Background determinations
Any release and in particular if it includes a relocation of animals need to be based on sound knowledge of the species’ habitat requirements and high familiarity with the proposed release area and its various components such as threatening processes.
Also, every species has their own requirements, strengths and weaknesses and will react differently to management options. No method of release is superior for all species and the needs of all animals. (Moseby et al, 2014)
However, more often than not available human manpower and financial resources, availability of or access to adequate release areas where threats have been ameliorated sufficiently and the number of animals needing release play the truly determining role in the way animals are released. This is unfortunately true for all relocations of western ringtail possums no matter whether conducted officially through the department or by community people.
The current recovery plan for the species acknowledges a lack of knowledge but fails to mention how broad this lack actually is. We have no scientifically sound methods of estimating carrying capacity for western ringtails in the various habitats as we lack studies on their natural history. The same applies to virtually all currently threatened or conservation dependent species. (Hayward et al, 2014)
Apart from this lack of ecological knowledge habitat is very limited for these specialised arboreal folivores and whatever habitat area is chosen either important components are missing or threatening processes are operating that present a major risk to translocated possums.
Western ringtail possums (and other species) can survive in sub-optimal conditions; however long-term success is highly doubtful. Quick interpretation of short-term survival as success can also lead to misjudgment of the site and poor selection of similar areas for subsequent releases. (Chauvenet et al, 2015)
Likelihood of success increases with learning from previous translocations. (Canessa, 2015)
Unfortunately rehabilitators/members of the public are not the only ones that avoid analysing and publicising their failed translocations even though this could provide vital direction for refining future programs and prevent others from repeating the same mistakes. (Parker et al, 2015)
It is for instance quite stunning how little information is publicly available regarding the translocation of western ringtail possums from Busselton to Perup Nature Reserve. This is one of the unfortunate projects that could at least have given us insight whether coastal ringtails can survive long-term in areas inland without any peppermint and if so under which conditions.
The small population paradigm claims that there is an inherent risk of extinction in populations with a low number of individuals. (Southgate, 2014)
The minimum viable population (MVP), the number of individuals with a reasonable probability of surviving for a specified time (Soule et al, 1988, Lindenmayer and Burgman, 2005), is however not only species and habitat dependent but also on an estimate of the absolute extinction risk. So far, we only have a single PVA (population viability analysis) for one of the largest populations in the Busselton area (Yokochi, 2015) which might not be transferrable to other habitat areas and populations and which would only estimate the extinction risk for this particular population.
Genetic variability and maintaining evolutionary potential are at the basis of any calculation of an effective population size but would necessarily suggest quite high numbers – usually in the range of several thousands. (Reed and Bryant, 2000)
Following this reasoning, the habitat area to be reserved for western ringtail possum conservation would have to be tens if not hundreds of thousands of hectares. Unfortunately those areas would heavily overlap with densely populated (by humans) regions.
Also, the estimate of the remaining numbers is already too low to allow for enough animals in all those areas where they still persist. The theory is very valid, but unfortunately in the case of our western ringtail possum it cannot be extrapolated to the reality we face.
However, Allee effects occur when populations are too small. The choice of a mate is then highly limited which could in the longer term result in inbreeding depression.
The search for a mate might involve long-distance travelling which increases the animals’ inherent vulnerability to predation further (Clarke, 2011) while the per capita risk of predation is generally smaller in larger populations. Active dispersal away from already low-density populations would further increase the predicament (Drake and Kramer, 2011) and numbers can also quickly drop below the viable margin through stochastic events such as fire. (Hayward et al, 2014)
These issues are most critical in social species that are cooperative in breeding, feeding and defence, (Drake and Kramer, 2011) which our ringtails are not.
Judy Clarke hypothesised however that due to the limited and irregular availability of ringtails for translocation, her numbers might have been too low for establishing breeding populations. Earlier translocations in that wider region with larger numbers had been more successful. (Clarke, 2011)
I would hypothesise that the best habitat patches were occupied by earlier releases and the habitat was not adequate to feed a larger population.
Clarke released 68 animals over a 2-year period, a number a community-driven program would rarely reach, so that the issue of small founder/population sizes will always apply.
However, even though larger release groups can obviously improve the likelihood of success (Tracy et al, 2011) high numbers carry the risk of increased competition for limited resources, degradation of the habitat and resulting dispersal with heightened predation risk.
Habitat quality (food and shelter), predation pressure, possible territorial conflicts with resident animals and the particular vulnerability of our clientele – mostly rehabilitated possums – all need to be taken into consideration when deciding releases.
Whatever we do will happen in a complex web of site conditions and results can therefore always be site-specific. Strategies will therefore have to be site-specific to some extent too.
