Abundance and decline
With only very limited survey data available, there are no truly reliable estimates of current total western ringtail possum numbers. According to the Action Plan 2012 the overall number of mature individuals is below 8000 and therefore meets IUCN requirements for a listing as critically endangered (Woinarski et al, 2014). The official estimate for the region Bunbury to Dunsborough gives a vague 2-5 thousand animals without specification whether that refers to mature adults or includes (very vulnerable) juveniles. (Shedley and Williams, 2014)
The nomination summery for reclassification under the EPBC Act estimates approximately 3400 mature individuals over 5 subpopulations with the largest subpopulation of approximately 2000 mature individuals on the Swan Coastal Plain. (EPBC 2016)
However, there seems to be a discrepancy in the number of subpopulations as the Gelorup/Dalyellup population was identified as a discrete subpopulation (Wilson, 2009) but not included in the above determinations.
Previously occurring in a wide range of vegetation types – peppermint (Agonis flexuosa), peppermint/tuart associations, eucalypt forest (jarrah, marri, wandoo) – western ringtail possums are now mostly restricted to peppermint woodland.
This decline pattern suggests that threatening processes have different impacts in different habitat types. (Jones et al, 1994a)
Habitat loss and predation caused the most dramatic decline in numbers and range and abundance declined with increasing fragmentation. (Wayne et al, 2006)
Habitat quality is a pivotal factor in abundance, particularly the dominance of peppermint over eucalyptus, canopy continuity, a higher level of leaf nutrients and low numbers of common brushtail possums. (Jones et al, 1994a, Jones and Hillcox, 1995).
In areas where A. flexuosa is dominant and eucalyptus is almost excluded, western ringtail possums are often the only possum species present. The most significant remnant of this type is Locke Nature Reserve where ringtails are still fairly abundant. (Jones et al, 2004)
Abundance is also higher near watercourses and cleared land as we tend to clear the most fertile areas for our human needs. (Wayne et al, 2006)
Very low fire intensity or absence of fire for at least 20 years might however be the most potent driver of higher abundances.
The coastal peppermint stands that fringe the southern margin of Geographe Bay had a very low fire exposure for the last 45 years and harbours the densest population. (Jones, 2016)
The semi-urban coastal strip around Busselton/Dunsborough is a major stronghold for the species (Jones et al, 1994a, Jones et al, 1994b), but this area has been subject to extreme development pressures in 2002-12 and this trend is ongoing. (Jones, 2016)
In 2002 the total extent of peppermint stands in the Swan Coastal Plain has been estimated to be approximately 350 ha. The most significant remnants were however small (up to 5 ha) and under development pressure. (Jones et al, 2004) A more current assessment calculated an available 630 ha of prime habitat that can sustain good population densities. (Shedley and Williams, 2014, Burbidge, 2016)
Since about 2010, the population on the Southern Swan coastal plain is clearly the largest of all those remaining and in some patches the density is even unsustainably high.
From Peppermint Grove Beach to Locke Estate the populations have declined little in comparison to those in other areas (Jones, 1992) but this is of course ambiguous as the Upper Warren population suffered a likely decline of 99%. (Woinarski et al, 2014)
Declines in the peak dry era 2006-2010 on the Swan coastal plain still mean a serious reduction in numbers, even in core populations in reserves. (Jones, 2016)
The reasons for the massive collapse of the high conservation value population in the Upper Warren habitat are unclear as long stretches of continuous habitat with older growth stands have mostly been undisturbed for decades. The formerly high rainfall at times when ringtails were common (1990s) has however significantly decreased which might have been the driving cause of the decline.
Most populations around Albany persist in low density populations (Jones, 2016, Burbidge, 2016) but neither abundance nor decline figures seem available due to a lack of consistent surveys.
Formal specific monitoring programs for ringtails had not even been in place during western Shield baiting. (De Tores et al, 2004) This will hopefully be rectified in the key management zones in the near future. Continuous spotlighting monitoring over a longer term to examine population changes would be highly advisable for all areas with high conservation value.
After Leschenault Peninsula and Yalgorup National Park failed, Ludlow Tuart Forest (30-55% decline between 2002 and 2012, (Woinarski et al, 2014) Locke Estate, Perup Nature Reserve and two Peoples Bay on the south coast are now the main conservation estate areas for ringtails.
The habitat in the Perup sanctuary seems still suitable and is free from introduced predators; (Wayne et al, 2012) however the drying trend could change this. It also seems obvious that animals from the almost exclusive peppermint habitat around Busselton should not be translocated there.
Diagnosing the cause of a species’ decline is highly challenging as there is rarely only one reason behind the trend but many factors that interact.
In most serious mammal declines and extinctions, predation by cats and foxes is usually the most noticeable and fairly easily researchable end result. However, predation most often acts in concert with other threats and disturbances and a population might decline even in the absence of introduced predators - e.g. Perup Sanctuary.
Also, a decline might stay unnoticed due to lack of monitoring (e.g. Leschenault Peninsula) which could add serious bias as the investigation into the end stage of the decline might only reveal current factors that could be different to those acting in the early stages. (Fancourt, 2016)
Particularly when environmental factors such as long dry phases are involved, diagnosing is difficult. A population could have been reduced to unsustainable levels during the dry spell and render the remaining animals unable to withstand other threatening processes. (Fancourt, 2016)
Successive and/or simultaneous threatening factors can act together and the final threat will push the species over the brink.
For the Upper Warren area it could for instance be hypothesised that the first severe decline through disease (Abbott, 2006) weakened the population’s resilience to the negative effects of logging and associated threats. Declining numbers meant increased predation per capita which in a low density population in a low carrying capacity habitat might restrict reproduction and recovery options.
Reduction in rainfall and food quality could then lead to changes in distribution and abundance (Fancourt, 2016) and finally the extinction of the population.
