INVASIONS by alien species鈥攁lready one of the most serious threats to
biodiversity worldwide鈥攁re set to worsen in the next few decades as the
world continues to warm.
鈥淲ithout any question, global change is going to exacerbate the problem,鈥�
says Harold Mooney, an ecologist at Stanford University who chaired an
international workshop on invading species and global change, held last week in
San Mateo, California. However, experts warn that too little research has been
done to predict exactly what will happen.
The outcome will depend on regional variations in climate
change鈥攕omething that climatologists still cannot predict with any
confidence. But almost everyone agrees that most regions of Earth will get
warmer, and this is likely to lead to more species invasions.
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鈥淚t opens up the door to new exotic species that were totally excluded from
the environment,鈥� says James Carlton, a marine biologist at Williams College in
Mystic, Connecticut. The tropical alga Caulerpa taxifolia, for example,
which invaded the Mediterranean Sea in the mid-1980s, could move up the Atlantic
coast of Europe if ocean temperatures rise, he says.
On land, a 3 掳C rise in average temperature could be enough to allow the
Mediterranean fruit fly (Ceratitis capitata), a major pest of citrus
crops, to spread northwards into northern Europe, says Robert Sutherst, an
entomologist at the University of Queensland in Brisbane.
Warming could also cause problems among species that have been intentionally
introduced. British oyster farmers, for example, grow Japanese oysters (
Crassostrea gigas) in coastal waters, but the species cannot reproduce
because the water is too cold. Warmer water could transform it into a pest, as
it already is in Australia. 鈥淚 suspect it would only take something like 2 or 3
degrees,鈥� says Susan Utting, a biologist at the Centre for Environment Fisheries
and Aquaculture Science鈥檚 lab in Conwy, Wales.
Climate change may also have more subtle effects. A longer growing season may
give some invading weedy plants time to flower and set seed where previously
they could only spread asexually. This new-found ability could allow the weeds
to adapt more quickly to combat insects that eat them. 鈥淭his could well make
biological control a lot more difficult on perennial species,鈥� says Spencer
Barrett, a botanist at the University of Toronto.
Rising levels of carbon dioxide in the atmosphere鈥攖he main contributor
to global warming鈥攕hould favour plant species that are physiologically
able to take advantage of the extra CO2 to make more sugar and
therefore grow faster. One such species, at least in laboratory experiments, is
cheatgrass or Bromus tectorum, an introduced species that now dominates
vast areas of the American West. 鈥淭hat鈥檚 kind of ominous,鈥� says Jeff Dukes, a
biologist at Stanford who studies how elevated CO2 affects plants.
Participants say this month鈥檚 workshop was the first time researchers have
pulled together so many threads to give a broad picture of how global change may
affect invading species. So far, their work has produced more questions than
answers. 鈥淲e are really entering a new field here,鈥� says Carlton.