Major threat: predation

The decline and critically endangered status of western ringtail possums is most often attributed to predation. There is no doubt that this was the main reason for mortality in monitored radio-collared ringtails.
Hand-reared animals with no predator awareness that grew up in a safe environment close to humans and are then transferred to completely new and unsafe surroundings are consequently highly at risk. Animals inexperienced in even recognising danger will fail to react or defend themselves effectively.

All sites in which predators are present can be expected to suffer some losses especially in the establishment phase. Effective predator control of cats as well as foxes would be ideal, however even in funded and monitored relocations, the level of control was insufficient. (Clarke, 2011)
Native predators of western ringtail possums – e.g. pythons, raptors, owls, chuditch, ravens – cannot be controlled and might already set the predation threshold the species can tolerate without succumbing on a population level. Complete or near complete exclusion of exotic predators would only be achievable in a fenced sanctuary, but breaches by cats or foxes would then have the most devastating effects. 

If eradication is unrealistic, reduction seems the only option. However the benefits of isolated baiting are likely to be fleeting. Any fox removed by baiting might be quickly replaced by foxes from surrounding unbaited areas. (Bengsen, 2014)
Longer-lasting and more widespread fox population reductions would need a coordinated approach from landowners over large areas – a very difficult task if no financial incentives are provided. 

Landowners’ interest in predator control is highly dependent on their use of the property. Sheep are fox prey and farmers running sheep are usually amenable to control efforts. Foxes also eat grapes which would entice grape growers to engage in control programs, however most cattle farmers show no interest in predator control and those who just enjoy a rural lifestyle are also hard to convince of the necessity.  The cruelty of killing through poison bait and the low reported success rate add to the list of disincentives.

Rabbits, when plentiful, are one of the most common prey of both foxes and cats and large rabbit numbers will attract large predator numbers and/or lead to increased breeding of predators.

One of our cameras monitors a section with high rabbit numbers and it confirms high fox numbers but we were never able to actually watch a fox catch a rabbit.

If foxes and cats cannot be adequately controlled, the question is whether a severe reduction in rabbits might not just lead to prey switching and increased predation of the species we want to reintroduce and protect. However, western ringtail possums might be more naïve and therefore easier to catch than rabbits, in which case a large number of rabbits would only lead to higher fox numbers without any benefit for the ringtails. Ringtails are ‘protected’ to a degree through their arboral nature but particularly in areas with low canopy connectivity, they tend to come to the ground far more often than we would like to see.

In addition to providing food, rabbits also build warrens that when big enough are occasionally used by feral cats and foxes for shelter (Moseby et al, 2009), however in a hot summer they might also be used by ringtail possums. Ripping warrens would then not only remove a refuge for predators but also for heat intolerant ringtails. I would suggest never to release ringtails (or actually any kind of possums) in an area with high rabbit numbers

In general, rabbits and foxes should be controlled simultaneously and the control efforts should be implemented well before the threatened species is reintroduced.

On our release sites mainly buried eggs are used for baiting but occasionally meat baits are hung up in positions so that smaller reptiles would (hopefully) not get to them.
How effective these meat baits are is hard to judge as our footage either shows foxes that completely ignore the bait or they even sniff it to then walk away. Whenever a bait was taken, the camera failed to capture the taker. However, some photo sequences hinted at brushtail possums.

Cats are even harder to control and only intensive and persistent control actions would keep cat numbers low. During low rainfall years trapping and baiting is usually more effective but still a constant uphill battle. 
Shooting has a possible safety risk for humans, is difficult on a site with abundant shelter for cats and because of  their inherent wariness. (Short, 2016)
If fox control is fairly successful, cat numbers might increase.

We have established a thriving colony of around 50 animals on one of our release sites. Predator control is as intensive as landowners can manage. Trapping, baiting, shooting are all highly labour intensive and costly activities. Without support measures predator control can only be  patchy and limited to the actual property. Upsettingly, fox incursion is high from an adjacent national park but several attempts to convince the relevant department to bait the area were unsuccessful. 

Regarding foxes, traps are probably the most unsuccessful measures. There are big traps on the market designed to catch foxes, however the closest we came to an actual catch so far was by a small trap meant for cats.

Traps also attracted skinks, bobtails, phascogale, brushtail possums, neighbour’s dogs and a cat. The visit by a ringtail possum was supposedly unrelated to the offering in the trap.

 

An article (Short, 2016) about the reintroduction of bandicoots which failed in the longer term due to cat predation provides a dire warning. 
After reintroduction bandicoots established and spread well even with a low number of predators around.
However, numbers fluctuated greatly and even after peaking at about 250 animals, they declined to apparent local extinction when fox baiting intensity was gradually reduced because of various logistic difficulties or funding shortfalls. When cats also escaped control efforts their numbers grew substantially and bandicoot numbers declined quickly.
Releases of more animals led to another even higher peak (467) but after just 8 months only 4 survivors were detected. Predation by feral cats was named as the primary cause of the local extinction.
As cats can climb trees, arboreal species such as ringtails might not have a significant advantage compared to bandicoots.

Interesting cat bait was used for the research –mouse carcasses with an oat grain coated with 1080 embedded in their throats. This labour intensive baiting method did not overcome the problem that cats prefer live prey and the more abundant alternative prey species were, the less bait uptake by cats was recorded. (Short, 2016)

No matter how serious the threat of predation, it is a habitat issue and should always be examined in the context of all habitat issues.
We relocate ringtails to a new environment which previously has not sustained a population of a significant and therefore viable size. The whole range of reasons need to be examined and mitigated in order to be able to provide a long-term chance of survival for the new releases. (Read et al, 2015)

As predator reduction on a landscape scale is unrealistic, habitat with a complex vegetation structure and good understorey that reduces the hunting efficiency of these predators should be chosen.  (also see: 'Desciption of release sites')
According to Jones (Jones et al, 2004) fox predation only contributes significantly to declines in western ringtail possums when canopy continuity is low. She however concedes that this may not apply to newly translocated animals during the orientation and establishment period.
This also seems to contradict Yokochi’s prediction of a 93% risk of extinction due to fox predation in a 20 year timeframe in a high quality habitat with good canopy connectivity.
Yokochi particularly stresses the high canopy fidelity of the species which to me would make fox predation less likely than cat predation.  (Yokochi, 2015)

A high percentage of private landowners have well trained dogs that ideally do not roam freely or at least only during the day when they mark the territory with their urine.
The smell of dog urine could suppress the presence of foxes and feral cats as in particular foxes dislike the smell of dogs. (Fleming et al, 2014)
Even sprinkling of dog urine or ammoniac around release areas seemed to have a positive effect.
Predator scent-marking of release areas would be another but more difficult option. It might however limit the incursions as the areas would seem occupied and interlopers might try to avoid conflict with dominant animals. This type of bio- fence has not yet been trialled in Australia though. (Ausband et al, 2013, Jackson et al, 2012)

Pre-release predator aversion training might be beneficial and establishing an effective training program for animals already in captivity would be possible if cooperation between rehabilitators/shelters and the relevant department/NRM groups could be achieved.

Diligent monitoring of areas where foxes/cats are likely – e.g. near rabbit warrens - and along fire breaks, cut lines and internal paths (e.g. camera monitoring) is crucial to be alerted early to predator incursions as foxes prefer using pathways to bush with denser understorey.

Camera monitoring over longer periods of time can also give an idea of fox numbers. Particularly sequences of photos make identification of individuals possible. The size, body shape, shape of the head and facial markings, the ears and of course the tails can be quite distinctive – not only if a tail shows injuries such as in the photo below.

Paths should be checked for cat or fox scats to indicate presence. Fox scats look similar to dog scats and are usually narrow and long with one pointy end or resemble a slightly twisted cylinder. The colour and shape varies with the prey they have eaten. Scats will contain fragments of prey or other food sources such as fruit. (Triggs, 2004)
Foxes love grapes and sometimes there is almost nothing else than grape skins in a scat.
It would be helpful to identify the species if fur is seen in scats, but possum and rabbit fur looks very similar.  

Foxes deposit their scats on the track where they walk, while cats scratch the soil, leaf litter or whatever is there to hide their scats. Cat scats are fatter, shorter and more rounded than fox scats.

Both species  leave a strong and characteristic odour and one of our landowners is an expert fox sniffer! This will however take some time and practice to develop.

If remains of a ringtail should be found, it can be difficult to distinguish between cat and fox predation.
A fox would most likely remove the head and eat most of the thoracic and abdominal organs as well as the flesh. Uneaten remains would be covered and cached in an excavated hole in the ground or taken back to the den.

A cat would usually not eat intestines and leave a partly eaten mangled corpse or just a few parts like the head and/or paws. 

If the skull of an animal is intact but devoid of soft tissue and it is found under trees, a raptor is most likely the predator.

The heath monitor (Varanus rosenbergi ) would probably only be a potential predator for the youngest of ringtails but clearly at risk itself from baits even if hung in a shrub as they are good climbers.

We have seen tiger snakes and dugites on our release properties but never a carpet python.  

Discussion: Fox numbers
3 sites:
A. Continuously baited, monitored by landowners and no figures available
Reported sightings of foxes and cats and baits taken and own opportunistic, occasional (camera) footage of both and a personal sighting of a fox during daylight in January 2018
Site A is bordered by (an unbaited area of) national park and a vineyard, both seem to lead to high fox numbers. Rabbit numbers are medium.

B. Occasionally baited, figures see below
Site B is in the vicinity of a rubbish tip, which clearly attracts cats and the monitored area is extremely high in rabbit numbers.

Fox sightings (several consecutive photos counted as one event):

 

C. Unbaited,  occasional report by landowners of fox sightings, no opportunistic sightings so far in person or on any camera
Site C is bordered by unbaited national park (but with a very wide gap with almost no vegetation in-between) and farmland. Rabbit numbers are low.

Monitoring on site B always followed the same pattern and detection probability should therefore be comparable.
Fox sightings tend to be highest in summer but we do not have any explanation for the extreme spike in sightings from September 2018. The lower number in December 2017 is probably caused by camera issues and not representing a real drop in fox numbers. The increase coincides with the release of a new variant of RHDV virus (rabbit haemorrhagic disease or rabbit calicivirus) which seems not to have had any effect at all so far.
The only possible explanation we can provide is that similar to the increase in breeding activity of ringtail possums after 2 wet winters and 2 mild summers, the foxes might have bred up too.
Rabbit density was also unprecedentedly high in the monitored spot. Rabbit numbers seem to have started decreasing in March 18 though.

Some foxes have distinctive features and are therefore recognisable. It is evident that those keep coming back on a very regular basis. Their home ranges seem to have shrunk significantly as the generous supply in food (rabbits) does not require larger ranges.

The area also seems to attract ‘unwell’, scrawny foxes that might not survive in an area with high competition due to low food supply.