Mesopredator release

Foxes and cats are having a huge impact on vulnerable critical weight range wildlife species; however they may also impact each other, either through competition for resources or through direct aggression. Those two most prevalent predators have strongly overlapping patterns of resource use, particularly dietary overlap and can therefore be seen as potential competitors. (Molsher, 1999)

In direct encounters foxes usually dominate cats, even attack and injure or kill them. Studies have also found cat remains in fox stomachs (intraguild predation). (Risbey et al, 1999)

In ringtail translocation programs predation by cats seems to increase in the presence of fox control. Consistent with this finding, many studies claim that foxes limit the abundance and distribution of cats. (Risbey et al, 2000)

The term ‘Mesopredator Release’ was created by Michael Soule (Soule et al, 1988) to describe the interaction between predators of medium size (such as cats and foxes) and large apex predators such as dingos. Apex predators limit the abundance of smaller predators and that way reduce the negative impact on prey species.  

There are no apex predators (those at the highest trophic level) left in the south-west of WA and the fox is not of a higher order than the cat and not an apex predator (Doherty et al, 2014) but the term is nowadays used in a more general way when any predator weighing 3-20 kg influences the effects of another middle-rank predator on prey. (Brashares et al, 2010)

Apex predators are historically deemed ‘undesirable’ because they typically have a more carnivorous diet than mesopredators and would prey on livestock. 
It can however be argued that Australia-wide control and eradication programs led to the unleashing of mesopredators.

Whether mesopredator release is at work is often hard to prove as other factors such as natural seasonal population variability or diminishing habitat quality could have resulted in similar outcomes.
Trophic bottom-up processes (food availability) could influence population changes more strongly than trophic top-down processes like predation. (Brashares et al, 2010)
During periods of drought depleted populations could be driven to extinction by predation but it would be difficult to ascertain whether the issue was predation or mesopredator release or basically a consequence of the drought. 
Also, not all studies agree that a stronger predator (fox) will suppress a weaker one (cat) and its impact on prey. Predators can adjust their activity patterns and use the same hunting ground, just at different times. (Mitchell and Banks, 2005)
The potential loser of a fight might flee (and wait) or adopt vigilance behaviour. (Molsher, 1999)
Some studies noticed that cats avoided some areas within their home range. As prey abundance was not different there to other sections, avoidance behaviour due to earlier fox encounters could be the reason. (Brashares et al, 2010, Buckmaster, 2011)