The presence of logs, large branches and other woody debris in the water is a very positive habitat feature for platypus populations, contributing to foraging success. Sizable debris piles sometimes also provide these animals with sheltered places to sleep (Serena 1994).
The APC has conducted three studies to assess whether the amount of woody debris present at sites that are intensively used by platypus differs from the amount present at less heavily used sites. In each case, the amount of woody debris was estimated by counting the number of logs and large branches projecting above the water surface during normal base flow.
In the Yarra River catchment, channel segments that were regularly used by radio-tagged platypus contained on average 21 logs and large branches (≥ 20 cm in diameter at the surface) in 100 metres of channel (Running Creek) or 26 logs and large branches (≥ 10 cm in diameter at the surface) in 100 metres of channel (Little Yarra River). Similarly, channel segments along the upper reaches of the Wimmera River in western Victoria that supported reasonably high amounts of platypus usage held on average 22 logs or branches (≥ 20 cm in diameter at the surface) in 100 metres of channel. In contrast, channel segments that were rarely used by platypus held on average 11 logs and large branches (≥ 20 cm in diameter at the surface) (Running Creek), 12 logs and large branches (≥ 10 cm in diameter at the surface) Little Yarra River) or 8 logs or branches (≥ 20 cm in diameter at the surface) (Wimmera River) in 100 metres of channel.
In addition, research has found that adding large pieces of dead wood to the Little Yarra River to address deficiencies in woody habitat resulted in macroinvertebrates rapidly colonising the new material and platypus foraging frequency increasing significantly at sites where wood was deposited (Coleman 2006).
What can be done to protect the platypus?
- Logs and large branches should be retained whenever possible in creeks, rivers and lakes in order to contribute to the quality of platypus habitat.
- Based on the studies described above, consideration should be given to augmenting the amount of instream woody habitat along creeks or small rivers where fewer than 20 logs or sizable branches project on average above the surface in 100 metres of channel during normal base flow. A potentially inexpensive approach may be to cut through fallen trees that bridge the channel or otherwise lie mainly out of the water to submerge more of their volume. To help inform future management, programs that augment woody debris should ideally include a monitoring component to assess environmental costs and benefits.
Photos courtesy of P. Sandercock (above), APC (below)
Literature cited
Coleman R (2006) Ecological and physical responses to large woody debris reintroduction in the Little Yarra River, Victoria, Australia. MSc Thesis, Monash University.
Serena M (1994) Use of time and space by platypus (Ornithorhynchus anatinus: Monotremata) along a Victorian stream. Journal of Zoology 232, 117-131.