As the inexorable march of the cane toads draws closer to the Top
End, Dennis Schulz asks if the animal will wreak havoc on the
wetland systems of the Top End—and in particular Kakadu
National Park. By Dennis Schulz
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This toad was measured at 16.5 cm; impressive, but lead cane toads
that head a feral migration can measure more than 20 cm Photo: Greg
Calvert
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With cane toads on the threshold of Kakadu National Park and
heading towards Darwin, scientific opinion remains divided over
what impact the introduced and highly poisonous species will have
on the Top End environment. While some experts forecast an
ecological holocaust with a wide range of native species facing
drastic declines, others are more sanguine.
“That’s scare-mongering,” charges NT Parks and
Wildlife Commission director, Dr Bill Freeland. Having done over a
decade’s research on cane toad impacts Dr Freeland says toads
cause massive species declines when they first appear but the
numbers of all affected species bounce back within a few seasons.
A just-completed preliminary assessment of the toad’s impact
on Kakadu
1
is uncertain about the long-term effects, simply because so few
studies have been done. A general lack of detailed ecological
information on many of Kakadu’s animals adds to the
uncertainty.
However, the report found that more than 150 predator species were
potentially at risk, some such as the northern quoll at a very high
risk because of its diminishing range.
When the toads do arrive, they will occupy almost all the habitats
within Kakadu National Park, although saline regions and open water
habitats were identified as being of less concern.
As Parks Australia staff at Kakadu National Park are currently
gearing up for the onslaught that’s almost certain to take
place this wet season, Savanna Links talked to the people who have
already experienced the invasion. Pastoralists, Aborigines and
other land managers from western Queensland and the
Territory’s Gulf region can provide an insight as to how this
toxic intruder will impact on the sensitive wetland ecology of the
Top End.
Chris Holt was invaded twice. The owner/manager of Mainoru station
in Arnhem Land is now in her second season of dealing with invading
toads. A decade ago she and husband Malcolm owned Balbarini
station, closer to the Gulf of Carpentaria, where they were
attacked en masse. “They came by the road, not in the creeks
like we had been told they would,” she recalled. “It
was just disgusting. The road was like a moving carpet at night. It
was if they sent in big shock troops because some of the ones at
the front were as big as bread and butter plates. They were
huge.” Ms Holt remembers the initial effects were
devastating. “We found lots of dead freshwater crocodiles in
the shallow water holes in the river. We found dead goannas and had
no snakes at all that year,” she recalled.
She said the tree frogs nearly disappeared, but by the end of the
dry the toads had no more food and the Holts found themselves
surrounded by dead cane toads. They assumed that by the next wet
the size of the toads would be just as large, but they
weren’t—instead they were quite small.
The Holt family, like many others caught in the westward march,
watched as native species managed to survive. “The native
animals seem to learn quite quickly that you can’t eat
them,” explains Ms Holt. “In our experience they had no
long-term effect on the wildlife whatsoever.”
That experience was duplicated on Mallapunyah Springs, where Louise
Martin witnessed their arrival. “When they came we found a
lot less goannas and poisonous snakes . . . (they) pretty much
disappeared,” she remembers, “but the goannas have all
come back. They got used to the toads.”
Many of those living in the Gulf region report how quickly
predatory birds learned to adjust to the poison sacs the cane toads
carry on their backs. “The crows and the kites have learned
to live with them,” says Ms Martin. “They pick the cane
toads up and turn them over and pick their guts out.”
Paul Zlotkowski, owner/manager of Wollongorang station, just inside
the NT border, was introduced to the toads in dramatic fashion.
“We couldn’t understand why all our chooks died,”
recalled Zlotkowski. “We must have had 20 die the first night
and the rest died the next night. They drank the water where toads
laid their eggs. Then the goannas and the green frogs disappeared
and the snakes disappeared. Never saw one snake for years and
years. But it’s all come back to normal now.”
Many of those canvassed who witnessed the initial invasion remarked
on how large and formidable the lead toads were. “When they
first came across they were a fair size but now they’re a lot
smaller. Just about twice the size of the little green tree
frogs,” observed Bill Olive, owner manager of Hell’s
Gate Roadhouse in western Queensland. “They die off by the
thousands when it gets really hot at the end of the dry
season.”
While toads persist everywhere they travel, they are usually only
seen during the night. “We have lights around the motel at
night and they line up in the light like Dad’s Army,”
reported Olive. “They clean up the insects and bugs. You
often wonder what we’d do if we didn’t have the toads
to eat all the bugs.”
Kakadu, however, is not the only national park threatened by the
advance of the toads. Nitmiluk National Park and its world-famous
Katherine Gorge are also in their path; the Eva Valley community of
Manyallaluk are already feeling the toad’s presence. Nick
Dicandilo, advisor to the community reports that they have been
sighted travelling by the hundreds at night on the gravel road just
10 kilometres from the settlement. They are already impacting on
indigenous bush foods.
“There hasn’t been many frill-necked lizards this year
and that’s a huge food source. So people are really
concerned,” said Dicandilo.
While this anecdotal evidence for the survival of native fauna may
seem reassuring, it is unlikely it takes into account many of the
Australian animals that are active at night. A positive outcome is
by no means assured and, as the Kakadu report notes, cane toad
control options are extremely limited and broad-scale control not
possible at this stage.
Reference:
1. van Dam, R.A., Walden D., Begg G., (2000), A preliminary risk
assessment of cane toads in Kakadu National Park, Final Report to
Parks North, Supervising Scientist, Darwin, NT., 89pp.
