Fwd: Alien Species Often Fit In Fine, Some Scientists Contend

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    Alien Species Often Fit In Fine, Some Scientists Contend



    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
    unscientific approach.

    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
    to 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
    to eradicate.

    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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