The Fate of Translocated Urban Possums
by
Rod Pietsch

 

This article highlights some of the realities of managing wildlife.  The solutions are not always simple ones.  It is essential that we are guided by an understanding of the biology of the animals involved if we have to do best by them.  Rod Pietsch reports on his study of the fate of urban possums relocated to the bush.

The common brushtail possum is one wildlife species that has flourished in the face of urban development.  This highly adaptable animal has successfully established in urban areas.  However, urban areas have a shortage of trees bearing hollows that can be used as dens.

Brushtail possums have adapted to this shortage of hollows by using house roofs and the dark recesses in buildings as dens.  However, possums and humans are not always compatible house companions.  Brushtail's can be very noisy as they move about in a roof at night, particularly if there is more than one possum using the roof, and I'm sure many of you can venture this!  Possums are also notorious for fouling the ceilings of houses and buildings.

Other problems arise from the feeding behaviour of the possums.  Possums browse on garden plants and have a reported liking for roses and other ornamental species.  Fruit trees are also subject to possums damage is possums will try unripened fruit and then discarded it.  Possums have been known to eat pet food that is left out and will scavenging in bins for discarded foodstuffs.  Some people find it very difficult to tolerate these behaviours and so live trapping and subsequent translocation to outline native bushland has been used in an attempt to alleviate these problems.

In this situation the possums are being translocated on the basis that it is more socially acceptable and a humane way of dealing with nuisance possums rather than some form of euthanasia.  It is believed that the welfare and well-being of the possums are better served by moving them to a location where they can settle and no longer come into conflict with humans.  This perception is held because it is commonly believed that urban possums translocated to a native forest will survive and readily establish in a new habitat.  However, no studies have evaluated the survival of the Common Brushtail Possum after translocation from urban to forest areas, and furthermore, the impact of the translocated possums on the wildlife residents at the site of release is not known.  With more than 2000 Common Brushtail Possums being translocated from Melbourne each year, this is by no means a small problem.

In an attempt to address the lack of information regarding the fate and effects of translocated urban possums, I conducted a study as a part post-graduate research project at Deakin University.  This study investigated the fate of urban Common Brushtail possums trapped in urban areas of Melbourne, and translocated to nearby sclerophyll forest at Silvan in the Dandenong Ranges east of Melbourne.

The total of 64 urban caught possums were released an over a three-month period into the sclerophyll forest study site at Silvan.  All released possums were fitted with reflective ear tags and twelve were fitted with radio transmitters.  The resident possum population had been monitored for three months prior to the first release of urban possums, and monitoring continued for three months following last release.

The results were not good for the released possums.  Spotlighting revealed that translocated possums rapidly disappeared from study area following release, probably due to a combination of predation and dispersal.  No ear-tagged animals were detected in the study area after 15 days following release.  More specific information on the survival of the translocated animals was obtained from radio collared possums.

Only two of offer 12 collared possums survived more than two months.  Eight of the collared animals died, and the status of the remaining two animals could not be confirmed.  Of the confirmed mortalities, 88% died in the first week of release.  The other known mortality occurred 30 days following release.  Five of the mortalities were attributed to predation, probably by foxes.  Two collared possums died from stress related to the trauma associated with the translocation and arrival in a foreign environment.  The remaining mortality was attributed to roadkill.

The high mortality and the low rate of establishment of the released possums in the study area appeared to be related with unfamiliarity with the released site and the naivety of the urban possums to the forest environment.  More than half of possums that tried to climb trees immediately following release either slipped or fell from the tree whilst climbing.  They were not used to climbing eucalypts would lose bark!

Another interesting finding was that the activity patterns of translocated possums differed markedly from those of resident animals.  In general, forest resident animals spent more than 90% of their time in the trees, whereas the translocated possums in the study spent more than 40% of their time on the ground.  Translocated possums also spend considerably more time traveling.  In addition, during the day the translocated possums were frequently found in log piles on the ground or sitting in tree forks with no cover, rather than denning in tree hollows.  These factors suggest that translocated possums were more likely to be accessible to, a detected by, predators.

Other incident observations suggest that low survival may be partly attributed to the naivety of urban possums.  In urban environments, possums can become habituated to people, noise and pets.  This habituation may reduce the normal evasive behaviours or wariness of the urban possums, rendering them more vulnerable to predators than the forest adapted animals.  It was not unusual to see the released urban possums standing for walking along vehicle tracks and it was possible to approach them to within a few metres without visibly creating excessive nervousness or abrupt changes in behaviour.  This was not observed in resident animals.  Resident animals that were encountered on the ground rapidly retreated out of sight or climbed a tree to a safe distance.

Another interesting finding of the study was that dispersing animals were likely to use houses or buildings as dens sites.  One of radio, possums took up residence in a house three kilometres from the site of release.  If possums occupy houses or buildings following translocation, then clearly translocation has not solved initial problem, but merely transferred it.

While the fate of released possums is apparent, and effects on the habitat and resident wildlife is more difficult to assess.    The spotlight monitoring throughout the study revealed that there was no significant change in the resident animal density following the release of the translocated possums.  This suggests that the survival of the residents was not impaired by the release of the urban possums.  However, issues related to the carrying capacity of the habitat where the possums were released, social disruption, possible disease transmission and genetic implications also warrant consideration.  Increasing the density of animals in the release area may result in habitat alterations or some form of environmental disturbance.  Mixing animals from different populations can alter the naturally occurring gene pool which is undesirable in view of a lengthy process by which local adaption occurs.  Similarly, mixing animals in different environments may expose animals to disease and parasites which they are not accustomed to, and consequently could reduce the survival fitness of the released or resident animals.

The principal reason for translocating Common Brushtail Possums from urban environments to rural forest locations is to reduce human-possum conflicts without killing the possums.  The reality is this choice is that translocation may result in the deaths of more than 70% of those possums in the first week of release.  The low survival rates for translocated possums in this study make it difficult to justify translocation on the grounds that it is humane.  Being killed by predator or suffering from translocation stress and trauma, is no more humane than some of the more conventional methods of euthanasia.  In addition to possible habitat, genetic and disease impacts associated with the process.  If the reduction of human possum conflict is warranted, alternative methods of dealing with the nuisance possums should be considered and evaluated.  If saving the possums is a primary concern, and in some other way of reducing the conflict, or perception of conflict, need to be sought.

Wildlife in the suburbs are a real bonus for urban dwellers and people should be encouraged to tolerate, live with, and appreciate to urban wildlife because in some parts of Australia and native wildlife is disappearing.

Acknowledgements

The study was supported by a grant from Holsworth Wildlife Research Fund.

Rod Pietscsh’s findings are supported by study by Barbara Smith and Michael Augee on hand-reared and relocated Ringtail Possums.  Of 82 released in Ku-ring-gai Chase National Park, there were two survivors.  Most had been killed within 70 days.  38 were killed by foxes and 27 by cats.

Source: conference on reintroduction biology of Australian fauna, Healesville Sanctuary, 19 to 21 of April 1993, Stephen Platt.

This article has been reprinted from Land for Wildlife Vol.2 No., with acknowledgements to LFW and the author.

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