Environmental Engineering Reference
In-Depth Information
Zebra Mussels: Invasive Species
With the increase in trade between nations, there has been an increase in the number of nonnative species intro-
duced throughout the world. The impact from the introduced species can cause major economic issues, physic-
al damage, and ecosystem alteration. For example, the zebra mussel is an invasive species that has caused ma-
jor damage in the United States. Zebra mussels first arrived in North America attached to the hulls and in the
ballast water of ships from Europe, populating Lake Clair in Canada in 1988, and then spreading to the Great
Lakes and their connected rivers, tributaries, and lakes. The ability of their larvae to drift far in flowing water,
and for adults to attach to many structures and then be transported, allowed their quick spread. And since zebra
mussels were nonnative to both the United States and Canada, they had no natural predators, competitors, or
diseases in the region, allowing them to proliferate without check.
Impacts from the spread of the mussels were far-reaching. Due to their small size, the zebra mussels not only
damage boats, fishing gear, and docks, but also can damage and/or clog engines and pipes. Ecosystems can be
altered because mussels consume large amounts of phytoplankton and zooplankton, reducing energy available
to other species, and native mollusks can be suffocated when the mussels attach to their shells. Because of the
large amount of food consumed by the zebra mussels, they deposit high quantities of nutrients on the bottom,
which feeds the benthic population. Too much of this, though, can lead to eutrophication of the water. Because
of invasive zebra mussels, large expanses of the Great Lakes that once teemed with life are nearly barren.
Zebra mussels have caused extensive physical damage, and there are high costs associated with repair of this
damage and with the eradication of this invasive species.
Extinction of the Passenger Pigeon
At one time, passenger pigeons were the most abundant birds in North America, nesting in huge colonies and
flying in enormous flocks, sometimes numbering over a billion individuals. Unbelievably though, the passen-
ger pigeon is now extinct, mainly due to deforestation, leading to loss of habitat and food sources, and from
overhunting. Pigeons were edible, and people also used them for other purposes, including using feathers for
pillows and bones for fertilizer.
Because of the vast number of large flocks, commercial hunters could kill many birds easily, sometimes over a
thousand in one hunting session. Pigeons were seen as disposable—killed freely, easily, and indiscriminately.
During a 70-year period, pigeon populations declined from their historic billions to only a few thousand. Be-
cause of the breeding habits of the birds, the regeneration of the species was slow. A female laid one egg per
nest per year.
The last passenger pigeon died in 1914 in the Cincinnati Zoo. This is a prime example of how humans can
single-handedly bring a species to extinction within a brief period of time.
DDT
Diphenyl-trichloroethane (DDT) is a pesticide that was widely used from 1939 until 1973, when it was found
to be toxic to humans, wildlife, and ecosystems. In 1973, it was banned in the United States and eventually be-
came illegal in many other nations as well. The author Rachel Carson raised awareness of the dangers of DDT
and other poisonous pesticides with her topic
Silent Spring,
published in 1962. DDT effectively kills insects in-
cluding malaria-carrying mosquitoes, but the risks of its use outweigh the benefits in most situations.
Nations at extreme risk of malaria still use DDT. Despite a treaty to reduce and phase out DDT, its use will
likely continue until a safer, effective, inexpensive alternative is developed. Much of the DDT used internation-
