con-
nects this lake to the Gulf of Finland in the eastern Baltic Sea at the port
of St. Petersburg, Russia. Thus, the North American populations were
apparently introduced in ballast water of cargo ships coming from St.
Petersburg.
Studies of these cladocerans have helped identify a major invasion cor-
ridor for exotic invaders of the North American Great Lakes. In particu-
lar, they have revealed the importance of the connection between the
Caspian Sea and the Baltic Sea (Berg et al. 2002; Ricciardi and MacIsaac
2000).The Volga River, which flows into the Caspian Sea, is connected by
a series of rivers, canals, and lakes to the Gulf of Finland in the eastern
Baltic Sea. Several alien species from the Caspian region, including
Cerco-
pagis pengoi
,have followed this corridor and made the final stage of the trip
in ballast water of commercial ships traveling from the eastern Baltic Sea
to North America. These and other planktonic cladocerans have shown a
rate of invasion roughly 50,000 times that of natural invasions of North
America from Eurasia (Herbert and Cristescu 2002). At least 12 other
species that have dispersed along the Eurasian portion of this corridor are
poised to make the final jump to North America (Berg et al. 2002).
Disproving Cases of Supposed Human Introduction
Molecular genetic analyses have also been used to determine that some
populations are not recent human-assisted introductions, as once thought.
The European periwinkle (
Littorina littorea
), widespread on the Atlantic
coast of North America, has been considered by many to be a recent his-
torical introduction because of its sudden appearance in many locations in
the late 1800s. Analyses of mitochondrial and nuclear DNA, however,
showed many differences between North American and European popu-
lations (Wares et al. 2002).The most recent date for the likely spread of the
European periwinkle to North America is about 16,500 years before pres-
ent. It now appears likely that this species was present but very restricted
in distribution along the North American coast and rapidly expanded its
range in response to ecological changes that occurred in recent time.
Source Areas and Biological Control Potential
The ability to determine the precise source areas of introduced aliens has
major implications for classic biological control. Source areas represent
the most likely places to find effective biocontrol species. In the case of
