384 CHAPTER 15 Biodiversity and Conservation
Asian tiger mosquito European gypsy moth Asian bighead carpSea lamprey Zebra musselWater hyacinth Fire ant European wild boar Purple loosestrifePuerto Rican frog Burmese pythonBrown tree snakeNutriaScott Camazine/Alamy George Grall/NG Image Collection © M. Spencer Green/AP/Corbis kaniwi/E+/Getty Images, Inc.James L. Amos/NG Image Collection© NHPA/Photoshot Michael Melford/NG Image Collection Patrick Lynch/Alamy Richard Nowitz/NG Image Collection David Chapman/AlamyJoel Sartore/NG Image Collection Joel Sartore/NG Image Collection © Peter Steiner / AlamyÛ>ÃÛiÊëiViÃÊUÊ}ÕÀiÊ£x°ÇÊ
Selected examples of the many established foreign species accidentally or deliberately introduced into the
United States. Invasive species cause great ecological and economic harm.
Cargo-carrying ocean vessels carry ballast water from
their ports of origin to increase their stability in the
ocean. When they reach their destinations, they dis-
charge this water into local bays, rivers, or lakes. Ballast
water may contain clams, mussels, worms, small fishes,
and crabs, along with millions of microscopic aquatic
organisms. These organisms, if they establish themselves,
may threaten the area’s aquatic environment and con-
tribute to the extinction of native organisms.
One of North America’s greatest biological threats—
and its most costly aquatic invader—is the zebra mussel,
a native of the Caspian Sea, probably introduced through
ballast water flushed into the Great Lakes by a foreign
ship in 1985 or 1986. Since then, the tiny freshwater
mussel has clustered in extraordinary densities on hulls
of boats, piers, buoys, and, most damaging of all, water
intake systems (Figure 15.8).
The zebra mussel’s strong appetite for algae and
zooplankton reduces the food supply of native fishes,Invasive Species The introduction of a non-native
or foreign species into an ecosystem in which it did not
evolve often upsets the balance among the organisms liv-
ing in that area and interferes with the ecosystem’s normal
functioning. The foreign species, which typically do not
have any natural predators in their new habitat, may com-
pete with native species for food or habitat or may prey on
them. Generally, an introduced competitor or predator has
a greater negative effect on local organisms than do native
competitors or predators. Foreign
species whose introduction causes
economic or environmental harm
are called invasive species (Fig-
ÕÀiÊ£x°Ç). Although invasive spe-
cies may be introduced into new
areas by natural means, humans
are usually responsible for such
introductions, either knowingly or
unknowingly.
invasive species
Foreign species
that spread rapidly
in a new area if
free of predators,
parasites, or resource
limitations that may
have controlled their
population in their
native habitat.