The cane toad (Bufo marinus) is a native of South
America. Australia has no native frogs in the family
Bufo.
Approximately 100 cane toads were introduced from Hawaii to
Gordonvale, north Queensland, in 1935 to control the greyback cane
beetle. Toads soon established as a pest and expanded their
range across the north of Australia at a rate of 27-50 kilometres
per year.
Cane toads now occupy more than 500 000 km2 of
Australia and have reached densities of 2000 toads per hectare in
newly-colonised areas of the Northern Territory. They now
occur down the north coast of New South Wales as far south as
Yamba, with one breeding colony near Lake Innes, near Port
Macquarie. Expansion west into the Northern Territory
occurred in the 1980s, reached the southern end of Kakadu National
Park in 2001, spread across Arnhem Land, the Gulf of Carpentaria
lowlands and the Katherine River watershed. Present
estimates have them between 250 and 300 kilometres east of the
Western Australia border. See Kimberley ToadBusters press
release in the documents module at right.
Computer modelling of climatic variables against toad adaptive
capacities suggest that they may eventually permanently inhabit the
wet coastal areas of the eastern and northern parts of the
continent and south to Broome in Western Australia, and possibly
with isolated populations further south. This would seriously
impact the Kimberley region.
Cane toads are large ground-dwelling amphibians with a dry warty
skin. They have a bony head with bony ridges over their eyes, sit
upright and move in short rapid hops. Their colour ranges
from grey to olive brown and they have a pale belly with dark
irregular spots.
The average size of an adult is 10-15cm long but females have
been recorded growing up to and over 23cm. They breed in still or
slow-flowing water and can tolerate salinity levels up to
15%. Adult cane toads are active at night during the
warm months of the year and can survive temperatures ranging from
5-40 degrees celsius. Cane toad spawn occurs in long
gelatinous strings with double rows of black eggs with females
laying between 8-35 000 eggs at a time. Tadpoles emerge from
water bodies as metamorphs after 6-8 weeks. Cane toads have a life
span of about five years, breed twice a year, and take between 6-18
months to reach sexual maturity.
The cane toad is poisonous in all its life stages, from egg to
adult. Almost anything that eats a toad dies rapidly
from heart failure. The poison is absorbed through body
tissues such as those of the eyes, mouth and nose, so that even
mouthing the toad may cause death.
Cane toads have also been known to transmit diseases such as
salmonella.
Bufo marinus has been listed as a key threatening
process under the Environment Protection and Biodiversity
Conservation Act 1999. The northern quoll (Dasyurus
hallucatus), monitor lizards (Varanus spp),
freshwater crocodile (Crocodylus johnstoni) and some snake
species have been severely impacted because they prey on
frogs. While data are still lacking, early indications
are that toads compete with native terrestrial ground frogs for
habitat and food resources.
The Australian Department of Environment, Water, Heritage and
the Arts reports that:
One study also found that cane toads caused a significant
longer-term reduction in the abundance of small lizards, possibly
by reducing their invertebrate food supply (Catling et al.
1999).
One study undertaken in the western Gulf of Carpentaria,
Northern Territory, reported that the abundance of beetles was
significantly lower, in the short term, in sites colonised by cane
toads compared to cane toad-free sites (Catling et al.
1999). Apart from this study, there have been no studies to
investigate the impact of Cane toads on invertebrate prey
communities in Australia or elsewhere.
A study on ground-nesting rainbow bee-eaters, Merops
ornatus, found that cane toads were one of the most
significant sources of mortality for bee-eater chicks. Cane toads
destroyed 33% of all nest attempts by blocking the nest entrance,
or by eating fledglings and eggs. When cane toads were removed from
the nests and placed up to 1200m away from the nest sites, 75%
returned to the same burrow overnight or within 2 or 3 nights.
Toads were less likely to return to the nest with increasing
release distance. This behaviour may have implications for the
red-cheeked dunnart, which also has its young in ground-level dens.
If cane toads have a memory and use olfactory cues to locate prey
as indicated in this study, there could be implications for peak
breeding times in a number of ground-nesting or denning animals
(Boland 2004).
In northern Australia, competition between cane toads with
native species for food and shelter sites is likely to be highest
near permanent water bodies during the dry season. While predation
and competition effects are suspected, due to the high densities
and dietary ecology of cane toads, these are yet to be sufficiently
confirmed, and it is difficult to adequately justify, at this
stage, a case that these aspects of the threatening process could
cause any native species or ecological community to become eligible
for listing under the Act.
Recent studies in Kakadu National Park have demonstrated that
local extinction of northern quolls is occurring following cane
toad invasion. Over the last two years this work has shown a
significant decline in northern quoll populations with declines
considered to be a direct result of lethal ingestion of cane toad
toxin. Two recent studies are notable: one involving dedicated
surveys of the northern quoll at two sites and another that
undertook broad scale fauna surveys.
Over the next 10 years, the rest of the mainland Top End
population of northern quolls are expected to also disappear,
along with much of the Kimberley mainland population. These
areas are estimated to constitute a further 30% decline in the
species' pre-toad distribution. With the exception of some of
its island locations, an almost total cane toad colonisation of the
northern quolls range is expected.
For more information on the summary of cane toads as a
threatening process, visit
www.environment.gov.au/biodiversity/threatened/ktp/cane-toads.html
What are we doing?
A team at the University of Queensland's Institute for Molecular
Bioscience are investigating the toad's own toxins to try and find
chemical cues which might be used to attract toads for trapping
purposes, or perhaps alarm signals which could be used to deter
these pests from sensitive water bodies. Researchers
are also looking at adapting the 'daughterless' technology being
pioneered in fish to the cane toad. This is a long-term
project aimed at reducing the breeding rate.
Professor Rick Shine's group at the University of Sydney are
investigating parasites that are endemic in our native frogs, but
are showing promising signs of slowing down toad movements by
compromising their health.
See the project links at right.
Our Participant Pestat Pty Ltd.
has developed a spray can product that anaesthatises and then
humanely kills cane toads - HopStop®.
