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Northern quoll at the Mary
River study site , February 2002. This quoll population is now
extinct. Photo: Katrina Reid
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Northern quoll Dasyurus
hallucatus
Quolls are rabbit-sized marsupial
carnivores, found only in Australia and New Guinea. There are four
Australian species, and two species restricted to New Guinea. All
Australian species have declined considerably, and three (the
eastern quoll D. viverrinus , the western quoll D.
geoffroii and the spotted-tailed quoll D. maculatus )
are listed as nationally threatened.
The northern quoll is the smallest of quolls,
with a maximum weight of 1.2 kg. It shelters during the day in rock
crevices, tree hollows, logs or termite mounds and forages at
night, both in trees and on the ground, for invertebrates,
vertebrates and fruit. Northern quolls have an annual highly
synchronised mating season; shortly after mating, all the males in
the population die off. Home range sizes vary from 35 ha (in
females) to over 1 km 2 for males in savanna
woodland.
— John Woinarski & Meri
Oakwood
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View with quoll den in
foreground, Mary River district, Northern Territory. Quolls
frequently den in rocky boulder piles, often at the highest points
of hills or outliers. Photo: Meri Oakwood
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Surveys of quoll populations reveal grim news
as cane toads invade Kakadu, writes Meri Oakwood
Profile of the
northern quoll | Study sites | Toads the cause? | References | More information
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AS their range has expanded into the Northern Territory,
there has been considerable debate over what impact cane toads will
have on the native fauna (See: ‘The cane toad dialogues:
disaster or disruption’, Savanna Links, 16, p. 1).
Some experts, such as Professor Mike Tyler from the University of
Adelaide, forecast an ecological disaster affecting a range of
predatory species.
Other scientists, such as Dr Bill Freeland, former head of the
NT Parks and Wildlife Commission, believe that although species may
suffer massive declines initially, they will bounce back within a
few seasons. Unfortunately, in the areas of Queensland that
experienced toad invasion years ago, no methodical scientific
monitoring was conducted. We only have a couple of field studies in
the Gulf country 1,2 and anecdotal stories from
Queensland on which to base predictions.
Cane toads reached Kakadu in 2001, and were first recorded in
the south-east at the junction of Gimbat Creek and a creek from Mt
Evelyn in April of that year. Since then they have spread
northwards and westwards across the Park with sightings now
occurring in Jabiru.
There have been several projects instigated in the Park to
monitor the effect of cane toads on the native fauna. Professor
Gordon Grigg (University of Qld), Dr Andrew Taylor (UNSW) and
Hamish McCallum (UQ) installed audio monitors at six sites to
determine whether the composition of the native frog community
changes. Dr Dan Holland has radio-tracked the two largest species
of woodland goannas, Varanus panoptes and V. gouldii.
Michelle Watson (Charles Darwin University) has been conducting
broad-scale fauna surveys before and after toad invasion. I have
been monitoring two populations of northern quolls, Dasyurus
hallucatus , considered to be a high-risk species of
carnivorous mammal.
Northern quolls are opportunistic predators, consuming anything
that moves that is within a size range that they can manage. They
eat several species of native frogs 3 and cane toads are
easy prey. Unfortunately, all the quoll has to do is mouth the toad
to cause it to exude poison from its parotoid glands (the swellings
on each shoulder behind the eardrum). The poison is then ingested
by the quoll. An individual of a closely related species of quoll
was observed mouthing a toad and dying ten minutes later
4 . Some social native species, such as crows, have been
reported to be able to consume toads safely by turning them over
and only devouring the stomach region. Unfortunately, northern
quolls are solitary hunters 5 , so there is little
chance that they will learn safe toad-consuming behaviour by
observation of other individuals.
The quoll/cane toad project has two study sites: one in southern
Kakadu (Mary River District) and one in northern Kakadu (East
Alligator District) 6 . Trapping, radio-tracking and
post-mortem examinations are being used to monitor the quolls and
road surveys are conducted to monitor the progress of the toads.
The cane toads reached the Mary River District in December 2001 and
were first recorded at the study site in February 2002.
Immediately some of the radio-tracked quolls were found dead of
apparent toad-poisoning. However, the wet season is normally a time
of high quoll abundance and high mortality as the juveniles become
independent and compete for the limited number of territories, so
these deaths had little impact on the population as a whole.
The dry season arrived, the toads became less obvious as they
retreated to shelter sites near water and the quoll population
followed the usual pattern of slight decline as the dry season
progressed. The mating season was normal, all of the females had
pouch young and by October, the young were kept in nursery dens. In
October the population was similar to that of the previous year, at
its lowest abundance for the year. But then the rains began, the
toads began dispersing again and it appears that as mothers died
from poisoning, whole litters of young starved to death. By
December, the population had crashed with only three individuals
detected during trapping. In January 2003, there were still only
three. In March, there were none. No quolls were caught in May and
July. Toads are still increasing in numbers.
Are toads the cause?
How can we be sure that this sudden decline is due to toads? To
begin with, quolls that appeared to have been poisoned by toads,
began dying the same month and in the same area that toads were
first observed at the site. These individual quolls had been
monitored intensively, being trapped every two weeks, and they were
healthy with no disease and no heavy parasite infestations. At
death, they had no signs of predator damage or accidental injury,
the only unusual sign being red irritation on the lips in some
animals. There were no obvious changes in the habitat at the time
except for the arrival of the toads. The most compelling evidence
is that the toad-free East Alligator area still has a
super-abundant quoll population.
The quolls at East Alligator are currently being intensively
monitored in preparation for the cane toad invasion, which may
occur this coming wet season. We expect that the pattern of decline
will probably be similar. This is very sad as East Alligator has
the highest density of quolls that I have ever observed in 12 years
of working on this species.
This massive decline of quolls to the point of local extinction
was also observed in Michelle Watson’s study in Kakadu which
found that quolls were not recorded at all in quadrats invaded by
toads, though they had been present the year before 7 .
So, it certainly appears that in the short-term at least, the
arrival of the toads has been disastrous for the northern quoll.
Quolls were already declining throughout the Top End and the toad
has certainly hastened the process. So, will numbers bounce back as
predicted by Dr Freeland? Only time will tell.
We will re-trap at Mary River at least once more this year, and
hope to continue to monitor the site (and East Alligator)
throughout 2004. The only way to address the question of long-term
impact, is to conduct methodical long-term monitoring, so hopefully
we will be able to revisit these sites at least once a year until
about 2010.
1. Freeland, WJ, & Kerin, S H 1988, ‘Within-habitat
relationships between invading Bufo marinus and Australian
species of frog during the tropical dry season’,
Australian Wildlife Research 15, 293–305.
2. Catling, PC, Hertog, A, Burt, RJ, Wombey, JC, &
Forrester, R ‘The short-term effect of cane toads ( Bufo
marinus ) on native fauna in the Gulf Country of the Northern
Territory’, Wildlife Research 26, 161–185.
3. Oakwood, M 1997, The ecology of the northern quoll,
Dasyurus hallucatus , PhD thesis, Australian National
University.
4. Prof. M Archer, Director of the Australian Museum, pers.
comm.
5. Oakwood, M, ‘Spatial and social organisation of a
carnivorous marsupial, Dasyurus hallucatus (Marsupialia:
Dasyuridae)’, Journal of Zoology, London 257,
237–248.
6. Oakwood, M 2003, ‘The effect of cane toads on a
marsupial carnivore, the northern quoll, Dasyurus hallucatus
’, Report to Parks Australia North.
7. Watson, M., and Woinarski, J. 2003.
Vertebrate monitoring and re-sampling in Kakadu National Park,
2002. Report to Parks Australia North.
