Why are drought refuges important to platypus conservation?
Sizable pools have disappeared from many waterways in the platypus’s range due to increased sedimentation, reduced woody debris in the channel and depleted (or unnaturally constant) surface flow. In turn, this undermines the capacity of aquatic systems to support a viable platypus population during drought. A platypus needs to consume a large quantity of aquatic macroinvertebrates each day to remain healthy. Predation risk is also predicted to increase as water levels drop, particularly if platypus are forced to travel across dry land between foraging areas.
In average-to-wet years, platypus drought refuges generally provide exceptionally favourable conditions for foraging, often attracting high levels of use by breeding females. A significant positive relationship between the distribution of platypus foraging activity and the occurrence and/or size of pools has been documented in many different parts of the platypus’s range, including creeks on the Atherton Tableland in Queensland (Milione and Harding 2009), the lower Hastings River (Grant 2004) and upper Macquarie River (Ellem et al. 1998) in New South Wales, and the upper Wimmera River and its tributaries in Victoria (Worley and Serena 2000).
Key attributes of a platypus drought refuge
- It should be large enough to support at least one platypus until flow resumes. The minimum required surface area presumably equates to the average area used by a foraging platypus in a given 24-hour period (around 0.4 hectare in fairly high quality Victorian creeks, though perhaps as little as 0.1 hectare in more productive habitats such as the upper Shoalhaven River). However, smaller pools or backwaters can certainly also contribute collectively to platypus survival, particularly if two or more pools occur reasonably close to one another. As noted above, pools and backwaters of almost any size are expected to promote successful reproduction in average-to-wet years and thereby assist post-drought platypus population recovery.
- It must support a reasonably productive macro-invertebrate community, and ideally retain water at the preferred platypus foraging depth of around 1-5 metres; this depth will also be advantageous in restricting encroachment by vigorous emergent littoral plants.
- It must provide some appropriate habitat for platypus burrows, defined by a section of bank that rises more or less steeply to a height of at least 60 centimetres above the water surface (though ideally more). The water should be deep enough that it doesn’t recede from the bank even during extended drought, and the bank profile where it meets the water should be vertical or concave (not slumping or convex). Protective cover should be present in the form of vegetation overhanging the water along with possible logs, undercut tree roots or large embedded rocks or boulders.
Some examples of platypus drought refuges. Top left: off-stream storage, Black Flats Dam, Tidbinbilla Reserve, ACT. Top middle: on-stream storage, Bakers Gully (near Bright VIC). Top right: on-stream storage, Emerald Golf Resort (Dandenong Ranges, VIC). Bottom: on-stream storage, Belgrave Lake (Belgrave VIC) .
Can off-stream ponds serve as a platypus drought refuge?
Platypus are most likely to use an off-stream pond or lake if it’s located fairly close to a natural creek or river (in our experience, generally within 100 metres and ideally within about 30 metres). Animals are most likely to discover an off-stream pond in the first place if it’s sometimes linked directly linked to the creek or river; this is particularly likely to be true on the Australian mainland where (apart from young juveniles) a platypus rarely spends time wandering across dry land. Animals that learn to access a productive off-stream pond via an intervening gully or channel may well continue to do so even when the connecting habitat no longer holds water. To reduce predation risk, dense low-growing shrubs should be encouraged to grow along the length of the intervening habitat.
How can landowners improve the quality of platypus drought refuges on their own properties?
- To maintain good water quality and prevent burrows being damaged by trampling, install fencing around a refuge pool to exclude livestock. Alternative stock watering points may need to be established.
- The ungrazed strip of land around the water should ideally be wide enough to support shrubs and trees that shade the water, provide cover from predators and provide inputs of leaves, etc. to feed aquatic macroinvertebrates (platypus prey). Appropriate indigenous vegetation should be planted if not already present. To provide cover from predators, shrubby vegetation should also be encouraged to grow between neighbouring pools in a chain of ponds.
- Removing willows at the water’s edge will mean that more dissolved oxygen is available to macroinvertebrates, especially in summer when exposed willow roots avidly consume such oxygen. Willows should be removed with care and any bare soil replanted as soon as possible to avoid bank erosion.
- Consider introducing fallen logs or large dead branches to the refuge pool to provide extra habitat for aquatic macroinvertebrates.
- Aquatic plants can contribute to platypus habitat quality as long as they don’t encroach unduly on the extent of open water available for platypus to swim and dive. Control measures may therefore be warranted to restrict the spread of very vigorous emergent plants at platypus refuge sites.
How can platypus drought refuges be promoted across the landscape?
- Assess the likely contribution made by a sizable pool habitat to local platypus population viability – both as a drought refuge and as a focus for successful breeding – before eliminating it for management reasons.
- Retain small on-stream weirs in places where platypus occur, particularly if these do not pose a significant barrier to fish movement (or can be modified at relatively low cost to restore longitudinal connectivity). If a pool cannot be retained, consider developing a new pool or backwater of comparable size and permanence elsewhere along the creek.
- Identify natural depressions or basins located on public land near creeks or rivers (i.e. sites that are likely to have functioned as natural billabongs or backwaters in the past) that could be rehabilitated at relatively low cost to serve as drought refuges. Improve connectivity of existing billabongs to adjoining natural water courses if this is likely to promote access by platypus on a seasonal or permanent basis.
- Ensure that new man-made water bodies developed near creeks and rivers (e.g. to retain and treat stormwater) are designed with a view to being accessible to and providing additional permanent habitat for platypus and other wildlife.
Photos courtesy of Pete Walsh (at top) and APC (below)
Literature cited
Ellem B A, Bryant A and O’Connor A (1998) Statistical modelling of platypus, Ornithorhynchus anatinus, habitat preferences using generalised linear models. Australian Mammalogy 20, 281-285.
Grant T R (2004) Depth and substrate selection by platypuses, Ornithorhynchus anatinus, in the lower Hastings River, New South Wales. Proceedings of the Linnean Society of New South Wales 125, 235-241.
Milione M and Harding E (2009) Habitat use by platypus (Ornithorhynchus anatinus) in a modified Australian Wet Tropics catchment, north-eastern Queensland. Australian Mammalogy 31, 35-46.
Worley M and Serena M (2000) Ecology and conservation of platypus in the Wimmera River catchment: IV. Results of habitat studies, summer 1999. Report for Rio Tinto Project Platypus. Australian Platypus Conservancy, Whittlesea VIC.