Ripples is the
quartlerly newsletter of the Australian Platypus Conservancy. It provides
updates on research in progress and other APC news. Members of Friends of the
Platypus automatically receive each edition
of Ripples.
Ripples Newsletter of the AUSTRALIAN PLATYPUS CONSERVANCY
Issue 21 Summer 2002
COUNTING ON CONDITION
To fuel its active lifestyle, a wild
platypus must consume around 20-30% of its own body weight in food each day,
mainly in the form of aquatic invertebrates.
In other words, an average-sized (1.5
kg) platypus needs to find and capture more than 2 kilograms of small freshwater
organisms (worms, snails, larval mayflies and caddisflies, water bugs and
diving beetles, peashell mussels, etc.) each week to stay healthy and happy.
So what typically happens when waterway
productivity is reduced because of pollution or habitat degradation: do
the platypus living along the waterway simply become skinnier, or do their
numbers decline?
To help answer this question, APC biologists
have for many years routinely assessed the physical condition of all platypus
handled in the course of live-trapping surveys.
Each animal is weighed, and the amount
of fat found in the tail (where a platypus stores up to 60% of its total
body fat) is assessed by squeezing the tail edges gently. The results of
this "squeeze test" are scored using a standard five-point scale developed
by Tom Grant and Frank Carrick in the 1970's: the tail of a very fat platypus
(class 1) will be firm and difficult to bend inwards, while the tail of
a starving animal (class 5) will be soft and limp, with the shape of the
tail bones clearly outlined beneath the skin.
More than a thousand platypus have been
examined by APC researchers since 1989, including animals associated with
lakes and backwaters as well as streams and rivers, and national parks as
well as heavily modified agricultural or urban landscapes. Interestingly,
platypus fat reserves have proven to be remarkably consistent across waterways
and habitat types, with the vast majority of animals (93%) belonging to
tail fat classes 2 or 3.
Of the remainder, 5% were assessed as
being in top condition (tail fat class 1), with roughly equal numbers of
males and females assigned to this category. A disproportionately large
number of juveniles was included among the 2% of all platypus rated as very
thin (class 4) - presumably reflecting their lack of experience in foraging
as compared to adults. Class 5 animals have not yet been encountered in
the wild by the APC.
In turn, these findings suggest that
platypus are remarkably good at matching population size to the amount of
food available in their environment.
This finding is not completely unexpected,
given that platypus are top order predators-as a group, predators tend to
avoid over-crowding by means of social behaviours such as marking their
home range boundaries with scent-backed up by aggression, if necessary.
From a practical point of view, this
means that there is no reason to expect that platypus will ever outstrip
the capacity of their local habitat to sustain them, or otherwise greatly
overpopulate an area.
As well, it provides a scientific basis
for choosing platypus population density as a useful indicator of the relative
productivity of a stream or river-and a way to monitor how waterway health
responds to conservation activities or other forms of habitat change along
the banks or channel.
MACKENZIE RIVER UPDATE
The Mackenzie River arises on the slopes
of the Mount Difficult Range in Grampians National Park and, in theory,
joins the Wimmera River near the town of Horsham.
In practice, a large proportion of the
Mackenzie's water is diverted for irrigation or urban use, with flow along
the natural channel regularly ceasing in summer below the off-take channels
located at the Laharum distribution heads.
In November 1997, the APC undertook
a platypus survey along 25 kilometres of the Mackenzie, sampling sites within
the national park as well as farther downstream on privately owned farmland.Just
three platypus were encountered, including a male and a female recorded
near the park boundary, and a second female captured on a grazing property
not far from Wartook township (about 10 kilometres downstream of the park).
In November 2001, the APC again undertook
platypus survey work along the Mackenzie, with funding provided by the Wimmera
Catchment Management Authority. A number of CMA staff also worked with
Conservancy biologists to set survey nets and check them after dark.
The results confirmed that platypus
continue to reside along the Mackenzie, although at very low density-just
one adult male was captured overnight at a site close to Wartook. Furthermore,
this animal appeared to be unusually old, given his heavily scarred bill
and tail and the fact that both of his spurs were broken off at the base-the
first time that this has been observed by the APC. In recognition of the
male's status as a venerable member of their community, local children who
attended his release early the next morning decided to name him "Old Bill".
Based on the relatively small amount
of survey information available, there is no scientifically valid way to determine
if platypus population size changed along the Mackenzie from 1997 to 2001.
However, it would not be surprising if the number of animals did decline
over this period, given that only a very small environmental flow was allotted
to the Mackenzie River channel downstream of Grampians National Park in the
last three (exceptionally dry) summers.
A third platypus survey will be carried
out along the Mackenzie later this year.
Meanwhile, the Mackenzie findings help
to highlight that platypus conservation on private land can be critically
important even when substantial parts of a river catchment are protected
within a large and carefully managed park.
In the case of the Mackenzie, while
the quality of the river environment in Grampians National Park appears
to be very good, the total amount of platypus habitat reserved within the
park is actually not that great when one considers that the home range of
an adult female typically encompasses at least one kilometre of waterway,
with males claiming even larger areas.
Furthermore, the ability of the Mackenzie's
upper reaches to support platypus may be reduced by a series of spectacular
waterfalls, which are likely to restrict the animals' movements.
Accordingly, the survival of platypus
along the Mackenzie over the longer term is predicted to rely mainly on
boosting the population's numbers through improved river management outside
the national park, in the waterway's middle and lower reaches.
Did You Know That....
A platypus
can reduce its use of oxygen underwater by lowering its heart rate dramatically-from
an average resting rate of 140-150 beats per minute while on the water
surface to sometimes fewer than 2 beats per minute while diving.
NEW PLATYPUS BOOK FOR SCHOOLS
"There Are Platypuses in Our Creek"
is a new text book for primary school students, produced by Barrie Publishing
as part of its Year 4A Australian Series of non-fiction titles.
The authors are Alan Lane and Virginia
King. Alan previously worked as a research scientist for the CSIRO and
participated as an enthusiastic Earthwatch volunteer in a Conservancy platypus
fieldwork expedition in 1999. Virginia's background includes many years
of experience as a primary school teacher and as a writer, with more than
50 books for children and teachers published in Australia and overseas.
As well as presenting a range of factual
information about platypus biology and ecology, the book features sections
about research and platypus habitat requirements which are designed to stimulate
the interest of young students in practical conservation activities.
Alan and Virginia have very generously
decided to donate royalties from sales of the book to the Conservancy's
research and conservation programs.
Copies of "There Are Platypuses in Our
Creek" can be ordered from Barrie Publishing, Suite 304, 89 High Street,
Kew, VIC 3101 or email info@barriepublishing.com.au.