Rivers and streams are natural corridors which promote
travel by a wide variety of species, including platypus. The presence of
artificial barriers along waterways can therefore be a problem to animals
if dispersal or migration patterns are restricted, or populations become
split into smaller units that are more prone to extinction than would otherwise
be the case.
Although platypus can walk, they clearly prefer to swim
when travelling from one point to another - a very sensible decision, given
how vulnerable they are to predators when out of the water. Forcing the
animals to leave the water regularly to negotiate a manmade barrier can be
a significant problem for platypus conservation along a waterway.
Culverts and tunnels
Platypus burrows are known to reach a length of 30 metres
(or possibly even 55 metres, based on circumstantial evidence). It is therefore
not too surprising that the animals will move through pipes and culverts.
Platypus have been recorded entering PVC pipes which are only 10 centimetres
in diameter, though some evidence suggests that the animals may not be able
to back up or turn around in such a confined space if it becomes blocked
at one end (for example, by a recently closed valve). In addition, very
strong flows through constricted pipes may well preclude (or at least discourage)
travel by platypus, especially in the upstream direction.
Accordingly, we recommend that pipes associated with
waterways in areas where platypus occur should have a minimum internal diameter
of 20 centimetres if they are accessible to the animals at one or both ends.
As well, project designers should be aware that placing very narrow grilles
or fine mesh barriers across culverts (for example, to keep out unauthorised
persons or filter out litter) may also prevent platypus and other aquatic
species from travelling through the structures.
The maximum length of pipe that a platypus will use
is not known, although a radio-tracking study in the Dandenong Creek catchment
near Melbourne found that the animals regularly move through a 45-metre-long
concrete culvert (1.3 metres in diameter) conveying creek water through
an embankment. To ensure that platypus do not drown while traversing relatively
lengthy pipes or tunnels (particularly ones that are sometimes filled or
nearly filled to capacity after storms) consideration should be given to
incorporating vertical "breathing bays" periodically along their length.
The effect of roads
Studies undertaken along several streams in the Melbourne
suburbs have shown that platypus will make use of culverts conveying water
under busy two-lane roads, with animals recorded to travel below one such
roadway (10.3 metres wide, with a 4.3 metre gap between the bottom of the
bridge and the creek below) while on average up to seven vehicles per minute
were passing overhead. As well, radio-tagged platypus have repeatedly occupied
burrows located within 100 metres of two-lane commuter roads, with the nearest
known burrow site found 40 metres from such a roadway.
The possible effect of multi-lane freeways on platypus
behaviour has not yet been assessed.
Manmade water bodies - features to
be avoided
Vertical concrete or metal surfaces at the exit or entry
point to a water body - whether built as dams or drop structures - are often
extremely difficult (in some cases impossible) for platypus to negotiate
and can represent a real environmental problem in terms of limiting the animals'
movements. The scope of the problem will be progressively greater as the
height of the face (or depth of the drop) increases. Possible strategies
to address this issue are as follows:
* Provide an easier alternative route, preferably carrying
at least a trickle of water throughout the year to bring it to the animals'
attention and with plenty of overhanging vegetation or other cover to reduce
the risk of predation.
* Reduce the angle of the problematic face (and provide
a series of steps or footholds) so animals can scramble up the surface.
Although platypus are surprisingly good climbers, this may not help the
animals significantly if the face is very high and/or very large volumes
of water cascade down the surface.
Manmade water bodies - positive features
for platypus
Platypus are known to make use of a wide range of artificial
water bodies, including water supply reservoirs, flood retarding basins,
recreational lakes and farm dams. Such manmade habitats are most likely
to be used by platypus if there is a suitable channel or pipe (holding at
least a few centimetres of water throughout most or all of the year) linking
the reservoir or pond to the nearest natural river or creek. Without such
a connection, the animals may never discover the new habitat in the first
place.
It is believed that platypus do not regularly forage
in water more than about five metres deep - at greater depths, conditions
are generally not appropriate to support many bottom-dwelling invertebrates
for them to feed on. As well, because platypus are naturally buoyant, they
need to expend considerable energy to dive deeply. However, as long as a
manmade water body includes some shallower areas (preferably with a range
of depths to create a diversity of aquatic habitats) it may offer good platypus
feeding habitat.
Along with many other freshwater species, platypus are
expected to benefit above all from the presence of substantial amounts of
indigenous vegetation growing at the edge of a manmade water body, both
on the bank (trees, shrubs and ground cover plants) and in the water (rushes,
sedges and other species). In the case of water bodies which are largely
surrounded by grassy lawns in parks or other public reserves, consideration
should be given to replacing introduced grasses with native grasses around
at least half the perimeter of the lake or pond and leaving these areas
unmown. As discussed elsewhere in this document, the presence of native
vegetation along the bank will both provide platypus with protective cover
and contribute to the productivity and health of their aquatic environment.
Encouraging the presence of natural organic materials
in a new lake or pond - for example, by not removing trees and shrubs that
are "drowned" by rising water - will increase habitat diversity (which is
often in short supply in manmade water bodies) and provide decaying matter
to be eaten by invertebrates.
The presence of one or more islands in an artificial
lake is also expected to be a positive environmental feature for platypus,
providing extra "edge" habitat as well as protected areas for burrows.