273
Since their introduction to
Australia in 1935, cane toads have
out-competed native frogs because
they reproduce far more quickly.support for the enemy release
hypothesis. Garlic mustard also
successfully competes with native
plants for resources, fulfilling the
resource availability hypothesis.
The plant even exudes secondary
compounds that may “attack”
native plants by inhibiting their
germination and growth. This
supports the “novel weapons”
hypothesis, proposed by ecologists
Wendy M. Ridenour and Ragan M.
Callaway in 2004, which posits that
invasive species have biochemical
weapons that give them a key
advantage over native species.The art of control
Successful invasive species are
extremely difficult to control and
almost impossible to eradicate.
If the species is a plant, the most
obvious way to remove it is to
pull it up or cut it down, but such
methods are highly labor-intensive,
especially over a wide area. The
use of chemicals to destroy
invasive species is often
successful, but it can also kill
native species and undermine soil
health, with the added threat of
harm to humans.
One frequently used method
of control, known as biological
control, or “biocontrol,” pits an
invasive species’ own enemies
against it. In an early success,
the cactus moth was introduced
to Australia from South America
in 1926 to feed on the prickly
pear. This plant had itself been
introduced in the 1770s and was
choking farmland in New South
Wales and Queensland. By the
early 1930s, most prickly pears
had been eradicated.Not all biological controls are
effective, and some measures have
had disastrous consequences. For
example, in 1935 cane toads were
introduced to Australia to control
the invasive grayback cane beetle,
which was destroying sugar cane
fields. The cane toad had been
effective in controlling beetles
in Hawaii, so the assumption was
that it would be equally successful
in Australia. However, grayback
cane beetles feed primarily at the
top of sugar cane stalks, which is
out of reach for the cane toads.
A lack of understanding of the
different environments favored
by the two creatures meant that
the cane toad was the wrong
choice as a biological control. By
the time the mistake was realized,
the toad had spread throughout
Australia, poisoning any predator
species that tried to eat the
toxic amphibian.
Even when biological controls
curb an invasive species, they may
create imbalances in ecosystems orTHE HUMAN FACTOR
the economy of local communities.
Regulators are, therefore, often
hesitant to support biological
controls without extensive prior
research. No magic bullet exists
that can control every invasive
species. They are dependent on
complex ecosystem interactions,
and scientists continue to design
field experiments to test their
hypotheses of how invasive
species function in the wild. ■Now is the time to take action.
The costs to habitats and the
economy are ... out of control.
Bruce Babbitt
US Secretary of the Interior ,
(1993 –2001)US_270-273_Invasive_species.indd 273 12/11/18 6:25 PM