Throughout history, species invading the
territories of others have fuelled the motor of extinction. And as
human economic activity spreads to every corner of the globe, the
process is speeding up.
After direct habitat destruction invasive species
now pose the second greatest threat to global biodiversity. The
international response to control and eradicate these aliens and
restore the balance in disrupted ecosystems.
Rats in the Seychelles:
The tropical ocean islands were the first
ecosystems to become victim to alien invaders. Rats arrived aboard
ships with the first human visitors to this bountiful paradise -
and they've been competing successfully for the food of the islands
endemic species ever since. The drastic measure of laying poisonous
bait is being applied in order to eradicate rats once and for all
and restore the Seychelle's delicate natural balance of
species.
Hedgehogs of South Uist
The hedgehog is the first documented case of a
species threatening an internationally important bird population.
In 1974, seven hedgehogs were introduced to the Scottish island of
South Uist to control garden slugs. Thirty years on, with no
natural predator, their numbers have exceeded 6,000. With an
appetite for almost anything, the population of hedgehogs has been
expanding on food normally eaten by local bird colonies, causing
the numbers of Dunlin to crash dramatically. The RSPB estimates
that within 20 years the Dunlin could disappear entirely from the
island. Urgent conservation measures are being considered,
including a hedgehog-barricade to halt their progress to other
Hebridean islands. But the only long-term answer is a final
solution of total eradication.
Leafy spurge:
As well as the huge ecological impact of invasive
species there's also the financial impact. In the United States,
alien weeds cost ranchers 144 million dollars a year. Theoretically
one alien species can be eradicated by a carefully selected second
alien species, a process commonly known as 'bio- control'.
In America's North West, rangelands are being
cleared of cattle-toxic leafy spurge by the European flea beetle
which feeds exclusively on the weed. And it seems to be working.
Now ranchers are locking away their shotguns and rifles and taking
up sweep nets in a bid to bio- control this poisonous weed.
Bio-control 'out of control'?:
Today, bio-control species are tested
exhaustively for ecological 'side-effects'. But this wasn't always
the case. Without proper testing, bio-control species could destroy
the ecosystems they were meant to restore. The Eurasian
weevil, introduced to stamp out musk thistle on Nebraska's
prairies, is now attacking native American thistles and out-
competing native insects such as the picture-winged fly.
Global trade in aliens:
It's clear that increased global trade inevitably
raises the risk of transporting organisms to new environments where
they may become invasive. The World Trade Organisation's
agreements to control the spread of 'pests' offer only limited
protection. A country must provide proof that a species is harmful
if they wish to introduce regulations to protect their territory.
With 32 million species in the world, finding out which are going
to be a problem is a tall order and, once they've arrived,
eradicating them is difficult and costly.