Environmental Engineering Reference
In-Depth Information
Whether alien species are deemed to be bad neighbors seems remarkably dependent on the whims
of fashion and scientific inquiry. Ecological disaster stories of the past often rapidly fade. The villains
cease to be villains. Back in the 1970s, coco grass, an old-world herbal remedy also known as purple
nutsedge (
Cyperus rotundus
), was given top spot among the “world's worst weeds” in a six-hundred-
page inventory put together by LeRoy Holm of the University of Wisconsin-Madison. But it does not
even merit a place on the list of top hundred worst invaders compiled by today's generation of inva-
sion biologists for the International Union for the Conservation of Nature (IUCN). In fact, only two of
Holm's top ten make it into today's top one hundred: cogon grass and water hyacinth.
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Similarly, of
the seven prime case studies cited by Elton in his 1958 topic, only two—starlings and Chinese mitten
crabs—make it into today's top one hundred.
Behind the demonology, what is the truth? One of the most common charges against alien species is that
they almost inevitably cause extinctions among their new hosts. They cause a decline in biodiversity.
The theory behind this is rooted in ecological thinking about how natural ecosystems are well-oiled ma-
chines in which every native species inhabits a unique “niche” with a particular job to do, whether as
prey or predator, pollinator, or processor of waste. The ecosystem is “saturated” with species, and there
is usually little room for interlopers to make much of a contribution. So if a new species takes root, then
it will usually supplant a native. Expelled from the system, that native may become extinct, at least loc-
ally.
A handful of researchers have tried to quantify this extinction threat from aliens, and their findings
have been widely quoted. Britain's Non-Native Species Secretariat, a government agency charged with
addressing the problem of aliens, gave prominence on its website in late 2013 to the “fact” that invader
species “have contributed to 40 percent of the animal extinctions that have occurred in the last 400
years.”
14
I was intrigued by the confidence of this assertion. So I went in search of the evidence that
backs it up.
The site sources it to the second Global Biodiversity Outlook report, published in 2006 by the sec-
retariat of the UN Convention on Biological Diversity (CBD) and the UN Environment Programme
(UNEP). That document is a bit more nuanced. It says that invasive species are involved in “nearly 40%
of all animal extinctions
for which the cause is known
” (my emphasis). It says nothing about four hun-
dred years, nor does it offer any research to back up its claim. UNEP referred me to the CBD's program
officer Junko Shimura, who in turn said that the source was a 2005 paper from Cornell ecologist David
Pimentel. That paper in turn gave as its source a paper by Princeton ecologist David Wilcove published
seven years earlier, when Wilcove worked for the Environmental Defense Fund, a US-based NGO.
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Wilcove's 1998 paper, in the journal
BioScience
, is a widely quoted text on invasive species. Google
Scholar found more than two thousand citations for it in other papers. But Wilcove was not saying what
UNEP, CBD, or the British government claimed. He was making a rather different point. He was ar-
guing, on the basis of a review of papers by invasion biologists, that 49 percent (not 40 percent) of the
extinction “threat” to “imperiled” species came in some part from invader species. So he was not talking
about actual extinctions but an extinction “threat,” as judged by invasion biologists. Even more surpris-
ing, given the global claims being made by the international agencies, he was only reviewing papers
about US endangered species. He made no claims that the conclusion might apply more widely.
Mark Davis, of Macalester College in Minnesota, who has reviewed the research that Wilcove col-
lected for his paper, says the paper is not even truly a reflection of the situation in the United States.
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