Competition with brushtail possums

Ringtail and brushtail possums inhabit a similar niche in the woodland habitats of the South West and seem to be able to co-exist. (How and Hillcox, 2000, Jones and Hillcox, 1995)
Based on their foraging preferences they partition some habitat types quite effectively. Heavily peppermint dominated areas are usually almost exclusively used by ringtails while there is a clear overlap in mixed bush areas.
In Yalgorup National Park ringtail possums showed a preference for dense peppermint understorey, while brushtails preferred tuart woodland with sparse peppermint understorey. (Clarke, 2011)

In the tuart/peppermint woodland of the Tuart National Park ringtails seem to use the lower canopy level of up to 15 metres while brushtails also utilise the upper canopy which shows some degree of partitioning, but also some degree of overlap that might lead to competition.  (Grimm and De Tores, 2009)

As natural drey builders, ringtails have the advantage of not relying on tree hollows like brushtails. In areas such as the Tuart Forest where hollows are the preferred nesting site of both species, the use of different hollows in the same tree or adjacent trees by both species was observed. Prior occupation by the other species was also no deterrent for use. (Grimm and De Tores, 2009)

In Judy Clarke’s study in mixed bushland habitat home ranges of the two species overlapped significantly and there was potential for inter-species competition for food and shelter. Survival of translocated ringtails was clearly lower when large brushtail populations were present. Competition with brushtails could have led to increased exposure to predation through reduced energy resources and longer searches for adequate rest sites. (Clarke, 2011)
High numbers of brushtail possums were in fact the second most important issue limiting ringtail proliferation.

Competition could have been one of the factors in the severe ringtail decline in Leschenault peninsula (after 1999), but as the increase in brushtail numbers was only minor, it was deemed very unlikely that this played a major role in the decline of ringtail possum density. (De Tores et al, 2004)

Efficient fox baiting can lead to an increase in brushtail numbers (Wayne et al, 2006) as they come to the ground more often, are therefore more threatened by foxes and profit accordingly more from the reduction of foxes. During the early ringtail translocation program to Leschenault spotlight monitoring revealed a dramatic increase in brushtail number after intensive fox baiting with 1080. (De Tores et al, 1998)

As ringtail possums declined heavily after nearby brushtail habitat was lost in a fire and the displaced animals shifted into the unburned ringtail habitat (Jones, 2003) it is obvious that any severe increase in brushtail numbers will negatively affect ringtail populations.

For instance, harvesting the pine plantation in Ludlow Tuart Forest could pose a major threat to the viability of the ringtail population as the brushtail possums will disperse from the pine plantation into ringtail-occupied habitat and cause major competition. (Grimm and De Tores, 2009)

(Also see: 'Competition with brushtail possums' in 'Practical Applications')