For more than 150 years, Australia has been plagued by rabbits. First introduced by an English settler as hunting fodder in 1859, the European rabbit population soon ballooned to an estimated 10 billion, contributing to extensive environmental damage and the extinction of some native species. Over the past century, biologists tried—and largely failed—to stem the tide with fences, poisons, and pathogens.
Now, an accidental approach seems to be taming the invasion. Since scientists unintentionally released a deadly rabbit virus in 1995, it has wrought havoc on the bunnies—and allowed some endangered native mammals to recover, according to a new study in the journal Conservation Biology.
Ironically, Australia’s viral progress began with a mortifying error. Government researchers were experimenting with the rabbit haemorrhagic disease virus (RHDV) on Wardang Island, off South Australia’s coast, when renegade flies picked up the pathogen and transported it to the mainland. Luckily, the containment failure became a smashing success: The virus eradicated an estimated 60% of Australia’s rabbits, acting with particular lethality in arid areas. The government officially released the disease in 1996.
As RHDV spread, researchers documented encouraging ecosystem changes. Native vegetation bounced back, and populations of large herbivores such as kangaroos increased.
Still, no one was quite certain how RHDV’s advance was affecting some groups of animals, including Australia’s small desert mammals. Several rodents, such as the dusky hopping mouse and the plains mouse, had nearly vanished during the rabbit takeover. So had the crest-tailed mulgara, a hamster-sized marsupial that preys on lizards and insects. Both the dusky hopping mouse and the plains mouse are considered vulnerable by the International Union for the Conservation of Nature (IUCN), and the mulgara is listed as endangered in South Australia.
Recently, however, scientists have noticed hints of a startling comeback. Driving through the desert at night, for instance, biologists now “see dusky hopping mice in your headlights everywhere,” says Reece Pedler, an ecologist at the South Australian Department of Environment, Water and Natural Resources. “It’s become pretty clear that something major has changed.”
To get a clearer picture, Pedler and colleagues pulled together 45 years of mammal trapping surveys conducted by the state government, mining companies, and nongovernmental groups. When they compared records from before and after RHDV’s arrival, they found that small mammal populations had skyrocketed in the years following the introduction of the rabbit virus. The crest-tailed mulgara increased its “extent of occurrence”—the area in which a species has been sighted—nearly 70-fold. The dusky hopping mouse and the plains mouse more than tripled and doubled their occurrence, respectively.
The surveys suggested that RHDV deserved credit for the resurgences. South Australia’s small mammal populations tend to boom after heavy rainfalls, but Pedler notes that the biggest recoveries happened in dry years. Correlation may not equal causation, but, Pedler says, “everything is pointing toward the reduction in rabbits” as the cause of the rebound.
The findings fit with what other biologists are seeing. According to Brian Cooke, an ecologist at the University of Canberra who was not involved with the new study, “there have been remarkable changes in arid zone vegetation since RHDV was released 20 years ago.” Cooke’s own research has documented ecological benefits in a different suite of species: When he removed rabbits from experimental plots, he observed recovery in kangaroos and wombats. “With data for small mammals pointing in the same direction, it is making an even stronger case” for the benefits of RHDV, Cooke says.
Why would a bunny bust lead to a native mammal boom? Pedler offers two explanations. First, the end of rabbit overgrazing allowed native vegetation to grow back, providing food and shelter for mice and mulgaras. But that’s not all. When Pedler and his team poured over the trapping records, they observed precipitous declines in rabbit predators such as feral cats and foxes. The virus had likely rippled through the food web, starving these invasive predators, which had been devouring native species along with rabbits—a chain reaction called a trophic cascade.
Australia annually spends millions of dollars removing cats and foxes, which are responsible for most of the country’s mammal extinctions. According to Pedler, however, further viral introductions offer an efficient ecological alternative to traps and poisons. “Cats and foxes could be controlled much more cheaply, at a much larger scale, by removing their rabbit prey,” Pedler says.
Thanks to RHDV, the small mammals now recovering from the rabbit menace may qualify for downgrading on IUCN’s “red list.” And viral control could soon become even more effective. Later this year, Australia plans to unleash a new strain of RHDV, which is expected to work better in moist climes. “Rabbits are so ubiquitous they are just regarded as part of the background by many people,” says Cooke—but perhaps not for long.