Densities, home ranges and dispersal
Ringtail densities vary widely and even experienced ringtail researchers are often unable to explain this variation, particularly in areas that are known as high quality habitat. (Shedley and Williams, 2014)
However, there is agreement that high quality habitat is scarce but of paramount importance for the species to reach high rates of reproduction. Species recovery and even viability of populations depend on good reproductive output.
Peppermint as the dominant tree species is the most important feature in all high density habitat areas and determines their carrying capacity. (Shedley and Williams, 2014)
However, even individual trees can differ in their quality as a food source in a habitat patch and the whole make-up (canopy connectivity, crown density, understorey, other vegetation) determines the value of the habitat.
Ringtail density could well be regarded as the ultimate measure for quality of habitat – at least if habitat was not shrinking so dramatically that the animals are boxed into smaller and more unconnected patches with nowhere to disperse to.
The trend of greater density of ringtails in spots with very little surrounding habitat even when the vegetation is in fairly poor condition as observed by Shedley and Williams could just be an indication of constant habitat reduction and fragmentation through developments for humans. High quality ringtail habitat is quite often in highly sought- after areas close to the foreshore and constantly invaded and eroded.
There is nothing surprising in the fact that decreasing habitat will increase the numbers of ringtails using it in the short term and that those last patches will be extensively used no matter how degraded they become. Also, a mature peppermint tree is highly likely to attract and sustain many more animals than newly planted trees. A survey in Kookaburra Caravan Park (Dec 2014) showed that up to 5 animals were crowded in single mature trees while the young trees were devoid of ringtails.
Prior to the foreshore development and a halving of numbers that has not been adequately explained so far, densities in Kookaburra Caravan Park have been estimated in the range of 26 animals/ha which is an extreme density. However, similar densities have also been detected in some development areas in Abbey/Broadwater during the peak development era 2002 onwards as shown in various management plans for development applications.
In comparison, a density of 8-10 animals per hectare would be exceptionally high in conservation estate and most densities are lower than 1-3 per hectare (pers. communication B. Jones).
The average density for Locke Nature Reserve, one of the last strongholds of the species in conservation estate, was estimated at 5.84 ringtails per hectare in 2010 (De Tores and Elscott, 2010) while earlier research in 1994 only reported densities of up to 4.5 animals per ha. (Jones et al, 1994b) Another survey in 2014 reported a density within the bush area (Nature Reserve) of only 1.03 per hectare and 4.8 per ha within a campsite area opposite the reserve (Harring-Harris, 2014) which I assume was not surveyed in the other 2 studies.
These huge differences can of course have a myriad of reasons from real fluctuation in numbers to differing survey methods (highest and lowest count used the same method – distance sampling - though), experience of the researcher, different time frames for the research (4 months versus 1 month) and different areas surveyed and then used for mathematical analysis. For me personally, these differences add a big question mark to what scientific evidence can truly tell us. However, the 2 counts derived from distance sampling were used for the only PVA (population viability analysis) we have for the species and for illustrating the alarming decline.
In comparison and as an indication of the vast fluctuations, a density of 2.5 animals per ha was estimated in Abba River (1994) and 13.5 per ha in an urban Busselton reserve. (Harewood, 2008).
Preliminary research during 2008 in the Tuart Forest National Park, another remaining conservation estate, calculated 2.34 ringtails/ha and 1.25 ha at Gelorup. (Clarke, 2011)
I was unable to find density figures for the third protected conservation estate – Perup Nature Reserve – however, it can safely be assumed that they are very low. The same will apply to the entire Upper Warren region.
There is very little information available for the other urban-dominated habitat, the area around Albany. The only density estimate I was able to find went back to 2008 and was given as 0.7 animals per hectare. (Van Helden et al, 2017)
As far as I know, densities in the Margaret River area have never been researched. The mainly mixed bush habitat with only smallish peppermint- dominated patches and a therefore low carrying capacity will most likely have densities no higher than 1 or 2 animals /ha. East Augusta will be the exception to the rule but I cannot find any density figures in the literature.
Densities can and will not only change between years as for instance recruitment will be less successful during drought; the method the density figures are derived also make them hard to compare. Capture rates, line or point transect counts, distance sampling (information theory), drey and scat counts have all been used in ringtail research so far and cameras as fairly new tools have now been added.
Every method can also be influenced by the proficiency of the surveying person and by detection probabilities which change between locations and can even change from day to day.
Home ranges
Densities and home ranges show similar variation. There is no such thing as an average home range of a ringtail as it is highly dependent on habitat values.
The scarcer adequate food supplies are, the larger the home ranges have to be in order to feed an animal and the lower the density will be. Populations in the Jarrah Forest need far larger home ranges than those populations around the coastal strip around Busselton/Dunsborough. (Wayne, 2005a)
Wayne determined average home ranges in the Jarrah Forest to be 5.01 ha for males and 1.26 ha for females (Wayne, 2000), while Yokochi found average home ranges for males to only cover 0.3 ha and for females a tiny 0.16 ha in the densely populated Locke Nature Reserve region. (Yokochi et al, 2015a)
Differences in home range size between occupied sites mainly relate to food quality and availability but other habitat values (shelter/protection, predation, competition, floristic composition etc.) are also important and could limit the carrying capacity of an area. Research in this area is still very limited.
In high quality habitats, territories are usually small and overlapping (How and Hillcox, 2000, Wayne et al, 2005) but, outside of the mating period, adult ringtail possums rarely show much interest in one another. (Clarke, 2011)
Home ranges overlap most strongly for females and their daughters while males inhabit a larger area that overlaps with those of females and other males (Jones et al, 1994b) but does not include the central part of a female’s home range. In those ‘best spots’ males are only welcome when in the company of the resident female. (Jones et al, 1994b) Males are usually restricted to lower quality areas around and between the best patches. (Jones et al, 2007)
If at all possible, ringtails will remain in close proximity to their natal range when dispersing and attempt to stay in their chosen range as adults. Sub-adults occasionally check out surrounding areas. It was found that females sometimes stayed in their natal home range for their first breeding season and then dispersed. (Jones et al, 1994b) Males seem to always disperse. (De Tores et al, 2004)
Dispersal
Density, home ranges and dispersal are related issues that should be examined in context with each other.
Dispersal is essential for gene flow and the maintenance of healthy population sizes. If dispersal is reduced due to habitat loss or fragmentation, a loss of genetic variation can occur, which might threaten the ability of a population to persist and/or recover.
Habitat patches need to be linked at least by corridors to improve the connectivity and promote successful dispersal between remnants. (Frazer et al, 2015)
Dispersal behaviour is also an important factor when choosing relocation sites as immediate dispersal of released animals can lead to early failure of the reintroduction/restocking (Richardson et al, 2015) while continued dispersal away from the (badly chosen) site, could create a bottleneck (significant temporary population reduction). (Frazer et al, 2015)
Animals that disperse long distances and are vulnerable to introduced predators will be at considerable risk.
Dispersal patterns for translocated ringtails seem to differ from ‘natural’ dispersal. After translocation of ringtails to Leschenault Peninsula and Yalgorup National Park, Judy Clarke observed that some of the animals travelled long distances – up to 9.5 km in total or 2.5 km in a single 24-hour period. (Clarke, 2011)
Limited food resources and lower quality food at the release sites than in Busselton from where they originated, competition with formerly translocated animals that already occupied the best patches (overstocking) but also stress and disorientation were assumed to be contributing factors to this pattern, which also led to reduced post-translocation survivorship. (Clarke, 2011)
Clarke’s ringtail translocation research confirmed that low fidelity to a release site translates to low survival. Animals dispersing widely will most likely travel for a significant time on the ground, which will make them vulnerable to predation, particularly if they move through or into less dense vegetation. (Clarke, 2011) (link translocation)
We need good knowledge of the nutritional quality of the vegetation at a release site and the size and home ranges of the existing population to be able to make good decisions where to release animals and in which numbers.
Best case scenario for relocations would be a well-managed site with adequate food/shelter and a low threat level, connected to a wider landscape with further appropriate habitat areas.
If the site selection should lead to high dispersal, follow-up translocations to compensate for individuals being lost are often suggested, however, threatened species such as ringtail possums are not readily available and if dispersal is ongoing the chosen habitat is most likely not suitable in the long-term and the attempt should be abandoned (Richardson et al, 2015).
Natal dispersal patterns of resident animals can also differ from those stressed by translocation. The latter often show more pronounced or different dispersal behaviours than the species would under normal conditions (Dickens et al, 2010, Richardson et al, 2015).
A lack of dispersal and in consequence of long-term genetic and demographic exchange with other populations can however also lead to translocation failure in the long-term. (Weeks et al, 2015)
