The lack of early reports of possum-induced tree decline in Victoria and reports of dense understoreys in many areas at the time (Hateley 2010) suggest that they could.
Furthermore, many areas elsewhere in southeast Australia including sites supporting Swamp Gum currently have dense understoreys and native and introduced predators (Victorian Biodiversity Atlas, data) with ringtail possums and tree canopies apparently in balance.
Certainly the best management response on the Mornington Peninsula now is to reopen understoreys as culling of possums and reintroducing predators are impractical.
In the Melbourne region, however, large River Red Gums Eucalyptus camaldulensis with open grassy understoreys have been killed by brushtail possums that den in natural hollows.
Several factors control populations of ringtail possum including availability of shelter, density of understorey vegetation, predation (originally mainly by humans, dingoes, spot-tailed quolls, large raptors and large owls, and now mainly by cats, foxes, large raptors and large owls where they occur), fire, food quality and availability, and heat waves.
However, being ground-based predators they are not efficient at controlling ringtail possums in dense understorey vegetation.
Algal information was recorded under four categories based on height and growth patterns: (1) canopy forming algae; (2) understorey (large bottom cover; 15 cm-2 m); (3) turf (short bottom cover, <15 cm); and (4) encrusting.
The understorey algal species were numerous and sometimes difficult to determine which species listed in the field notes was dominant.
For each dataset (QCI or CC or combined), two sets of predictor variables were used: (1) all habitat variables; and (2) all, excluding Understorey Sp 2, Canopy %, Understorey %, and Encrusting %, which were considered the least reliable (estimated most often from field notes).
setchellii, Pleurophycus gardneri, or Costaria costata as the second dominant understorey species.
There were data problems with understorey species and we considered that using the understorey species as first level predictor variable was unwise (e.g., CC dataset with all variables and mean SL as response variable: Table 3).