Predation introduction
A significant number of animals that went recently extinct or are now on the list of threatened mammals have a body mass of 35 g to 5.5 kg which was defined as the ‘critical weight range’ by Burbidge and Mckenzie (Burbidge and Mckenzie, 1989).
These animals seem to be disproportionally vulnerable to predation because of their small size.
22 Australian mammals went extinct since Europeans arrived and foxes and feral cats, which heavily prey on small to medium-sized species, are blamed for the majority of them. (Burbidge and Mckenzie, 1989) Only mammals in heavily wooded habitats that also receive higher annual rainfall have declined less sharply.
Most of the vertebrate species listed under the EPBC Act 1999 (vulnerable and endangered) are susceptible to predation from feral cats and the south-west of WA was declared a priority area for intervention. (Dickman, 2014)
The Perup forest area in Western Australia is a good example for the interconnection of threatening processes and the difficulty of pin-pointing predation as a clear causal agent in ringtail possum decline.
Perup had an almost intact mammal fauna (Abbott, 2008b) until the 1930s even though the cat had been established there since the 1890s.
Epizootic disease in 1913 and 1930 (Abbott, 2006) had far more devastating effects than the decades of cat predation and decimated populations of both large possum species.
Even though the fox had been established in that region in 1931 and increased predation pressure, western ringtails were still locally common around 1970. (Dickman, 2014)
However, serious monitoring seems to only have commenced in 1972 and in 1974 ringtail sightings decreased. Reduced use of rabbit control was seen as a major reason for the decline as this resulted in higher fox numbers. (Abbott et al, 2014)
Fox control since 1977 led to a apparent recovery of native species until 1999.
Today up to 99% of ringtails in that area are lost (Wayne et al, 2012) and it is probably fair to say that there is no single culprit.
Higher abundances of ringtail possums in the region were associated with areas that had a high level of fox control (Wayne et al, 2006), which might however have led to mesopredators release (also see 'Mesopredator release') and/or interactions between cats and other introduced species could have caused ‘invasional meltdown’. (Simberloff and Von Holle, 1999)
Predation is a ‘habitat feature’ and should not be discussed without evaluating habitat values and the interdependence with other threatening processes.
