Introduced carnivores have impacted strongly on our wildlife and rendered many of the translocation programs that were meant to save endangered species failures.
Efforts to control and eradicate exotic mammals go back at least 50 years (Moro et al, 2015), however the problem seems to be undiminished. It would be necessary to at least significantly reduce if not eradicate feral cats and foxes and the rabbit populations that sustain the predators from mainland Australia if we want in particular vulnerable ‘critical weight range animals’ (Burbidge and Mckenzie, 1989) to have a future.

Translocations to islands were often successful but can be detrimental to other species as the translocation of an insurance population of Tasmanian devils illustrates. Those islands are main breeding habitats for a number of seabirds which are devil prey. (Huxtable et al, 2015)

Until trapping and poisoning techniques and strategies improve drastically, it seems we need intensive conservation management projects such as large enclosures and sanctuaries that completely exclude predators for the persistence of the most vulnerable species and to avert an extinction crisis.  

The first large fenced area that excluded introduced predators and herbivores alike was Warrawong Sanctuary which opened in 1985.
Today, we have at least 32 securely fenced sanctuaries that are larger than 10 ha into which several threatened species have been reintroduced. (Dickman, 2011)
The most important in southern Western Australia are probably Karakamia (258 ha), Perup Nature Reserve (420 ha), Waychinicup National Park (84 ha) and Mount Gibson (6,000 ha), which is the largest fox and cat-free area on mainland Western Australia. Karakamia and Mount Gibson are privately owned and managed by AWC while Perup and Waychinicup are under the management of the WA environment department (with its changing names).
Bush Heritage Australia and Australian Wildlife Conservancy (AWC) are now the main non-governmental sanctuary organisations in Australia.
Karakamia, which was founded by Martin Copley in 1995, enabled reintroductions of more than 20 species – our western ringtail possum one of them.
Of all the others, only Perup Nature Reserve has also been used for ringtail relocations

Fenced sanctuaries can play an important role in the conservation of fauna and provide a research hub for those reintroduced endangered species that are often under-researched. (Hayward et al, 2014)
If the enclosure is large enough to mimic natural habitat, no artificial feeding or veterinary intervention is needed – a big advantage to zoos and other captive situations. As the animals are still exposed to natural climate patterns, they also retain some ability to adapt and evolve to changing conditions. (Hayward et al, 2014)
Enclosures should also be situated in areas of high conservation value, so that they can support a variety of species. Animals with highly specialised requirements such as our ringtails are rarely catered for as the best habitats are also prime human real estate areas.
The main obstacle for sanctuaries is that costs for the land and the predator proof fencing are significant and do not end with the establishment of the sanctuary. Ongoing maintenance and monitoring, repair and replacement are paramount and costly. 
Predator-proof enclosures might only last for 15-25 years without major reconstruction, a time frame that is hardly sufficient for providing security for threatened species in perpetuity. (Long et al, 2004)

Smaller enclosures which are easier and cheaper to manage are unlikely to provide persistent habitat for a population of a viable size.

Securely fenced, well monitored sanctuaries can provide maximum protection and reduce the risk of extinctions but it is questionable whether the animals will retain their potential for evolutionary change and long-term viability. (Hayward and Kerley, 2009)
For prey species it would also be highly desirable to evolve the ability to coexist with exotic predators at low densities particularly if eventual release beyond the fences is an aim. (Short, 2016) However, so far the threshold levels of predation that can be tolerated by the various species are unknown. (AWC, 2016)

Safely fenced- in animals will not be able to adapt to any new threats in the natural environment and naivety towards predators will increase with the absence of any predation risk. The animals might lose their ability to recognise predators and to develop appropriate defence mechanisms. (Blumstein et al, 2005) The result could be a loss of a species’ fitness. (Southgate, 2014, Moseby et al, 2015)

Fences are a barrier to dispersal and problems such as over-population, damage through over-grazing, habitat degradation, low genetic diversity and inbreeding are likely in the long run. (Moseby et al, 2015a)
These same factors also rendered populations relocated to predator-free islands inherently vulnerable to extinction in the long term. (Eldridge and Herbert, 2015)

It seems an unresolved problem how to estimate carrying capacity of any given area and there are examples in the literature of reintroduced species that have increased so many-fold that they significantly damaged the habitat through over-browsing. (Kemp et al, 2015)
Habitat management is clearly a continuing requirement.

I am unsure if there ever was any trial or whether this is merely a consideration, but the introduction of native predators could regulate populations of native species within an enclosure if they ever bred up to unsustainable numbers. In the case of ringtail possums, chuditch or carpet pythons would come to mind.
Some fence designs would also let animals leave the enclosure but not enter it from the outside. (Hayward et al, 2014).

Fences as such are not without risk. Deaths of night flying and ground-nesting birds, medium to large reptiles and kangaroos were associated with collisions or entrapment in fences. The inclusion of electric wires also adds the risk of electrocution (Hayward et al, 2014).
Fences also carry a risk of failure through erosion or if a tree should fall onto them in a storm.

As foxes are good climbers and can jump fairly high, a tall, strong fence also dug deep enough into the ground is necessary. A range of fence designs have been tested and are effective (Moseby and Read, 2006) but come at a high price. To exclude cats there also needs to be an overhanging top part to the fence that cannot be traversed.

The initial search and removal of the last individual cat or fox will add considerable expenditure of time and material resources. (Read and Bowen, 2001)
However, a reliable monitoring system that rapidly detects breaches might be the most important issue as a single breach by a predator could have catastrophic consequences. (Fleming et al, 2014) 80% of Australian predator-proof enclosures have recorded breaches. (Long et al, 2004)

However, fenced areas are very popular with the public which could lead to an excessive focus on enclosures and shift the scarce funding away from the development of new techniques that might help native species to coexist with predators – the ultimate goal of all conservation activities. (Hayward et al, 2014)