This section highlights issues that are important for platypus conservation:
- Habitat quality
- Water flow (droughts and floods)
- Water quality
- Dams and weirs
- Pumps, pipes and culverts
- Urban development
- Dangerous nets and traps
- Inappropriate angling practices
- Predators and disease
Platypus reproductive success and population density are most likely to be limited by the availability of nutritious food (mainly in the form of aquatic insects and other invertebrates), which in turn is influenced by the quality of in-stream and bank habitats.
A positive relationship has been found to exist between platypus abundance and/or feeding activity and the following habitat features:
- Indigenous trees (eucalypts and wattles) growing on the banks, especially large trees growing right next to the channel (as shown at right)
- The amount of cover provided by shrubs and lower-growing plants, especially those overhanging the water
- Stream and river banks that are both well-consolidated by plant roots and undercut to a depth of 8 centimetres or more
- The occurrence of relatively coarse inorganic bottom materials, such as gravel, pebbles, cobbles and large rocks
- The occurrence of coarse organic matter in the channel, such as fallen logs, branches, bark, twigs and leaves
- Water depth = 1 to 6 metres (ideal depth = 1 to 3 metres)
- The occurrence of sizable pools or backwaters (either natural or man-made) associated with a river or stream
Platypus populations are sometimes found in water bodies that are lined with large numbers of willows. However, two studies by APC researchers have found that a negative relationship exists between the occurrence of willows and the distribution of platypus foraging activity in summer and early autumn. The streams where this research took place were quite small and the willows were quite old and well established, so the channel under the trees was lined by a thick mat of tough, fibrous roots – undoubtedly making it difficult for a platypus to detect and capture prey.
From a platypus’s viewpoint, other negative habitat features are known to include:
- Lack of much vegetation on the banks
- Compacted bare soil on the banks
- Unconsolidated fine sediment (silt or sand) deposited in or next to the channel (indicative of active erosion farther upstream)
- Substantial amounts of bare clay in the channel bed (indicative of active erosion at the site)
In country landscapes, the main factor driving poor platypus habitat quality along streams and rivers is generally insufficient cover by vegetation – particularly along the edges of a stream or river, but also along adjoining gully lines. In many cases, this was originally caused by land clearing but has been maintained or made worse by excessive browsing and grazing, especially by rabbits and/or livestock.
For example, a number of longtime landowners have reported that platypus were often seen by their fathers or grandfathers in the upper reaches of the Avoca River in Victoria up until the First World War, when the number of sightings started dropping noticeably (becoming rare by about 1940). The population’s decline is partly attributed to the war happening halfway around the world: with most of the men gone away to fight, introduced rabbits proliferated and denuded river banks of grasses and other low-growing plant cover. The subsequent erosion caused vast amounts of sediment to clog the channel and fill the deeper pools – and fish and platypus populations dwindled.
Radio-tracking studies have confirmed that platypus burrows may be abandoned soon after cattle have been allowed access to the banks above, possibly because their weight damages burrows. More generally, substantial use of banks by livestock generally results in bare soil increasing and young trees and shrubs being lost through grazing and trampling. Especially in the case of cattle, animals often urinate and defecate in the water, degrading water quality both where this occurs and farther downstream.
In urban landscapes, poor platypus habitat quality is most commonly driven by a combination of vegetation loss and changed drainage patterns caused by unnaturally rapid runoff of rainfall from impermeable surfaces such as roofs and roads, especially when runoff is carried to waterways in concrete drains or underground pipes.
Water flow (drought and floods)
A platypus will starve in the absence of enough surface water in which to feed. However, animals can survive dry periods in an isolated refuge pool, as long as the pool is large enough to provide a reliable food supply in the form of aquatic insects and other invertebrates. If a pool continues to shrink, a platypus will have to weigh up the risk of starving versus the risk of being killed by a predator or dying of heat stress if it chooses to leave the pool and walk across land to find another place to live. Females may suffer more than males when competing for a dwindling food supply, as males are larger and have venomous spurs.
Flow plays an important role in promoting platypus breeding success and hence population survival, as shown in studies conducted along Melbourne streams and the Shoalhaven River in New South Wales:
- In both study areas, there was a significant positive relationship between platypus reproduction and the amount of flow in the five months before mating begins. This is believed to reflect the fact that insufficient flow reduces the platypus’s food supply, causing female fat reserves to decline in the lead-up to breeding.
- In the mainly dry to very dry years from 1997 to 2007, virtually the same proportion of adults died annually in declining platypus populations as compared to those that maintained their numbers near Melbourne. Instead, it was reproductive success that determined population resilience: the number of adults lost annually in stable populations was matched by the number of new juvenile recruits that replaced them.
- The most stable creek population studied near Melbourne (Olinda Creek, in which 16 resident animals were lost and replaced by 17 new residents in the decade from 1997 to 2007) benefited from a highly reliable environmental flow of about 2 megalitres per day throughout the period.
Flooding can harm platypus populations either directly (mainly when animals drown) or indirectly (by damaging their habitat). Although adults are sometimes found dead on the banks after flooding, the main mortality risk is to juveniles occupying burrows that are inundated. Flooding can degrade platypus habitat in a number of ways, especially by eroding banks and depositing sediment in the channel (as shown at right).
Photos courtesy of Ron Cosgrave (above), APC (below)
Platypus abundance near Melbourne has been found to be negatively related to the amount of dissolved nutrients (nitrogen and especially phosphorus) in stream water, as well as the amount of toxic metals (zinc, lead and cadmium) in bottom sediment. In other words, fewer platypus lived in creeks where nutrient and/or metal concentrations were high. This pattern most likely reflects the harmful effects of high nutrient loads and toxic metals on aquatic insects (the platypus’s main food source). In addition, high nutrient loads fuel algal growth that may make it harder for a platypus to detect and capture its small prey. It’s also possible (though not proven) that metals could become directly toxic to platypus after being ingested in food and accumulating in their tissues over time. Similarly, it’s been suggested (but again not proven) that platypus in urban creeks may be adversely affected by eating aquatic insects that contain measurable traces of drugs used to treat diseases such as depression and Alzheimer’s disease.
Platypus abundance in urban streams is also negatively related to the amount of fine sediment carried in the channel after storms (technically, the 90th percentile of total suspended solids). Given that a platypus normally shuts its eyes underwater, this is unlikely to be due to the water being murky. Instead, it’s believed to reflect the fact that suspended solids eventually drop to the channel bottom, and unconsolidated mud and sand provide a poor habitat for most aquatic insects to live. In addition, high levels of suspended sediment can trigger mass downstream migration of aquatic insects, reducing the density of platypus prey by more than 50% in 24 hours.
The electroreceptors used by platypus to navigate underwater and locate their prey presumably function over a limited salinity range and are predicted to perform best in fresh water. Although platypus don’t permanently inhabit highly saline coastal environments, the animals are found in rivers and stream where salinity can sometimes reach 10,000 to 14,000 Electrical Conductivity units (1 EC unit = 1 microsiemens per centimetre). To put this in perspective, horses and sheep can respectively tolerate up to around 9,000 and 16,000 EC units of salt in their drinking water without declining in condition. The salinity of ocean water is typically 51,500 EC units.
Photos courtesy of Sharon Wormleaton (below), APC (above)
Because a platypus mainly feeds on bottom-dwelling insects, much of its time is spent investigating the channel bed where litter tends to accumulate. A platypus also finds it very difficult to remove loops or rings that encircle its neck or body: its front feet end in two broad flaps of skin that make great paddles but are hopeless at gripping or grabbing. The back feet are more dexterous, but can’t remove a loop from around the body except by trying to pull it farther back, thereby making it even more tightly constricting. Any circular piece of litter that accidentally finds its way around a platypus’s head therefore tends to remain around the animal until the loop breaks or the platypus dies – often due to horrific injuries that develop as the litter gradually wears through its skin and underlying tissue.
Live-trapping surveys carried out by the APC from 1998 to 2007 found that nearly 5% of the platypus captured in suburban habitats around Melbourne had one or more loops of plastic, rubber or metal litter encircling their neck or chest. Virtually any sort of rigid or flexible loop with a diameter of up to about 8 centimetres is likely to be a problem if it ends up in the water.
For example, all of the following items have been recovered from animals (some alive, others dead): plastic cable-ties, six-pack holders, both wide and narrow elastic bands, a hospital identification wrist band, canning jar seals, knotted loops of twine, an engine gasket, miscellaneous circular fittings (in one case, apparently from a bicycle headlamp), a short off-cut of PVC pipe, tamper-proof rings from food containers, plastic bangle-type bracelets and elastic hair-ties. In rural areas, discarded loops of nylon fishing line are often the biggest problem.
There’s a lot that people can do to reduce the risk that a platypus dies a slow and horrible death after becoming entangled in litter:
- Pick up litter – particularly anything that looks like it could eventually get caught around a platypus’s bill, neck or body – whether or not it’s found near water.
- Spread the word, particularly to children, that carelessly dropped personal items such as plastic bracelets or elastic hair-ties can have lethal consequences for wildlife. Items dropped on sidewalks or in playgrounds may still end up in platypus habitats after being carried long distances through storm water drains.
- Make it your habit to cut through all metal or plastic rings or loops of any size before you dispose of them – just to be on the safe side.
Photos courtesy of J. Shaw (middle), APC (top and bottom)
Dams and weirs
Pools found behind small on-stream weirs often provide excellent platypus feeding habitat. They can be particularly important to platypus survival by supporting reproduction and serving as drought refuges. In contrast, because a platypus prefers to feed in relatively shallow water (ideally 1-3 metres deep), most of the area within deep water storages is typically not suitable for its use.
Large dams may also cause problems for platypus in other ways:
Firstly, although platypus are surprisingly good at scrambling up banks, they are generally unable to climb vertical (or nearly vertical) concrete structures. Many dams and weir walls are therefore likely to serve as barriers, particularly to dispersing juveniles engaged in long range movements. A platypus can leave the water to bypass such a barrier, but this will increase its exposure to predators and other dangers. The actual amount of risk will depend on how far it has to travel across land, how much protective cover exists along the way, and whether or not it needs to cross a road to reach water again.
Secondly, river regulation fundamentally changes the way a river operates. For example, water is typically retained behind dams through winter in southeast Australia (when rainfall is usually highest) and then released downstream in summer (when irrigation of crops and pasture occurs). The seasonal pattern of high and low flow downstream of a dam is therefore often the exact reverse of what animals have evolved to expect. Depending on the pattern of irrigation demands, the amount of outflow can also fluctuate much more dramatically and erratically than would have been the case historically – or be held unnaturally constant for extended periods of time. Water that’s released from the bottom of a deep reservoir will also be colder than would have been true before the dam was built.
Most fish ladders (as shown at left) that are built to help fish travel around dam walls can also be used by a platypus for the same purpose. River management is also becoming increasingly sophisticated as more becomes known about seasonal flow requirements of platypus and other aquatic species.
The biggest challenge is to ensure that sympathetic and effective management of shared water resources occurs even when surface water is in short supply, during long droughts.
Pumps, pipes and culverts
Given that a platypus normally spends a lot of time in a burrow, it’s not surprising that the animals will travel through pipes and culverts of considerable length. For example, radio-tagged individuals have been found to travel routinely through a 45-metre-long concrete culvert carrying creek water through an embankment. Although a platypus will enter a pipe that’s only 10 centimetres wide, it apparently finds it difficult to back up or turn around in such a confined space and can therefore die if the far end becomes blocked).
Problems can also arise when a culvert or other structure in the channel has a vertical face that forces a platypus to leave the water to continue moving upstream or downstream. While moving across land, it’s by no means uncommon for a platypus to be run over by a car or killed by a fox or other predator.
Cases are also known where a platypus has drowned after becoming wedged in a narrow opening in an irrigation gate or between overlapping wire mesh panels placed across a creek to catch leaves.
A surprisingly high number (8%) of all platypus mortalities reported to the APC from the 1980s to 2009 occurred after an animal was sucked into a mini-hydroelectric generator or irrigation pump (as shown at right). Juveniles and breeding age females appear to be particularly likely to die this way, especially in places where pump sheds are located on elevated banks that provide the best available habitat for platypus nesting burrows in low-lying irrigation districts.
The presence of trees and other vegetation encourages rain to filter into the soil, so it’s only gradually released to streams and rivers. Replacing plant life by hard surfaces (such as roofs and roads) encourages rain to run directly to the nearest stream or river, especially if it travels through a storm water drain or pipe. This rapid runoff promotes channel erosion and typically reduces flow in the channel between storms. It also carries noxious pollutants into waterways, including grease and oil from roads, litter, and toxic metals from sources such as metal roofing (zinc), tire wear (zinc, cadmium) and wear of other car parts (chromium, nickel, copper).
In practice, it’s been found that platypus disappear from urban creeks and rivers if directly connected catchment imperviousness (defined as the proportion of a stream or river catchment area covered by hard surfaces that are drained by sealed storm water drains or pipes) exceeds about 2.2%.
If you live in an urban area, you can help to reduce impacts of urban development on your local waterways in the following ways:
- Install one or more rain tanks to collect water from your house and/or shed roof.
- If possible, use permeable materials (such as loose gravel or porous paving) when developing low maintenance surfaces in your garden.
- If possible, encourage rain that falls on concrete or hard paving to flow towards a garden bed or lawn (rather than to the street or a concrete drain).
Dangerous nets and traps
A platypus can hold its breath for less than three minutes, and therefore easily drowns in a submerged net or trap. Numerous animals can die in this manner if nets or traps are set in a high quality platypus habitat or are abandoned after being set. For example:
At least eight platypus died in two illegal mesh nets that had been strung between dead trees standing in the middle of a large public reservoir located in north-central Victoria (as shown at left). The weight of the large fish also trapped in the nets made it impossible for the platypus to rise to the surface to breathe after becoming entangled.
The remains of 17 platypus were found in a single unlicensed fyke (or eel) net that had been illegally set and then abandoned along a small creek in southwest Victoria (as shown at left). This type of net contains a series of one-way funnels, making it impossible for a platypus to escape. The state of the platypus remains (mainly bones) indicated that the net had been stationed in the creek for many months and possibly years. Animals entering the net when it was submerged during high winter and spring flows would have drowned. Those entering the net in summer and autumn (when the top of the net extended above the water) would have starved to death.
Seven platypus died in an illegal spring trap (as shown at right), which would have been set to capture yabbies or crayfish in the Werribee River in southern Victoria. The animals are believed to have died over a period of about a week (the patchy white appearance of the platypus at the top of the pile is due to fur falling out as its skin starts to decompose), so the trap presumably had been left unattended for the same period. The reason why this trap design is so lethal is that the entrance is fitted with a tapered funnel pointing inwards. Studies have shown that a platypus held inside a trap only searches for an exit in the trap’s outer netting – it doesn’t expect the opening to be located internally within the trap and so fails to find it there.
Opera house yabby traps (as shown at right) are fitted with an internal funnel around each entrance. As in the case of spring traps, this means that a platypus will drown after it enters the trap either by accident or because it’s been attracted by edible yabbies inside. Opera house traps are also known to drown water-rats/rakali (as shown below at left) and freshwater turtles (as shown below at right).
Fortunately, if you want to catch some yabbies to eat at home, there are some effective options for recreational yabbying that pose no risk to air-breathing animals and are also generally legal (though always check local fishing regulations to confirm this is true). Apart from dangling a chunk of meat dangled in the water so yabbies are attracted to it and can be scooped up using a dip net, various types of open-top lift nets can be purchased (some collapsible, others with fixed mesh walls as shown at right). To maximise success, keep in mind that the number of yabbies captured in all types of baited traps generally peaks within 3 to 6 hours of their being set – after that time, the capture rate tends to decline as yabbies leave after eating their fill of bait.
Photos courtesy of P. Gerolemon (paragraph 2), Brad Smith (par. 4), Mal Doreian (par. 5), J. Spirek (par. 6 at left), other photos: APC
Inappropriate angling practices
The vast majority of anglers are highly responsible and would never knowingly set out to harm a platypus. Nonetheless, it is by no means uncommon for a platypus to be injured or even killed by fishing hooks or fishing line.
A loop of discarded nylon fishing line can easily encircle a platypus’s body as the animal searches for food and is almost impossible for the animal to remove due to the structure of its front and back feet. The line will gradually wear through skin and muscle until either the line falls off or the platypus dies. This can take a long time – for example, a dying platypus was discovered on the banks of a Victorian lake with one loop of fishing line around its chest that had actually sawed through the animal’s ribs, opening up its lung cavity.
We’ve also heard of many instances when a platypus has been accidentally hooked by an angler, most often when a hook becomes embedded in the animal’s bill or a front foot as line is reeled in.
The presence of a hook in its bill or foot is a huge and very painful problem for a platypus. The risk to the animal is compounded if the line is cut: we know of numerous cases where a platypus has died after line trailing from a fishing hook became tangled around a submerged branch. The animal then either drowned or died of exhaustion after unsuccessfully trying to break free. We also know of cases where a platypus has died of exhaustion after becoming impaled on a baited hook left overnight in the water to catch fish. If you fish in places where platypus occur, you can do a lot to prevent such tragedies from happening:
- Make it your habit to retrieve all fishing line and lures from the banks and water.
- If you notice that a platypus is feeding nearby while you’re fishing, either stop fishing until the animal moves away or relocate your activities a short distance upstream or downstream.
- If a hook becomes lodged in a platypus, don’t cut the line. Instead, reel the animal in gently and remove the hook (taking great care to avoid the animal’s spurs if it’s an adult male).
- Never leave fishing lines unattended for any length of time. (Apart from the danger it poses to wildlife, this is illegal in most places.)
Photos courtesy of M. Taillard (above), S. Byass (below)
Predators and disease
The earliest evidence that platypus were hunted and eaten by aboriginal Australians consists of bones found in caves occupied between 13,000 and 30,000 years ago.
Carnivorous marsupials (spotted-tailed quolls and Tasmanian devils), white-breasted sea eagles, wedge-tailed eagles, grey goshawks and carpet pythons have all been reported to capture and/or consume a platypus. Given that Australian water-rats/rakali can kill reasonably large waterbirds, it’s possible that they sometimes prey on platypus (particularly small juveniles), although there are no records of this occurring. Circumstantial evidence (the nature of injuries sustained by dead animals) suggests that domesticated or feral house cats may also be responsible for some platypus deaths.
The most significant platypus predators in recent decades are almost certainly domesticated dogs and introduced foxes (which often start by chewing off the head, as shown at left). Dog attacks were identified as the leading cause of platypus mortalities in a Tasmanian study conducted in the 1990s, and combined dog and fox attacks were identified as the second most important mortality factor in a Victorian study carried out by the APC from the 1980s to 2009 (in which 88% of predation-related deaths were deemed to be caused by foxes or dogs, with the rest caused by birds of prey).
While many parasites and microorganisms have been detected in or on a platypus, few are known to cause disease. In 1982, people began noticing that platypus living in parts of Tasmania were starting to develop skin ulcers which resulted in some animals dying. The causative agent was eventually identified to be a fungus, Mucor amphibiorum, which is known to infect frogs and has been identified in Queensland soil samples. It has therefore been suggested that the fungus may have been introduced to Tasmania via infected frogs transported from northern Australia in shipments of tropical fruit. Fortunately, mucormycosis appears to becoming less of a problem over time: the incidence of infection in Tasmanian platypus declined by a factor of four from the 1990s to 2008-2009, suggesting either that animals are becoming more resistant and/or that the fungus is becoming less virulent over time. Interestingly, this disease has never been reported to harm platypus anywhere on the Australian mainland.
As shown at left, the platypus has its own species of tick (known to scientists as Ixodes ornithorhynchi). The ticks are mainly found on the platypus’s lower hind legs and normally don’t seem to harm their host. They do serve to transmit two types of blood parasites, but these again rarely cause illness.
Photos courtesy of Ted Donelan (below), APC (above)