Biology Reference
In-Depth Information
Apache trout. Hybrids with cutthroats were found at 20 sites, hybrids
with rainbows at 16 sites, and hybrids with both aliens at 2 sites. Alto-
gether, more than 60% of the fish examined were hybrids. This analysis
reveals the extent to which careless introduction of alien fish can destroy
the genetic uniqueness of native species and highlights the complexity of
efforts that will be required to prevent the genetic extinction of this fed-
eral endangered species.
Extinction, Extirpation, and Endangerment
Extinction is the disappearance of species due to demographic failure—
mortality that exceeds reproduction until the last individual dies—or
genetic swamping—the loss of genetic identity due to interbreeding with
other species. Extirpation is simply the local extinction of part of an over-
all population of a species. Endangerment is the existence of conditions
that have not only reduced the population of a species to a critical level,
but also, if continued, are likely to lead to extinction. Extinction and extir-
pation of populations are natural evolutionary events, but they have been
magnified in severity one thousandfold by human influences.
The impacts of introduced alien species are the most rapidly growing
cause of extinction and extirpation of endemic, native species. Alien
species can cause extinction through predation, competition, or disease.
Introgressive hybridization, the interbreeding of two or more species to
produce fertile offspring, can also lead to the disappearance of species as
distinct entities. Reid and Miller (1989) attributed 20-42% of all historic
extinctions of mammals, birds, reptiles, and fish to introductions of alien
species. Wilcove et al. (1998) judged that alien introductions were a sig-
nificant factor in 47% of vertebrates and 27% of invertebrates that are now
imperiled in the United States (table 16.1).
Species of insular environments, including oceanic islands, lakes, and
streams, are particularly vulnerable to extinction. According to the World
Conservation Monitoring Centre (1992), 100% of reptile extinctions,
90% of bird extinctions, 79% of mollusc extinctions, and 59% of mammal
extinctions have been of island species. More than 36% of all historical
extinctions of plants and animals have been of island species (Reid and
Miller 1989). A large fraction of these extinctions has been due to the
introduction of alien species. For birds, King (1980) estimated that more
than 49% of extinctions on oceanic islands were due to predation, com-
petition, or genetic swamping.
As the global impact of human activities has grown, however, the pre-
