I am about to start a project here at Arid Recovery that will study how a new strain of a virus used to control rabbit numbers will affect the ecosystem here. What will a drop in rabbit numbers do to the population of cats and foxes, as well as our threatened native species?
The original strain of calicivirus is arguably Australia’s most successful biological control method – it had huge impact on rabbits, which have caused untold ecological damage across the country. When the virus was released in 1997, rabbits went from plague proportions to scarce in a matter of weeks.
Rabbits fighting back
Rabbit numbers have been slowly recovering since as they have been evolving immunity to the virus, but it is still out there doing its job such that rabbit numbers have never quite recovered to pre-1997 levels. The removal of so many rabbits from the landscape in a short space of time had a huge impact across the whole ecosystem.
Predators turn on the natives
Rabbit impacts have been studied in detail around Roxby Downs over the last 30 years - check this paper out! When the first calicivirus came through, the population rabbit numbers crashed to virtually nothing. This had major flow-on effects. With fewer rabbits, cats and foxes lost their main food source so not only did their populations plummet, but they also became a lot hungrier and turned to eating more native animals in a process known as prey switching (analyses found a greater proportion of native animals in fox and cat scats after calici).
This put extra pressure on many native species, and it is possible a population of rock-wallabies went extinct as a result (follow up here). In the long term however, the knockdown of rabbits appeared to have some incredible benefits. Cat and fox numbers fell and have stayed down since calici, and over the 20 years since many native animals have rebounded in South Australia including dusky hopping-mouse, plains mouse and the crest-tailed mulgara (read more here).
A new strain of virus is coming
There is a new strain of calicivirus on its way to Roxby, to which most rabbits here are yet to evolve any immunity. My project will look at how native prey species will be affected by controlling rabbit numbers with this new strain of the virus. I will be comparing the extent to which cats and foxes switch from hunting rabbits to killing native species, versus how much the populations of feral predator will fall or rise.
We have a perfect opportunity to test this near Roxby Downs in 2016/2017 as the new strain of calicivirus becomes established in the area. This new strain (RHDV2) is an altered form of the original, and has been killing many rabbits that are resistant to the original strain.
Originating in Canberra, it has moved across south-east Australia in a wave and is currently in the Flinders Ranges. It will create a neat before and after experiment. I will use the Arid Recovery Dingo Pen as a contained area where we can track exact numbers of cats, rabbits and native animals as the drama unfolds.
Cats with cameras
By tracking numbers and locations of native animals I can make a detailed analysis of exactly what happens when rabbit numbers crash. I will measure the extent of prey switching to find out how many more native animals are killed by cats after rabbit numbers fall.
To do this, I will need to follow cats on their daily journey of destruction. I’ve been fitting cats with video cameras to record what they hunt and how they behave, as well as using small GPS units so I can see where they go. Here is an example of one of my collared cats ready to hunt! This will enable me to measure the actual kill-rates of feral cats, and estimate the impact of their predatory behaviour on native animals before and after rabbit knockdown.
I will record the area covered by each feral cat, the number of rabbits and natives killed every 24 hours and the total number of animals killed by the whole population of cats. From this I can calculate the total predation pressure.
I may find that native animals are at their most vulnerable when rabbits decline and that during those times we should ramp up predator control. Or it might make no difference. What we learn will be useful for protecting vulnerable species that persist in refuges and for planning threatened species translocations.