Biology Reference
In-Depth Information
The direct and indirect economic costs of these invaders are enor-
mous. In the United States alone, alien plants, animals, and microbes are
estimated to cause economic losses of $137.2 billion annually (Pimentel
et al. 2000), with estimates for agricultural losses worldwide reaching
$248 billion (Pimentel et al. 2001). An additional $4.2 billion loss occurs
in the United States due to alien forest insects and pathogens. Additional
direct losses result from damage by aliens to fisheries, navigation, and
industry. Still other costs are incurred in fighting alien species that are
endangering native plants and animals. Invasions of natural ecosystems by
alien species are degrading their unique aesthetic and recreational values.
Invasions of alien species are certain to continue, in spite of increasing
awareness and prevention efforts. Much of this invasion is likely to occur
as a result of increasing international trade and travel (Ewel et al. 1999).
For the period from 1920 to 1990, for example, Levine and D'Antonio
(2003) examined the relationship between value of foreign imports to the
United States and the numbers of alien mollusks, plant pathogens, and
insects becoming established.The most conservative model predicted that
between 2000 and 2020 some three species of mollusks, five species of
plant pathogens, and 115 species of alien insects are likely to become
established in the United States. Less conservative models gave numerical
predictions more than tenfold greater. Increased internal trade and traffic,
including exchanges of organisms mediated via the Internet (see, e.g., Kay
and Hoyle 2001), also mean that the spread of alien species within the
United States and other countries will be great.
Alien Species and Contemporary Evolution
These threats, however, are only the tip of an ecological and evolutionary
iceberg (Palumbi 2001). The worldwide introduction of alien species is
leading to rapid evolutionary change in both alien species and the native
species with which they interact. Contemporary in the sense that they
occur over tens to hundreds of generations rather than millions of gener-
ations, these patterns of evolution are the result of profound human influ-
ences on the natural world (Stockwell et al. 2003). The introduction of
alien species is interacting with habitat destruction and degradation, over-
exploitation of plants and animals in natural ecosystems, and global cli-
matic change to create an evolutionary revolution. Patterns of rapid evo-
lution involve all groups of organisms and all patterns of organismal
interaction: plant-herbivore, prey-predator, and host-pathogen systems
(Thompson 1998; Gilbert 2002). Far from slowing evolutionary change,
humans have accelerated evolutionary processes.
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