The importance of native plants on
the banks
Recent studies have repeatedly found that the presence
of platypus is positively related to the amount of indigenous vegetation
growing along a waterway. For example, a radio-tracking study along Running
Creek near Melbourne (a small stream mainly bordered by either native forest
or open cattle paddocks) found that each 50-metre section of waterway in
the areas used by platypus supported on average 22 medium-to-large trees
growing within 8 metres of the water, of which 82% were eucalypts or acacias
(with the rest being introduced poplars or willows). In contrast, the same
amount of bank in the areas not used by platypus supported fewer than 9 trees on average, of
which only 21% were eucalypts or acacias (with 70% being willows). In the
Wimmera River catchment in western Victoria, waterways supporting reasonable
numbers of platypus had on average 6-10 medium-to-large river red gums (Eucalyptus camaldulensis) growing
on each 50-metre segment of the banks (within 20 metres of the water) as compared
to just 3-4 medium-to-large gum trees per 50-metre segment in habitats supporting
few or no platypus.
Both trees and understorey plants (shrubs, herbs and
grasses) are constantly shedding leaves, stems, bark, fruit and buds. On
land, this contributes to a mulch which helps to protect the soil from erosion.
In the water, it provides food for a diverse (and from the viewpoint of platypus,
delectable) group of aquatic invertebrates which are adapted to eat dead
plant material.
By controlling bank erosion, trees and smaller plants
benefit platypus by providing stable and protected sites for their burrows.
As well, inputs of sediment can damage the platypus food supply by smothering
invertebrates and eliminating their habitat when pools become choked with
silt, burying rocks and woody debris. Along with shrubs overhanging the
water, trees also provide shade in summer which helps to keep the water cool
and thereby improves the availability of dissolved oxygen to animals that
rely on gills to breathe (aquatic invertebrates as well as tadpoles and fish).
To improve the quality of platypus habitat, it is essential
that indigenous vegetation be planted or encouraged to regenerate in areas
where such plants have been lost. As well, weeds should be controlled along
waterways whenever possible and a substantial layer of ground litter be
allowed to develop. Along with helping platypus, these actions will also
improve water quality and assist the survival and reproduction of native
fish and other desirable aquatic species.
Removing willows
While some species of introduced willows remain confined
to Australian gardens, others have spread aggressively along the banks of
waterways, displacing native plants and progressively choking channels with
their roots. Several studies have shown that the amount of oxygen present
in the water under willows in summer is often much lower than the amount
of dissolved oxygen available under eucalypts - creating unsuitable conditions
for most fish as well as many of the small invertebrates eaten by platypus.
As well, research on platypus foraging behaviour suggests that platypus
may avoid feeding under willows because it's difficult to capture insects
and other small prey within the tough, tangled fibres of willow root mats.
In practice, the impact of both factors is predicted to depend on how much
of the channel bed has been colonised by willow roots: platypus habitat
along small streams may tend to be more badly degraded by willows as compared
to wider, deeper waterways where the central channel remains free of roots.
Studies carried out near Melbourne have shown that the
process of removing willows from stream and river banks does not disrupt
platypus populations over the short term - at least in cases where the trees
are first poisoned and then cut down, and the roots left in the bank to rot
gradually. Radio-tagged platypus remained resident in an area where willows
were being removed by this method and continued to use burrows located next
to willow stumps. As well, platypus reproductive success was not compromised
in the breeding season following willow removal works. However, it is important
to note that some native trees (mainly acacias) were found along this waterway
to provide a measure of cover and shade after willows were eradicated.
In situations where willows are the only (or nearly the only) woody plants
growing on the banks, special care should be taken to ensure that indigenous
trees and shrubs are planted as soon as possible after willows are removed.
(See also "Planning Work along Waterways".)
The value of snags, stumps and woody
debris
People sometimes mistakenly believe that platypus have
trouble finding their way through tangles of woody debris in the water.
In fact, platypus are remarkably good at slipping through such natural obstacles
- the animals will sometimes even spend the day sleeping inside a pile of
branches and leaves accumulated at the edge of the channel.
Radio-tracking research undertaken along Running Creek
near Melbourne has shown that the parts of the stream used by platypus contained
on average 11 emergent logs or large branches (20 centimetres or more in
diameter) in each 50-metre segment of waterway - as opposed to only 5 such
logs or branches in 50-metre segments of waterway that were not used by the animals. Similarly,
platypus habitat studies carried out by the APC in the Wimmera River catchment
in western Victoria found that 11-19 submerged logs or large branches were
present on average in each 50-metre segment of three different waterways supporting
platypus. The strong positive association of platypus with woody debris
makes sense because logs and fallen branches provide important feeding and
breeding habitat for many of the aquatic invertebrates eaten by platypus,
and also help to retain smaller pieces of invertebrate-friendly organic matter
(such as dead leaves, twigs, and bark) which would otherwise tend to drift
downstream. Large woody debris also provides important places for invertebrates
to shelter during floods, and promotes habitat diversity by maintaining scour
pools immediately downstream of the dead wood. In short, logs and large branches
lying in a waterway should be welcomed by landowners as a very positive environmental
feature - and generally only be relocated if they represent a problem for
human safety (for example, at a popular swimming hole) or specifically threaten
a valuable structure like a bridge or dock.