Biodiversity of Freshwaters - Freshwater Ecology: Concepts and Environmental Applications - page 196

nounced examples of the effects that a nonnative species can have, in terms
of both explosive invasion and economic consequences (see Sidebar 9.1).
Many other introductions of nonnative species to freshwaters have oc-
curred (Table 10.2) and many of these were intentional. Introduction of
fish species has led to homogenization of fish communities; many of the
same species are found across the continent now (Rahel, 2000). Hopefully,
this practice of purposely stocking exotic species will decrease in popular-
ity as it becomes clear that unintended consequences are common with
species introductions.

EXTINCTION

Sidebar 10.1.
Invaders of the North American
Great Lakes

Man has been reducing diversity by a rapidly
increasing tendency to cause extinction of
supposedly unwanted species, often in an in-
discriminate manner. Finally we may hope
for a limited reversal of this process when
man becomes aware of the value of diversity
no less in an economic than in an esthetic
and scientific sense.

The Great Lakes have a long history of inva-
sions of exotic organisms caused by humans
(Table 10.1). As of 1990, more than 139 alien
species had become established, including
plants, fish, invertebrates, and algae (Mills
et al., 1994). By 1998, 145 alien species had
been documented (Ricciardi and MacIsaac,
2000). Some of these introductions have had
massive economic impacts, such as the effect
of the zebra mussel on water intakes for mu-
nicipalities and industries and the crash of the
large lake trout fishery caused by the sea lam-
prey. Many of the other introductions have had
moderate impacts or impacts that do not di-
rectly affect humans in an economic sense.
Routes of introduction include intentional re-
leases, movement through the aquarium trade
and bait buckets, release of ship ballast water
taken from other areas, and connection of the
Great Lakes with the Atlantic Ocean by a sys-
tem of shipping canals (the Erie Canal and the
St. Lawrence Seaway). Attempts are being
made to limit the number of new introductions.
For example, ships from overseas that originate
in freshwater ports are required to exchange
ballast water while at sea to avoid additional
introductions of freshwater species. However,
species that are tolerant to salinity changes
will not be kept out by these methods (Ricciardi
and MacIsaac, 2000). It is likely that new
species will continue to be transported into the
Great Lakes, and damage associated with some
of these species and the existing nonnative or-
ganisms will continue to be considerable.

—Hutchinson (1959).
Humans likely will cause extinctions of
at least half of all the approximately 10 mil-
lion species on Earth in the next 50-100
years (May, 1988), despite the fact that the
problem has been recognized clearly for more
than 50 years. May calculated that world-
wide extinction rates are currently 1 million
times greater than rates of evolution.
In an even bleaker assessment of the sit-
uation, it has been argued that area-diversity
relationships (discussed previously) can be
used to estimate the ultimate effects of habi-
tat destruction, and that in the long term the
habitat destruction caused by humans to date
will result in the loss of 95% of the earth's
species (Rosenzweig, 1999). Freshwater habi-
tats are probably the most impacted by hu-
mans. Because they integrate the landscape,
aquatic systems suffer corresponding effects
from all extensive terrestrial disturbances.
Humans live near water and discharge their
wastes into it. Control of hydrology by hu-
mans destroys habitats for many species.
Consequently, losses of freshwater species
have been substantial. The outlook for sensi-
tive aquatic species is bleak (Folkerts, 1997).
Rates of species extinction caused by man
far exceed the rates of evolution of freshwater
organisms. More than 300 species of endemic
fish may have evolved in Lake Victoria, Africa,