ISSUE 26 - February 2004
PLATYPUS AND POLLUTION
A
number of studies undertaken by the APC along both rural and urban waterways
have shown that the distribution of platypus activity along a waterway—in other
words, where the animals mainly choose to spend their time—is related to the
physical qualities of their habitat.
For
example, platypus consistently favour sites where there are plenty of native
trees and shrubs growing on the banks and organic materials (logs and branches
as well as finer leaves, twigs and bark) accumulated in the channel. In contrast, the animals tend to avoid sites
choked by unconsolidated sand or silt (normally created by erosion of either
the waterway or nearby land surfaces).
To
investigate how pollution may affect the species, researchers working for the
APC and Melbourne Water recently embarked on a collaborative study to see how
the occurrence of platypus populations in the greater Melbourne area varies in
relation to water and sediment contaminants.
A
total of 28 waterway segments (or ‘reaches’) were considered in the analysis.
For each reach, information was available on the local status of platypus
(based on live-trapping surveys), a range of water quality variables (based on
long-term monitoring of urban stream health), and the concentrations of eight
heavy metals found in fine bottom sediment (described by Melbourne Water
ecologist Vincent Pettigrove mainly as part of his ongoing Ph.D. research).
Of the 28 reaches included in
the study (extending from as far west as the Werribee River to as far east as
the Tarago and Lang Lang Rivers), platypus are known to occupy 17 areas and be
absent from 11 areas.
The
results—presented at the international Symposium on Urban Stream Ecology held
in Melbourne last December—strongly support the conclusion that pollutants can
have an adverse effect on platypus populations.
Notably,
sites known to support a breeding platypus population had significantly lower
concentrations of dissolved nutrients (phosphorus and organic nitrogen) in
water and significantly less cadmium, lead and zinc in sediment as compared to
reaches without platypus.
One
of the most common symptoms of an oversupply of dissolved nutrients—the rapid
proliferation of strands and mats of algae—could well interfere with the
platypus’s ability to detect and collect food efficiently in the form of
aquatic insects and other small prey.
Many
aquatic invertebrates are also known to be highly sensitive to the presence of
lead and zinc in sediment, so excessive levels of either metal could result in
less platypus food being locally available.
Clearly,
an enormous amount remains to be learned about the direct and indirect impacts
of various sorts of freshwater contaminants on platypus. In terms of their conservation, a particularly
important priority is to refine our knowledge of the animals’ tolerance of a
range of common pollutants—that is, the maximum level in the environment which
is compatible with a productive, self-sustaining population.
In turn, this information is needed
both to help set targets for improved management of stressed waterways, and
develop the platypus’s potential to serve as a useful indicator of waterway
health.
PEERING INTO THE
PLATYPUS PAST
One
of the most rewarding aspects of the APC’s Platypus Care program has
been the opportunity to record information provided by longtime landowners,
based on their decades of familiarity with local waterways and wildlife.
The
memories of such persons often preserve a treasure trove of detail about the
changing status of platypus populations.
To
our knowledge, only one person can recall ever having seen platypus in the
Wimmera River downstream of Jeparit in western Victoria—the animals were
observed occasionally between the township and Lake Hindmarsh until about 1925,
when the person was 10 years old. The
animals were still abundant enough in the early to mid-1920’s that a boy
actually brought a platypus to school one day after capturing it at the edge of
Jeparit township. The headmaster, very sensibly, made him return the hapless
animal immediately to the river!
In
the Avoca River upstream of Avoca township, platypus are reported to have been
widespread and abundant up until the First World War (based on information
passed from father to son), with the animals continuing to be seen fairly
regularly at a few popular fishing holes until about 1940.
The animals’ decline is attributed to a
change in river structure linked at least in part to events happening halfway around
the world: with the men gone to war, rabbits proliferated and the river banks
were denuded of low-growing plant cover.
The ensuing erosion produced enough sediment to clog the channel and
fill most of the deeper holes—and the numbers of fish as well as platypus
dwindled.
Along
Cardinia Creek (a small, self-contained catchment draining directly into
Western Port, southeast of Melbourne), landholders have reported that platypus
were observed regularly in the stream’s upper reaches until about 20 years ago.
This timing suggests that platypus may
plausibly have disappeared in the wake of the 1983 “Ash Wednesday” bushfire, a
massive firestorm which devastated the Cardinia valley.
Even if relatively few platypus died due to
direct exposure to flames and/or extreme heat, mortality resulting from
increased post-fire predation (following the destruction of vegetation cover on
the creek banks) and other adverse impacts (such as the introduction of
substantial quantities of ash to the channel) may have been enough to extirpate
the local platypus population.
Platypus
Care is a two-year initiative, so
persons can (and are encouraged to) continue providing details of platypus
sightings to the program through the end of 2004. All records are entered into a secure database: to protect human
privacy and ensure that the animals themselves are not disturbed, specific
locality details remain strictly confidential.
Persons
wishing to contribute information about where platypus have been seen—either
recently or in the past—can pick up a reporting form from CMA (Catchment
Management Authority) offices or DSE/DPI information centres across
Victoria.
Platypus
sightings can also be registered either by mailing/faxing a description of when
and where an animal was seen directly to the APC office or by visiting the Platypus
Care website (www.platypus.asn.au).
On behalf of platypus
conservation, particular thanks are due to all of the following for providing
funding to Platypus Care: the State of Victoria, Melbourne Water, and
the Corangamite, Glenelg Hopkins, Goulburn Broken, North Central, North East,
West Gippsland and Wimmera CMA’s.
Did You Know
That....
The platypus’s front feet are equipped with blunt
claws that can be used to dig or remodel a burrow at the rate of around 0.5
metre per hour. Radio-tracking studies
have shown that an adult will most typically occupy several different burrows
(up to about a dozen) over a period of a few weeks.
OUTFOXING THE PREDATORS
The
platypus is believed to have been subject to predation by relatively few kinds
of terrestrial animal at the time of European settlement. Based mainly on information provided by
early naturalists, there is reason to believe that platypus were sometimes
eaten by birds of prey such as eagles,
and large pythons and goannas.
Under
the heading of native aquatic predators on platypus, there is at least one
documented case of a platypus carcass being found in the stomach of a Murray
cod. As well, it has been suggested
that predation by saltwater crocodiles may contribute to the absence of platypus
in Australian tropical waterways (north of about Cooktown).
It
also seems likely that platypus continue to be killed occasionally by
Australian water-rats (Hydromys chrysogaster), given that these highly
carnivorous rodents have been observed preying successfully on waterbirds such
as ducks and grebes.
In
particular, it would not be surprising if water-rats are sometimes responsible
for attacking small juvenile platypus in their natal burrow.
However, the fact that adult females
characteristically seal the tunnel leading to their young with a series of
hard-packed soil plugs presumably is effective in deterring the majority of
would-be intruders. In any event, it is
certainly the case that platypus co-exist with water-rats along many different
waterways, large and small.
Unfortunately,
the introduction of non-native mammals to Australia in the last 200 years
appears to have increased the number of platypus predators significantly.
Strong circumstantial evidence points to the
fact that both cats and dogs will sometimes kill a platypus, particularly near
towns. As well, the European fox is
almost certainly taking a toll on the species in both urban and rural
environments.
An example of the threat
provided by dogs and foxes was seen in December 2003 when a dead female
platypus was recovered about 50 metres from Monbulk Creek at Rowville, in
Melbourne’s southeastern suburbs. Importantly, Monbulk Creek supports the only
reasonably large platypus population surviving in the Dandenong Valley. APC surveys undertaken since 1996 have shown
that about 30 adults occupy this waterway, with most individuals found in the
upper reaches around Belgrave.
To
help boost platypus numbers, work to improve habitat quality along lower
Monbulk Creek has recently accelerated, most notably in association with the
development of the Waterford Valley golf course.
In
this context, the December mortality was particularly disappointing given that
the dead animal was the first female to be found in the lower part of the creek
since substantial habitat restoration work began to be carried out.
By
conducting an autopsy, APC researchers were able to establish that the dead
female’s skull had been crushed. Puncture wounds were also present on the neck,
consistent with having been grabbed and/or carried in the mouth of a fox or
fox-sized dog.
The
female was first recorded in the rehabilitated area in April 2003, when she was
classed as a juvenile (born in the spring or early summer of 2002, presumably
somewhere farther upstream). A Trovan
microchip transponder implanted when the animal was first encountered enabled
the corpse to be identified accurately.
The
young female seems to have done well in the intervening eight months, given
that her weight increased from 930 grams to 1170 grams and she was in excellent
physical condition at the time of her death.
It seems likely that she would have eventually gone on to breed
successfully, thus further boosting platypus numbers along the lower reaches of
the waterway.
Programs
to reduce fox numbers and minimise unsupervised (or poorly supervised) activity
of pets near waterways are both expected to assist platypus conservation.
However,
the most effective long-term solution to the problem of predation may well lie
in eliminating opportunities for foxes or domesticated predators to access the
water’s edge easily. The presence of
dense shrubs and lower growing plants overhanging the banks of rivers, creeks
and ponds provides vital protective cover for a large range of potential prey
species, including platypus.
In addition, the fact that
foxes often use cleared tracks to travel around their home range means that
bicycle routes or walking paths should normally be located at least 15 metres
from the banks of waterways, especially in areas where the water is
characteristically shallow.