The mixed messages on invasives that the public is receiving from the media and scientists only help to further blur the issue. A few vocal opponents, or deniers as some media label them, make it seem like there is more controversy than perhaps there really is. Their philosophy is, “they are here to stay, so get used to it.” This article will lay out some of the issues with, and science behind, invasive species and let you make an informed decision about whether or not you think invasive species are the villains that many portray them to be.
An invasive species, according to the federal government, is any species that is not native to the area and causes or may cause harm to human health, to the environment or to the economy. The key terms there are “not native” and “causes harm,” for there are many non-native species that remain neutral or beneficial in the environment (think of the many fruits and vegetables we eat). Some land managers argue that there are native species that can have invasive tendencies in the environment; we will get to that later in the article. For now, let us discuss the role of invasives in our ecosystems.
The small but vocal contingent of biologists and land managers that think that too much fuss is being made over invasive species management even go so far as to say that we may be underestimating the benefits these species may have. Mark A. Davis, a professor and Chair of Biology at Macalester College, has written the book Invasion Biology to challenge the notion that all exotic species are bad. In the book, Davis asserts that if some invasives are simply changing the ecosystem, not causing extreme harm, then altering one's perspective is certainly much less costly than any other sort of management program.” The public cost of invasives is estimated at more than $138 billion a year according to the article, Environmental and economic costs of nonindigenous species in the United States, in the journal Bioscience, so you can see where Davis is coming from.
Davis isn't saying that we disregard all invasive species though. For highly destructive species like kudzu,where the damage is great, he does support control programs, as long as the control methods stand a good chance of success. Unfortunately many times no one can predict the chance of success until large amounts of time and money have been spent.
 (Photo: John Byrd, Mississippi State Univ.) |
One of the more controversial assertions that Davis
makes is saying that invasives rarely drive native species
to extinction. On the contrary, he says that the opposite
is true; that exotics increase species richness by adding
new species to the native landscape. Yet many other
biologists disagree. Dan Simberloff, professor of
environmental science at The University of Tennessee -
Knoxville, wants us to think of the Asian chestnut blight, an introduced fungus that for all intents and
purposes exterminated the native chestnut tree from the
United States (some small root sprouts persist but are unable to reproduce). A variety of species that
depended on chestnut trees were impacted, including
“total global extinction of at least seven species of moth
that were host-specific only on American chestnuts.”
Research published in Bioscience by David Wilcove at
Princeton University found that invasive species are at
least partially responsible for the extinction or
threatened status of 49 percent of imperiled species in
the U.S. Invasive species have also been shown to
homogenize some habitats, leading to a loss of genetic
diversity.
Davis' use of the term exotics, rather than “invasives,”
could have a part to play in the difference of opinions.
Not all exotics are invasive. Confusing the two terms
can have negative results. Defining an exotic species as
invasive is a value judgment.
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