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But a closer look does not give confidence in the robustness of its findings. For a start, Paolucci
had to reject all but sixty-two of the fourteen hundred papers that he had found because they contained
no “quantitative descriptions” of the extent of any impact. And of the tiny minority that remained, few
directly compared aliens with natives. The majority were about aliens alone. Given the well-attested
penchant of invasion biologists for a bad-news story, Paolucci's conclusion seems to me rather doubt-
ful. The same concern about cherry-picking extends to a similar comparison by Simberloff of published
literature in the United States. He found that “six times more non-native species have been termed in-
All this feels like self-serving nonsense. Researchers whose working assumption is that alien species
do harm review their own literature to discover that more aliens than nonnatives are “termed” harmful.
It is hard to disagree with Lawrence Slobodkin, of the State University of New York until his death in
2012, who railed against the “reification” of untested ideas and unvalidated hypotheses among invasion
biologists. He asked: “What can it mean, 150 years after Darwin, to say that some species or communit-
ies are good and some are bad?”
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We will return to some of the ecological thinking behind such presumptions. But, for now, surely it
is at least as valid, and far more even-handed, to start from the belief that aliens are not in any sense bad
but are essentially indistinguishable from natives. Both groups are capable of causing harm, sometimes
dramatically, under some circumstances. Moreover, like natives, aliens can do good. If their being alien
can sometimes be problematic, then equally it can sometimes revive flagging ecosystems, creating new
space for natives and providing ecosystem services. Without very good grounds for taking a contrary
view, it is surely more reasonable to treat species on their merits.
A final element in the mythology of the badness of alien species is economics. Conservationists are in-
creasingly keen to translate their concerns about the fate of nature into terms more amenable to debate
in company boardrooms and ministerial briefings. They want to put dollar signs on the value of con-
serving nature and on the dangers of trashing it. This is not an easy task. Every economic assessment of
nature rests on numerous contestable assumptions, not least how to deal with the economists' conven-
tions on discounting future loss. I sympathize. But even by the conventional standards of environmental
economics, much work on alien species stands out as crude and sometimes plain daft.
I began with another “fact” that is frequently repeated: the annual cost of alien species to the global
economy is more than $1.4 trillion. This works out to 5 percent of the entire global economy. UNEP
boss Achim Steiner has used the figure a lot.
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Few question it. But the stat is, to put it mildly, built
on rickety foundations. The source trail goes back to a single paper, again the work of Cornell ecolo-
gist David Pimentel. His key paper in this case is a 2001 study that extrapolated the $1.4 trillion fig-
ure from data gleaned from six countries—the United Kingdom, Australia, Brazil, South Africa, India,
and—from some of his own more detailed work—the United States.
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Pimentel's studies put the total annual cost of aliens to the US economy at $120 billion. The three
largest elements were crop weeds at $27 billion, crop plant pathogens at $21.5 billion, and rats at $19
billion. The last valued each rat at $15, based on estimates of how much stored grain they eat. Thus the
total bill of aliens to the US economy was $420 per person. Taken together, he calculated that the six
countries had total annual damage from introduced species of something over $200 per human. Scaled
up to the world's population, that delivered a global bill of $1.4 trillion.
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