Studies have confirmed that a strong positive relationship exists between the amount of cover provided by shrubs, trees and low-growing plants on creek or river banks and the quality of platypus foraging and burrow habitats (e.g. Ellem et al. 1998, Serena et al. 1998, Worley and Serena 2000, Serena et al. 2001). On average, shrubs or tall grasses grew along 72% of the bank edge in platypus habitats supporting breeding-age females in the upper Wimmera River system in western Victoria (Worley and Serena 2000). Similarly, 70% of burrows used by radio-tagged platypus in Melbourne’s Yarra River occurred at sites where overhanging vegetation grew along more than 40% of the bank edge (Serena et al. 1998).
A self-sustaining corridor of dense riparian vegetation is of enormous value in stabilising banks and preventing erosion. Trees and overhanging shrubs shade the water and keep it cool in summer, thereby improving the availability of dissolved oxygen to macroinvertebrates that in turn are eaten by the platypus. Plants are also a crucial source of organic matter (fallen branches, twigs, leaves, buds and flowers) that helps to fuel the aquatic food web and promotes instream habitat diversity. In the case of urban waterways, a substantial corridor of trees and shrubs also acts to screen out noise and artificial light, trap litter and other pollutants, reduce trampling by humans and restrict access by unsupervised pets.
Platypus foraging activity has been found to be reduced at sites where numerous willows occur, particularly in places where a tough, fibrous layer of willow roots has invaded a large proportion of the channel (Serena et al. 2001). However, it is important that eradication programs for willows or other weedy plants do not contribute to subsequent bank erosion. In addition, platypus survival may be compromised (particularly along narrow creeks) if predation risk increases after cover is reduced by weed control activities.
What can be done to protect the platypus?
- Urban (or other) development should never be allowed to encroach unreasonably close to creeks and rivers where platypus occur. A review by Le Feuvre et al. (2018) concluded that, as a rough rule of thumb, a well-vegetated setback of 30 metres (or more) on both banks is recommended to protect water quality and productive instream processes in relatively small creeks, with wider setbacks needed to support the viability of larger streams or rivers (at least 50 metres on both banks). Even wider setbacks may be required to maintain populations of sensitive aquatic fauna, including many frog species. We agree with Le Feuvre et al. (2018) that the platypus is appropriately deemed to be setback-sensitive, due to the platypus’s high energy/food requirements throughout the year and (particularly in the case of seasonally shallow streams) vulnerability to predators such as foxes. See also Platypus mortality factors.
- Unrestricted livestock access often causes bank habitats to become degraded, sometimes very severely. To protect populations of platypus and other aquatic wildlife (such as fish), creek and river banks should be fenced to restrict their use by livestock, ideally on a permanent year-round basis.
- Willows should be progressively removed from river catchments and replaced with indigenous vegetation to improve overall habitat quality. To minimise disturbance and potential erosion, willow trunks should be cut and hand-painted with herbicide while the root system is left intact to rot naturally; sites where willows have been removed should be replanted as soon as possible with appropriate native trees and shrubs.
- In cases where one or more weedy species dominate both banks of a creek or river for a substantial distance (e.g. 70 metres or more), consideration should be given to staging removal programs so weeds are eliminated from one bank in one year and the opposite bank in a later year. A single-bank approach to weed removal is particularly recommended in places where water depth drops to 30 cm (or less) for part or all of the year, contributing to the risk that a platypus may be targeted successfully by predators in the absence of protective plant cover.
- Herbicides used to control riparian weeds should never be allowed to enter the water, either directly or through storm run-off. Areas of bare soil caused by herbicide use should be planted (or reseeded) as soon as possible with appropriate indigenous species.
- To avoid damaging platypus burrows, use of heavy machinery within about 15 metres of the water’s edge should be avoided whenever possible in platypus habitats. Special care should be taken not to disrupt banks or cause them to become compacted in spring and summer when females are raising their young, particularly in places where platypus are abundant.
Photo: APC
LITERATURE CITED
Ellem B, Bryant A and O’Connor A (1998) Statistical modelling of platypus, Ornithorhynchus anatinus, habitat preferences using generalised linear models. Australian Mammalogy 20, 281-285.
Le Feuvre M, Sutton F and Malony C (2018) Riparian setback widths: a review of recommendations for guidelines. Report to Melbourne Water by Ecology Australia: Fairfield VIC.
Serena M, Thomas JL, Williams GA and Officer RCE (1998) Use of stream and river habitats by the platypus, Ornithorhynchus anatinus, in an urban fringe environment. Australian Journal of Zoology 46, 267-282.
Serena M, Worley M, Swinnerton M and Williams GA (2001) Effect of food availability and habitat on the distribution of platypus (Ornithorhynchus anatinus) foraging activity. Australian Journal of Zoology 49, 263-277.
Worley M and Serena M (2000) Ecology and conservation of platypus in the Wimmera River catchment: IV. Results of habitat studies, summer 1999. Report to Rio Tinto Project Platypus by Australian Platypus Conservancy.