Charles Darwin University, Darwin
Ron Firth
Background | Aims | Habitat | Shelter
Requirements | Movements | Population | Diet |
Other
Top End populations | References |
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PhD student Ron Firth tracks down an elusive
Brush-tailed Rabbit-rat (Conilurus penicillatus ) on
NT’s Cobourg Peninsula. Below, the brush-tailed rabbit
rat.
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Photos: Ron Firth
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The Brush-tailed Rabbit-rat ( Conilurus penicillatus ) is
the only extant member of its genus: its sole congener Conilurus
albipes became extinct at the end of the 19th century before any
but the most superficial of studies could be carried out on its
ecology (Taylor and Horner 1971). There are three recognised
subspecies of the Brush-tailed Rabbit-rat; C. p. randi (New
Guinea), C. p. melibius (Bathurst and Melville Is) and C. p.
penicillatus (Top End and Kimberley) (Kemper and Schmitt 1992).
In all, Australia has lost 17 species of mammals in the last 200
years since European colonisation (Woinarski and Braithwaite 1990;
Smith and Quin 1996). Marsupials and rodents have suffered the
highest rates of extinction (Short and Smith 1994; Braithwaite and
Griffiths 1996), and especially so for those in the critical weight
range, 35.0 g to 5500 g (Burbidge and McKenzie 1989).
Weighing around 150g, the Brush-tailed Rabbit-rat falls clearly
within the critical weight range. The limited evidence suggests
that the Brush-tailed Rabbit-rat is in decline in the Top End. It
has not been trapped in the Alligator Rivers Region or mainland
Arnhem Land for at least a decade (Woinarski et al. 2001). Early
reports by naturalists such as Dahl (1897) and Collett (1897) noted
that Conilurus penicillatus was common all over Arnhem Land.
Tunney collected 40 individuals in 1902-03 (Thomas 1904). In the
Top End it can still be found on Bathurst and Melville Islands
(Tiwi Islands), and is patchily common on Inglis Island off
Northeast Arnhem land (Woinarski et al. 1999b). The only known
mainland population occurs on Cobourg Peninsula, where Frith and
Calaby (1974) found it to be common in the Port Essington, Port
Bremer area, as did Taylor and Horner (1971).
There is little known regarding the ecology of the Brush-tailed
Rabbit-rat (Bradley et al. 1987; Friend et al. 1992). Its decline
from its core mainland range to promontories and islands suggests a
successional pattern of extinction. Consequently some long-term
ecological information on this species is vital so that management
plans and strategies can be devised and implemented to prevent
further decline and possible extinction. This project aims to
provide information concerning habitat preference/ use, shelter
requirements, movements, population dynamics, life history
parameters, diet and the possible causes of decline. Due to its
relative accessibility and large population of Rabbit-rats, the
study is mainly being conducted at Garig Gunak Barlu National Park
(Cobourg Peninsula) and at the Mardugal campground in Kakadu
National Park.
On Cobourg Rabbit-rats appear to use an array of habitats from
tall eucalypt woodlands dominated by Eucalyptus miniata, E.
tetrodonta and E. nesophila with a sparse understorey
through to Casuarina equisetifolia beach dunes. There also
seems to be an association with perennial grasses from genera such
as Eriachne and Chrysopogon and animals appear to be
absent from areas with predominantly annual grasses such as annual
sorgum.
Radio-tracking has shown that animals spend the majority of time
foraging on the ground, only returning to trees when threatened or
to nest during the day. Some preliminary habitat modelling of a
large distributional data set from Melville Island indicates a
correlation with tall eucalypt forests.
Radio-tracking Rabbit-rats has revealed that most animals
shelter during the day in tree hollows and hollow logs. Most of the
nest trees have DBHs (diameter at breast height) that are on
average greater than 30 cm. Species of trees used to nest in
include Eucalyptus miniata, E. tetrodonta, E. nesophila, E.
bleeseri, E. porrecta, Erythrophleum chlorostachys and dead
trees. Hollow logs are also used as dens with animals denning in
hollow logs disproportionately more than was randomly available to
them. Rabbit-rats also prefer trees that are significantly wider
than the majority of trees available to them. Most animals use more
than one nest site over periods of a week.
The movements of animals are monitored in two ways:
- By looking at the captures and recaptures on the trapping grids
(the trapping grids consist of 20 trap lines with 20 traps in each
line and spaced 20m apart, so that a grid consists of 400 traps
approximately 380m x 380m).
- By radio-tracking animals at night and locating their nest
sites during the day. The radio-tracking data from 41 animals is so
far demon strating that animals are relatively sedentary with home
ranges on average being 0.77 hectares and ranging from 0.10 to 4.40
hectares. Males also have significantly larger home ranges than
females, with the largest male home range being over four times as
large as the largest females home range.
Population information is being provided by mark-recapture data
from the trapping grids. So far population estimates for both of
the trapping grids are around 100 animals per grid, which gives a
density of approximately six animals per hectare.
Faecal samples so far analysed macroscopically indicate that the
animals primarily eat plant stems, however a substantial proportion
of the faecal samples also contained grit. Animals often appear to
be foraging on bare ground when they are being observed, which can
account for the presence of grit in their faecal samples. Some of
the scats also contained root material and small amounts of
invertebrate exoskeleton fragments. I have also observed one
Rabbit-rat eating a relatively large spider, a grasshopper and some
herbaceous species.
I was also part of a preliminary survey of Croker Island that
failed to trap any Rabbit-rats.
However, Rabbit-rats have recently been re-discovered in Kakadu
National Park. In 2001, Pete Christopherson and Sandra McGregor,
local Aboriginal residents of Kakadu, reported their observations
of unusual looking rats at the Mardugal campground to Scientist, Dr
John Woinarski. At Peter and Sandra's suggestion, John's crew
conducted a trapping survey at Mardugal and confirmed that the
unusual rats were the Rabbit-rat. Subsequently, in July 2001,
Michelle Watson and myself trapped another 13 Rabbit-rats at the
Mardugal campground, confirming that a substantial population did
live in the area.
And this year, in conjunction with Cobourg, I am also conducting
parallel studies in Kakadu National Park. Results so far are
indicating that the population is a relatively small and isolated
one.
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