Environmental Engineering Reference
In-Depth Information
points. In the Indian Ocean, on Australia's Christmas Island, the dynamic new ingredient was yellow
crazy ants (
Anoplolepis gracilipes
) from West Africa. The ants have spread widely across the tropics,
hitching rides with passing trading ships, and are reckoned to have been on Christmas Island for the bet-
ter part of a century. But they suddenly went on the rampage in the 1990s, creating “super-colonies” of a
kind never seen before. Nobody is sure why this happened, but they may have benefited from the spread
of sap-sucking insects, since the ants eat the honeydew secreted by the insects. The ants and sap-suckers
certainly seemed to have formed a strong reinforcing relationship that ecologists call “mutualist,” even
though one is alien and the other native.
The ant super-colonies extended for hundreds of acres and contained billions of insects. As they
grew, things got out of hand. The ants came into ever greater contact with the island's previously dom-
inant species, red crabs (
Gecarcoidea natalis
), during their annual migration to the coast to breed. The
ants adopted their normal defense mechanism, spraying formic acid around. The acid blinds and even-
tually kills the crabs. Since 1995, some twenty million crabs have been killed by the ants, equivalent
to around a quarter of their total population. Though not much more than a tenth of the island has been
invaded by ants, their impact has cascaded through the island's ecosystems. Where the crabs have been
eliminated, many more forest seedlings germinate, and parts of the forest have seen a massive increase
in foliage in the understory.
23
Meanwhile, with fewer crabs to eat them, another long-established invader, the rat-sized African
land snail, is now also on the march across the island.
24
And the crazy ants—so-called because of their
long-legged physique and frenetic behavior—have won an instant place on the global list of the hundred
most invasive species, drawn up by the International Union for the Conservation of Nature.
25
Actually,
it is far from clear that they should be blamed. What caused the super-colonies to explode so suddenly
after the ants had been on the island for so long? Nobody knows.
Ants can travel the world easily and often thrive quite well on arrival—in part no doubt because
they arrive in crowds. One recent study suggested at least six hundred ant species may have made them-
selves at home in alien environments around the world. Notorious arrivals include South American rasp-
berry crazy ants, now swarming inside electrical equipment in the United States, and Argentine ants
(
Linepithema humile
), which have been building super-colonies in Europe.
26
But while invader ant pop-
ulations sometimes boom, they usually bust. The Argentine ant arrived in New Zealand in 1990 and
marched across both the north and south islands in short order, killing chickens, invading orchards, and
pushing aside native ant species. “They can even squirm under the edge of screw top jars and follow the
grooves until they reach the contents,” a government public warning advised. Officials set aside around
$60 million a year to confront the ant army. Then in 2011, most of the known super-colonies collapsed.
Why? Nobody is sure. But the panic is over, and native ants are recovering.
27
Many remote islands have nurtured unique collections of species. The Seychelles in the Indian Ocean,
for instance, have been isolated for more than sixty million years, gathering passing wildlife and turning
the itinerant species into their very own. Aldabra, one of the Seychelles' outer islands, is home to flight-
less birds, like the white-throated rail, and some hundred thousand giant tortoises, two-thirds of the
world's total. Aldabra remains remarkably unaltered, but the native species on many such islands have
suffered. They have nowhere else to go, and the simple island ecosystems are wide open to outsiders
with skills unknown to the naive locals. More than 90 percent of the hundred-plus bird species known to
have become extinct over the past four hundred years were endemic to islands, according to the World
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