When the calici virus culled rabbits in 1997, the rabbits’ predators (foxes and cats) are suspected to have wiped out a population of rock-wallabies in their attempt to fill their bellies. Hugh McGregor from UTAS is researching predation pressure in an attempt to understand the native species most vulnerable to prey switching.
A blog written by Dr Hugh McGregor
Postdoctoral Research Fellow
School of Biological Sciences, University of Tasmania
TSR Hub sub-project 1.1.6 Integrated management of feral herbivores and feral predators
Calicivirus is arguably the most successful biocontrol to have been released in Australia. When released in 1997, rabbits went from plague proportions
to scarce in a matter of weeks. Rabbit numbers have been slowly recovering since as they have become more and more immune, but the virus 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.
Rabbit impacts have been studied in detail around Roxby Downs over the last 30 years. When the first calicivirus came through the population, spotlighting
counts showed that rabbit numbers crashed to virtually nothing. This had major flow-on effects. Cats and foxes lost their main food source, so not only
did their densities 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. In the long term however, the knockdown of rabbits appeared to have some incredible benefits. Cat and fox densities
fell and have stayed down since calici, and over the 20 years since many native animals have rebounded in South Australia.
My project looks specifically at the mechanism that drives changes in native prey density with rabbit control by comparing the extent of prey switching
versus changes in feral predator density. 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 (RDVH2) is an altered form of the original, and has been killing many rabbits that are resistant to the original
strain. Released in Canberra, and 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.
By tracking densities 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 using small video cameras and fine-scale GPS data. This will enable me to measure the actual kill-rates of feral cats, and estimate
the total predation pressure they exert 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
Figure shows spotlighting transects. Densities of rabbits, cats and foxes before and after the original calicivirus (RHCV1) swept through central Australia.
This is based on regular spotlight counts conducted around Roxby Downs region.