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Alien Species Often Fit In Fine, Some Scientists Contend
By MARK DERR
Governments, private groups and individuals spend billions of dollars a
year to root out non-native organisms that are considered dangerous to
ecosystems and to prevent the introduction of new interlopers.
But a number of scientists question the assumption that alien species are
never acceptable in a natural ecosystem. While applauding efforts to
banish harmful organisms like the brown tree snakes that have destroyed
most of Guam's native species of forest birds or the star thistle, a
prickly weed that is toxic to horses and has invaded much of the West
they say portraying introduced species as inherently bad is an
Distinctions between exotics and native species are artificial, said Dr.
Michael Rosenzweig, a professor of evolutionary biology at the University
of Arizona, because they depend on picking a date and calling the plants
and animals that show up after that exotic. Ecosystems free of species
defined as exotic are, by default, considered the most natural.
"You can't roll back the clock and remove all exotics or fix habitats,"
Dr. Rosenzweig said. "Both native and exotic species can become invasive,
and so they all have to be monitored and controlled when they begin to
get out of hand."
At its core, the debate is about how to manage the world's remaining
natural ecosystems and about how, and how much, to restore other
habitats. Species that invade a territory can harm ecosystems,
agriculture and human health. They can destroy some native species and
supplant or threaten others. Next to habitat loss, these invasive species
represent the greatest threat to biodiversity worldwide, many ecologists
Ecologists generally define an alien species as one that people
inadvertently or deliberately carried to its new location. In the New
World, exotic species are those introduced after the first European
contact. That date, rounded off to 1500, represents what ecologists
consider to have been a major shift in the spread of species, including
crops and livestock, as they began to leapfrog with humans from continent
Only a small percentage of alien species cause problems in their new
habitats, said Dr. Daniel Simberloff, a professor of ecology and
evolutionary biology at the University of Tennessee. Of the country's
7,000 alien species out of a total of 150,000 species only about 10
percent are invasive, he said. The other 90 percent have fit into their
environments and are considered naturalized. Yet appearances can deceive,
ecologists caution, and many of these exotics may be considered
acceptable only because no one has documented their harmful effects. And
non-native species can appear innocuous for decades, then turn invasive.
One example is the Brazilian pepper, which landscapers introduced into
South Florida in the late 19th century. It started to spread widely in
the 1950's and has now crowded out native vegetation throughout the
Everglades. Once a species begins to run amok, it is extremely difficult
Faced with such uncertainty, many ecologists argue for strong steps. "I
think we should take a precautionary approach and be much more proactive
in trying to take these things out before they become problems," Dr.
Simberloff said. "You don't want exotics in natural ecosystems."
But a number of experts question the scientific wisdom of trying to roll
back ecosystems to a time when they were more natural.
Defining which species belong in an ecosystem is based less on science
than on historical, cultural, moral, geographic and theological
arguments, said Dr. Mark Sagoff, who studies the issue at the University
of Maryland's Institute for Philosophy and Public Policy. Science cannot
judge an ecosystem with exotics to be worse, or less natural, than one
without them, he said, without also taking into account the effects of
those species on their environments.
Even many ecologists who would like to rid ecosystems of all exotics
admit that the goal is impractical.
"We can't return to pre-settlement ecosystems," said Dr. Alan Holt, the
director of conservation programs for the Northwest and Hawaii at the
Nature Conservancy, a nonprofit group that acquires and restores land to
preserve the diversity of species. For one thing, he said, many exotic
species have become so integrated into ecosystems that animals, some
endangered, rely on them for food and shelter.
Dr. Rosenzweig said removing exotics might cause other problems. In
Australia's Northern Territory, for example, the eradication of the non-
native water buffalo that were ravaging vegetation led to the explosive
growth of a little-noticed plant, the giant mimosa, which was introduced
from Central America in the 1890's. The shrub has been more destructive
and harder to remove than the water buffalo.
In the March issue of the journal Evolutionary Ecology Research, Dr.
Rosenzweig, the editor, challenges the prevailing view that invasive
alien species reduces biodiversity. The exotics increase the number of
species in the environment, he wrote. Even if alien species cause
extinctions, the extinction phase will eventually end and new species may
then begin to evolve, he explained.
Ecologists should focus on managing the environments that include exotic
immigrants, Dr. Rosenzweig said, and creating new ones where necessary to
enhance species' survival and biodiversity.
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