MYTHS OF THE ALIENS - The New Wild: Why Invasive Species will be Natures Salvation - page 71

In the past decade or so, a new body of evidence has emerged suggesting that some of the most
widely used statistics in the canon of invasion biology do not stand up. When Dov Sax of Brown
University conducted a global study of species invasions on oceanic islands, he found that while the
number of birds did sometimes go down after invasions, plant biodiversity usually rose. Typically, such
islands now have twice as many plant species as before. 26 That study almost instantly put Sax at the
forefront of a quiet revolution in which a new generation of researchers questioned the demonizing of
aliens by Simberloff and others.
Even remote “problem” islands may gain overall biodiversity. In Hawaii, introductions heavily out-
number extinctions, and overall biodiversity has risen. In New Zealand some native species have been
ravaged by invaders. But there has been an overall doubling of plant biodiversity—from around two
thousand species to around four thousand—thanks to the newcomers brought by Europeans. The biod-
iversity of birds in New Zealand is as high as it has ever been. Yes, there have been some tragic losses. It
is often said that sparrows and starlings hardly make up for the loss of Haast's eagle, the world's biggest
eagle, or its principal prey, the large flightless moa. Indeed. But both those birds died out around six
hundred years ago, as a result of hunting by the Maori people rather than species introductions. 27
Other prominent figures questioning the widely circulated statistics on the biodiversity threat posed
by alien species included Mark Davis, who began to question the basis of invasion biology. It was, he
said, based on a false proposition. Except in rare cases, aliens bring greater biodiversity, not less.
When the Suez Canal opened in 1869, it allowed tropical species from the waters of the Indian
Ocean to move into the Mediterranean. And they did. Yet while 250 species of all kinds established
themselves, there has only been one recorded extinction. 28 Similarly, when the Panama Canal joined
the Pacific and Atlantic Oceans in 1914, biodiversity increased on both sides. North America has more
birds and mammal species than when the Europeans first landed. And the addition of some four thou-
sand plant species has added 20 percent to biodiversity and not, so far as is known, resulted in a single
plant species being lost. Likewise, the UK's twenty-three hundred additional species have not directly
caused any known local extinctions.
Davis questioned the claim that species-rich and mature ecosystems are “saturated” with species,
with every available niche taken. In fact, rather than being resistant to taking on more species, “typically,
species-rich communities . . . accommodate more introduced species than species-poor communities,”
he said. 29 More alien species means more natives, rather than fewer—a rule that seems to apply from
New Zealand to the Appalachians. 30 Aliens may find new jobs to do or share jobs with natives. But
either way, there is no shortage of tasks, and biodiversity usually increases. And rather than suffering
“invasional meltdown” as a result of a constant assault from new arrivals, these augmented ecosystems
will in effect become inoculated against threats from future incomers. The greater variety of species in
the system will make it more robust and better able to embrace and make use of any new invaders, an
idea termed “biotic resistance.”
In the face of this evidence, Simberloff and others offer a revised argument. Yes, they say, this boost to
biodiversity in invaded ecosystems sometimes happens. But this is a temporary phenomenon. There is
an “extinction debt” in many disturbed ecosystems. The arrival of aliens will have set some existing spe-
cies on a cycle of decline that will ultimately lead them to oblivion. It may take decades for new species
to establish themselves and maybe decades more for natives to succumb. But it will happen. Simberloff
points out how there is often a time lag of several decades between the arrival of new species and their