MYTHS OF THE ALIENS - The New Wild: Why Invasive Species will be Natures Salvation - page 74

That calculation begs some questions about how representative those six countries are. But the more
immediate question is whether the national calculations hold water. This was a pioneering first approx-
imation, so it would be unfair to be too picky. But my first concern was simply factual errors about ali-
ens. Some are relatively trivial. “In Britain,” Pimentel wrote, “efforts are being made to eradicate the in-
troduced muskrat and coypu. The populations have been significantly reduced, but the eradication effort
has yet to succeed.” 42 For the record, the last coypu in Britain was eradicated in 1989 and the muskrats
have been gone since the 1930s.
But some errors have significant implications for his trillion-dollar figure. Rats, mostly brown and
black rats, are responsible for just over half the environmental losses from alien species in his six coun-
tries, at $56 billion a year. Almost half of the rats' bill, $25 billion, comes from India, where local es-
timates put the rat population at 2.5 billion. Fine so far. And each Indian rat may, as he reckons, eat its
way through $10 of grain every year. But why, I wonder, is this marked down to alien species at all? The
brown rat ( Rattus norvegicus ) is thought to have originated in China, so though it has probably been in
India for thousands of years, it might just qualify as an alien. But the black rat ( Rattus rattus ) is widely
recognized as having originated in India and remains the main rat species found there. It is not alien to
India; it has no place in Pimentel's stats. And yet as the main rat found there it must make up a large part
of his $25 billion. I put this to Pimentel by e-mail but he did not reply.
My second concern was how Pimentel did his costing of aliens. His US papers give some useful
detail on this. Starlings are assessed on their consumption of grain and cherries, dogs for their livestock
kills and medical bills for bitten humans, and domestic cats ( Felis catus , originally from Africa and
domesticated in the eastern Mediterranean about three thousand years ago) for their bird kills. In what
seemed to me a rather arbitrary way—based mainly on the amount of money people spend to go bird-
watching or shooting—he assessed the cost of each bird caught by domestic cats and their feral cousins
at $30. Multiply that by the half-billion or so birds estimated to be killed by Felis catus each year, and
the annual cost of cats comes out at a staggering $17 billion. Well, maybe. But in what sense is this a
cost to the US economy? In what sense would each American be $50 or so richer without this cat-on-
bird carnage? 43
Some birds feature as a cost when killed by cats but also as a cost when they stay alive. In his 2001
study, Pimentel puts the cost to the six nations of three common avian invaders—pigeons, sparrows, and
starlings—at $2.4 billion. Starlings ( Sturnus vulgaris ) in the United States alone cost $800 million, he
says. Perhaps they do eat crops worth that amount. But, as Pimentel himself notes in passing, “these ag-
gressive birds have displaced numerous native birds.” Given that, we must presume that the starlings are
often eating crops that native birds would otherwise have eaten. But nowhere does Pimentel ask—still
less answer—whether their presence actually increases the crops taken by birds at all.
The basic problem, as I see it, is that Pimentel stacks up every cost he can find on the debit side, but
nowhere offsets this with an assessment of any mitigating factors, still less the possible economic bene-
fits of alien species. To be fair, he does not claim to have conducted a rigorous cost-benefit analysis. But
surely to leave out one side of the ledger is poor economics. And certainly others who have reused his
statistics are guilty of misrepresenting what they do and don't show.
Let's go back to cats in the United States. It is well known that they hunt mice as well as birds. By
one estimate they kill ten times more mice than birds. 44 Pimentel acknowledges that mice are “partic-
ularly abundant and destructive” in the United States, yet nowhere does he suggest that cats should get
a cash credit for eating mice. Taking a wider view, cats also make their owners healthier. A Canadian
study found that claims made to health insurance companies by pet owners were a third less than those