Letters: July 2005

Letters from the July 2005 issue.


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To bury Burdick . . .

In the May article “The Truth About Invasive Species,” Alan Burdick dismisses the impact of introduced species with the contention that “fifty years of research by invasion biologists around the world has failed to confirm it,” suggesting that all changes and impacts wrought by invaders merely threaten “our self-serving ideas of what nature is supposed to be.” Yet in addition to extinctions there are many dramatic, well-documented effects of introduced species, such as shifts in regional climate, reduced groundwater levels, altered nutrient cycling, and reduced oxygen levels in aquatic systems, which affect the abundance and distribution of native species. Burdick states that for most introductions, we have no evidence of negative consequences (“most invasions do no harm”). This results almost entirely from lack of study and not from studies documenting no effects. An absence of evidence is not evidence of absence. Burdick, quoting two sources out of context, implies that the invasibility of aquatic habitats equates with a lack of impact, but this idea also finds no support in the scientific literature. Many historical introductions for agricultural and fishery purposes have been beneficial, but the scientific record shows that the consequences of introductions are unpredictable and come with potentially enormous consequences.Edwin D. Grosholz

, Dept. of Environmental

Science and Policy, University of California at


James T. Carlton

, Williams College–Mystic

Seaport, Mystic,


Gregory M. Ruiz

, Smithsonian Environmental

ResearchCenter, Edgewater, Maryland

Alan Burdick’s assertion that introduced species generally have little harmful impact is misleading. He argues that most introduced species do not outcompete native ones, but invasion biologists have never claimed that most did. Resource competition is extremely difficult to demonstrate for most species, and introduced species are no exception. The species with the biggest impact, usually those that modify habitat or change ecological processes, are plants. Burdick cites example after example of the lack of impacts from animal species. It is a commonplace among invasion biologists that most introduced species do not have major impacts—perhaps about 90 percent. But the minority generating major change do cause huge problems. Much remains unknown about how most introduced species affect natural ecosystems. Further, many known impacts of these species were understood only after intensive research. Burdick is correct that the value to humans of a landscape dominated by natives is largely subjective, but he is way off base to claim that the changes wrought by introduced species are minor.daniel simberloff

, Department of Ecology and Evolutionary Biology,

University of Tennessee, Knoxville,


Don C. Schmitz

, Department of Environmental Protection,

Bureau of Invasive Plant Management, Tallahassee, Florida

Although no quotes were taken out of context, the rhetorical nature of some—“With a lot of invasions, it’s not ecology, it’s ‘Who cares?’ ”—may have been less self-evident than I thought. What Jim Carlton meant here was that despite measurable ecological impacts, scientists must often invoke human health and economic impacts to get the public and politicians to care. My apologies if that wasn’t clear. Carlton was an author of the 2003 Pew Oceans Commission report, which deemed invasive species a significant threat to today’s oceans. Some may be skeptical of the scale of damage done by invasives; Carlton isn’t among them. By the same measure, the researchers are incorrect to interpret my article as dismissing the impacts of aliens or implying that “the changes wrought . . . are minor.” I made numerous explicit references to the harm done by invasives, such as the loss of valuable beta diversity and priceless native species through predation, disease, and ecosystem alteration. (Editor’s note: See also Letter From Discover, page 27.) What invasion biology has failed to confirm, I wrote, isn’t their harmfulness but rather the disease metaphor inherited from the 1950s. Greater biological diversity does not (as Charles Elton posited) offer a biotic resistance against invasions. Ecologists now know aliens invade for reasons largely unrelated to ecology; the main cause is the pressure of human traffic. Disease is an evocative metaphor for alien species, but it obscures the real—and complicated—reasons why invasions occur and ecosystems change.

Alan Burdick

. . . And to praise him

I admire Alan Burdick for presenting an aspect of invasion ecology that has received little attention from most ecologists. Until recently, it was not common practice for ecologists to explicitly point out that only a small subset of introduced species cause problems. For example, if one reads the literature on biological invasions published in the 1990s, one will find few efforts by ecologists to present the full picture of the impacts of introduced species—for instance, that a small number cause great health, economic, and/or ecological problems, most have little known impact, and a small number have a positive impact. Instead, one finds a tendency to paint introduced species with a single broad brush, often accompanied by inflammatory metaphors and doomsday, scare-tactic language, such as “biological pollution” and “invasion meltdown.” Some ecologists argue that this is a necessary strategy when dealing with the public and policymakers. I couldn’t disagree more. Although one might be able to secure short-term gains this way, misleading the public and policymakers almost always backfires. The public doesn’t like its news manipulated, whether it’s by the Department of Defense or the biology department.

Mark A. Davis

, Department of Biology,

MacalesterCollege, Saint Paul, Minnesota

I enjoyed Alan Burdick’s willingness to think for himself and produce something more than yet another me-too boilerplate on alien species fear and loathing. The conservation biologists born of Earth Day have matured to dominate academic ecology. Neither they nor their allies in the environmental movement tolerate challenges to antialiens dogma. Having discovered the power and gratification of condemning, even exterminating, whole populations that violate their idealized model of the world, they can hardly be expected to balk at a little verbal abuse. In Charles Elton’s 1958 The Ecology of Invasions by Animals and Plants, there appear three kinds of reasons for conservation: “religious,” “aesthetic and intellectual,” and “practical.” Invasion biology’s florid competition to hyperbolize the alien threat suggests that Elton’s omission of “scientific” or “ecological” among his reasons for conservation was well considered. Read a few or a few hundred scientific articles about invasions and you will discover that most include some statement to the effect that “nativeness rules.” Alan Burdick didn’t explicitly question that faith, but he did temper it with experience and personal insight. That, under any rigid system of belief, is the first sign of heresy.MATT CHEW, Phoenix, Arizona

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