Competition with brushtail possums

Competition with brushtail possums has been identified as a major threatening factor in translocation research. However, most ringtail habitats with mixed bush vegetation are also habitat for the common brushtail possum. Cohabitation and habitat partitioning is clearly possible but we have not yet established the factors that enable or prevent it.

All 3 of our release areas are mixed bush and have also a brushtail population.
Releases of ringtail possums were restricted to the peppermint dominated patches, and we observed very little brushtail presence in the earlier stages. Cages also seemed to be less attractive to them than pure water stations even though there is/might be some food in them.
In the vicinity of human habitations brushtail presence clearly increased.

On property A with the largest ringtail population, the release area closest to the human dwellings had the highest number of brushtail sightings of all release areas.

Sightings of brushtails everywhere increased from November onwards and sharply decreased in May again.

5 years of camera observation – 2013 to 2017:
Percentage of nights when brushtails visited water stations:

The years with peak brushtail occurrence 2014/2015 were both particularly dry years. 
After 2 relatively wet winters and a very mild summer the reasons for the high brushtail number in 2017 will however have been different for their crowding in on ringtail habitat. The ringtail population had clearly increased and the same can be expected of the brushtail population which might be the reason for the high presence figures for them.

However, brushtail presence stayed generally moderate in all other monitored areas. The highest occurrence at all other release spots was in March 14 with 52% of nights with brushtail presence but this went down in April to 17% and then stayed low even in the peak summer months.

Ringtail releases on property B with a larger brushtail population started in late 2014 - again only in the clearly peppermint dominated areas. There was however an immediate brushtail presence.

 

The influx of brushtail possums again started in November 2015 with 53% of nights with sightings but decreased to 39% in December.

The percentages of brushtail sightings per month are comparable on both properties as are the peak months of occurrence. However, on property B ringtail sightings decrease considerably when brushtail sightings increase – up to a point when they seem to completely take over the habitat and become the dominant species even in peppermint dominated patches.
Competitive displacement of ringtails from the best quality habitat could be particularly threatening to the viability of the ringtail population during breeding.

While ringtails come in pairs or groups of 3 or sometimes even 4 (in the breeding seasons),  brushtails are always on their own or a mother comes with her one offspring. There were never any sightings of  more than 2 at a time.
Whenever interaction between the species was seen on camera, it was not aggressive. However, aggression  does not seem  necessary as ringtails immediately retreat if brushtails appear.

We know that fox baiting had no significant effect on ringtail possum survival (Clarke, 2011). Brushtails tend to come to the ground more often and are then probably easier prey for foxes then ringtails. Brushtails also have a wider range of food sources and are therefore spreading out further into all areas of mixed bush while ringtails concentrate in peppermint dominated areas.
Brushtail possums are even attracted to fox bait (see ‘Foxes and their control’) and frequent those areas baited because of the prolific fox-attracting rabbit population. Ringtail possums were never seen in that part of the property.

In consequence, ringtail survival in fox baited areas might be higher if there is also a brushtail population. However, the question is whether an abundance of rabbits and brushtail possums would not just lead to increasing fox numbers. The ‘competition’ between foxes and brushtails for bait might add another counter-productive element as foxes clearly prefer live animals to bait while brushtails even teach their offspring to find bait. In consequence we might lose the bait, lose the young brushtail offspring due to the toxicity of the bait and lose more brushtails to foxes.