Rodents – rabbits – prey switching

Rodents such as rats seem of low concern regarding ringtail possum conservation.
However, they are omnivorous predators and impact on native ecosystems in various ways. They prey on small reptiles and mammals, birds and their eggs, they compete for resources with native mammals and if plentiful they provide a food source for predators and that way keep their numbers up.

There is no doubt though that rabbits have directly impacted on native animals and are impacting on ringtail possum populations.
Usually occurring in high densities, rabbits potentially devour plants to a degree that regeneration of palatable trees and shrubs is inhibited. They destroy shelter and they support predators such as cats and foxes which can elevate their abundance and increase predation pressure. (Krull et al, 2015)

As early as 1950 the myxoma virus – the causal agent of the disease myxomatosis – was released by CSIRO into the environment in order to decimate rabbits that had reached plague proportions. Initially the virus was highly successful, however within a decade, co-evolution between virus and rabbits has let to genetic resistance and the survival of more and more animals. (Cooke, 2012)

Another viral pathogen, rabbit haemorrhagic disease virus (RHDV) or rabbit calicivirus also proved successful after it ‘escaped’ from laboratory conditions in 1995 but efficacy has been variable by region. (Parkes et al, 2002, Story et al, 2004) However, it at least temporarily not only resulted in the significant reduction in rabbit numbers but also in a decline in feral cats and foxes and reduced predation on native species. (Holden and Mutze, 2002)
Unfortunately development of resistance has been quick again (Parkes et al, 2008) and a new variant of RHDV which will hopefully override resistance developing in rabbits has been released in 2017. Months after the release rabbit populations are still extreme though and a multi-pronged approach – poison, warren destruction and fumigation – might still be needed.

When rabbit numbers are significantly reduced, introduced predators will turn to other prey such as our naïve wildlife. This prey switching could seriously threaten a small population of endangered western ringtail possums and even lead to local extinction. Predator numbers might only decline when there is nothing left to protect.
Any control of rabbits needs to work in concert with effective control measures for foxes and feral cats.