Other predators and their interaction
Research usually specialises on one class of predators – most often mammals (cats, foxes), but the suite of predators can include representatives from various taxonomic classes which all use similar resources.
If resources such as prey and den sites are limited and several predators have a profound niche overlap, they can influence other predators largely through competition and direct aggression.
Intraguild killing (e.g. fox kills cat) seems to carry more advantages than nutritional benefits for the killer as the victim is often left uneaten. A potential competitor is eliminated and others of the same species will avoid the area. (Glen, 2014)
The Jarrah Forest in Western Australia which once provided the habitat for the largest western ringtail possum population is home to a wide range of potential predators: cats, foxes, chuditch, south-west carpet pythons and goannas (gouldii and rosenbergi) plus owls and raptors. All these apart from the wedge-tailed eagle which is classified as an apex predator (Abbott et al, 2014) would belong to the mesopredator range.
Mammals dominate the diet of foxes, feral cats and pythons and are also an important food source for the chuditch. Pythons overlap extensively with foxes and cats and can have strong ecological impacts on prey populations. (Sutherland and Bryant, 2014) They had a devastating effect on translocated ringtail possums (Yalgorup National Park and Leschenault Peninsula).
The role and impacts of reptilian predators are less well researched and recognised than mammalian predators. (Sutherland and Bryant, 2014)
Native marsupial predators such as the chuditch are often seen as having little evident effect on populations of prey with which they have co-evolved (Glen and Dickman, 2005), whereas the impacts of introduced predators can be devastating.
However, one ringtail translocation had to be abandoned (in 1999) because of predation by chuditch. (Lane Poole Reserve/Keats Forest Block near Dwellingup) Translocation sites are usually controlled for foxes which might have given chuditch an advantage.
Foxes, feral cats and chuditch are generally crepuscular (most active at dawn and dusk) or nocturnal, although all can be active at any time. (Sutherland and Bryant, 2014) As carpet pythons mainly feed on mammalian prey that is crepuscular or nocturnal, I assume the same applies to them.
Unlike exothermic snakes, mammalian predators are active throughout the year.
Only Goannas are strongly diurnal, however they also pose the lowest risk for ringtails even though they are active predators and dietary generalists. (Glen, 2014)
The control of foxes can potentially not only lead to an increase in cat predation but also carpet python predation, as might have been the case in ringtail relocations to Yalgorup National Park and Leschenault Peninsula.
Interestingly, foxes in eastern states research cached dead radio-collared ringtails only during winter. These food stores might be necessary when competition with pythons is high in warmer weather. (Russell et al, 2003)
This illustrates the introduced foxes’ adaptability to environmental conditions and the subtlety with which native and introduced predators can interact.
Foxes kill (and sometimes eat) cats and in a study in the Jarrah Forest 3 of 8 radio-collared feral cats were killed by foxes. (Molsher, 1999) However, in other studies remains of a cat were also found in a chuditch scat in South West WA (Morris et al, 2003) and predation of a (smaller) cat by a SW carpet python (Bryant et al, 2012) has also been recorded.
In the Eastern States ringtail possums and other arboreal prey are a significant food source for powerful and sooty owls. Sooty owls have reportedly shifted to include arboreal species as terrestrial mammals have declined. (Glen, 2014) These 2 species do not occur in Western Australia but the principle could still hold true for masked owls if e.g. rabbits were declining (or are being controlled).
The minimal time overlap between nocturnal ringtails and diurnal birds of prey should reduce the risk of high predation; however Cherriman reported that in the eastern states ringtail possums constituted 20% of the prey of wedge-tailed eagles. (Cherriman, 2009)
Raptors were also implicated in western ringtail possum deaths in Clarke’s translocation study. (Clarke, 2011)
In an urban context, predation or more likely attack leading to fatal injuries by domestic dogs/cats and ravens pose a serious threat to ringtail possums.
