degradation ■ Physiological and behavioral limitations of the species
Introduced Species
Invasive or exotic species are animals and plants that are transported to any area
where they do not naturally live. The spread of nonnative species has emerged in
recent years as one of the most serious threats to biodiversity. It has undermined
the ecological integrity of many native habitats and pushed some rare species to
the edge of extinction. Some introduced species simply out-compete native
plants and animals for space, food, or water. Other negative impacts include
predation of nontargeted species and the disruption of food webs and/or
biogeochemical cycles.
About 15% of the estimated 6,000 nonnative plant and animals species in the
United States cause severe economic or ecological impacts and have been
implicated in the decline of about 40% of the species listed for protection under
the federal Endangered Species Act. Examples of introduced species include:
■ DUTCH ELM DISEASE (caused by a fungus) is transmitted to elm trees by elm
bark beetles. Since 1930, the disease has spread from Ohio through most of the
country, killing over half of the elm trees in the northern United States.
■ EUROPEAN GREEN CRABS found their way into the San Francisco Bay area
in 1989. They out-compete native species for food and habitat, and they
eat huge quantities of native shellfish, threatening commercial fisheries.
■ WATER HYACINTH is an aquatic plant, introduced to the United States from
South America. It forms dense mats, reducing sunlight for submerged
plants and aquatic organisms, crowding out native aquatic plants, and
clogging waterways and intake pipes.
■ ZEBRA MUSSELS first came to the U.S. from Eurasia in ship ballast water
released into the Great Lakes. Since 1988, they have spread dramatically,
out-competing native species for food and habitat. Zebra mussels can
attach to almost any hard surface—they clog water intake and discharge
pipes, attach themselves to boat hulls and docks, and even attach to native
mussels and crayfish.Endangered and Extinct Species
Since 1500 C.E., approximately 816 species that we know about have become
extinct, 103 of them since 1800—a rate 50 times greater than the natural
background rate. In the next 25 years, extinction rates are expected to rise as
high as 25%. According to the Nature Conservancy, about one-third of all U.S.
plant and animal species are at risk of becoming extinct.
Mammals that are listed as “endangered” rose from 484 in 1996 to 520 in