MYTHS OF THE ALIENS - The New Wild: Why Invasive Species will be Natures Salvation - page 70

Brook University in New York. Gurevitch and Padilla had argued that “available data supporting inva-
sion as a cause of extinctions are, in many cases, anecdotal, speculative and based upon limited observa-
tion.” 20 Alien species, they pointed out, often turn up in a new place at roughly the same time as natives
decline. But that does not prove cause and effect. It is equally likely that some other change—perhaps
to climate or the local habitat—messes with the natives, and the aliens just move in to fill the gap. Some
researchers say this makes aliens “passengers rather than drivers.” 21 That makes sense to me.
There is a threadbare laxness in the use of statistics by many invasion biologists. Almost wherever
I pursued a key claim, the trail fizzled out in obfuscation, false citations, unverified judgment calls, and
absurd leaps from the specific to the general and the local to the global. Several trails lead back to im-
portant and pioneering work by Pimentel. He is a revered figure among ecologists, and I have quoted
and cited him many times in writing about many scientific topics. He does pioneering work. But here
this status seems to have become a liability. He is so highly regarded that nobody tries to replicate his
findings or update them. Years later, his back-of-the-envelope calculations are handed down like tablets
of stone. That really is not his fault; it is the fault of the wider scientific community.
Just over a decade ago, Pimentel published a series of papers trying to estimate the costs of invasive
species to the United States and the world. For now let's stick to his observations about the environ-
mental problems related to nonnative species. In a 2001 paper on the global threat, he stated in con-
clusion that “an estimated 20-30% of the introduced species are pests and cause major environmental
problems.” 22 This statement has been widely cited by others, including (once again) the UK invasive
species secretariat and official documents of the CBD. But the figure did not seem to result from any
research by him in the paper, so I asked for his source. Pimentel's e-mailed reply said: “This 20-30%
figure is mine, based on my long-time experience.” That's it. We have to take his word for it, because
nobody has troubled to try to improve on it.
In a 2004 paper on US alien species, Pimentel noted in passing that “in other regions of the world,
as many as 80% of the endangered species are threatened and at risk due to the pressures of non-native
species.” 23 This too is widely quoted by the CBD and others. Here he had a source: “Armstrong 1995,”
which turned out to be one sentence in a news item by my old New Scientist journalist colleague, South
Africa correspondent Susan Armstrong. 24 She was referring specifically to plants in the fynbos area, a
small part of the Western Cape that is of exceptional biodiversity, but hardly a “region of the world.”
I checked back. Armstrong told me that her source was Brian van Wilgen of the Centre for Invasion
Biology in Stellenbosch. Van Wilgen told me: “80 percent is wrong; it looks like about 25 percent of
threatened species are [on the threatened list] at least in part because of aliens.” A UNEP study quoting
a report from South African scientists (among them van Wilgen) says that just 750 of the 8,574 nat-
ive plant species of the fynbos “are currently facing extinction [as a result of] pressure from invading
species.” That's less than 10 percent. 25 Whatever the truth, Pimentel's stat has entered the domain of
constantly recycled “facts” about alien species.
Despite the unreliable statistics, it might still be true that alien species often cause extinctions. Wil-
cove told me: “Given how harmful rats, cats, and other non-native species have been to islands' faunas,
I am sure the proportion of species driven to extinction by non-native species is high.” As an adjunct to
this, Simberloff argues that under sustained pressure from invaders, even mature ecosystems will even-
tually suffer “invasional meltdown.” He says that “chronic exposure to many invaders ultimately will
undermine almost any ecosystem.” But others say the carnage on a handful of islands, assiduously doc-
umented by invasion biologists, does not represent the wider picture. It is true that—as when rats meet
ground-nesting birds with no defenses—alien predators can cause extinctions. But competition between
species for jobs within ecosystems rarely results in one species disappearing, even on islands.