The Australian government unleashed a strain of a hemorrhagic disease virus into the wild earlier this year, hoping to curb the growth of the continent's rabbit population. This move might sound barbaric, but the government estimates that the animals—brought by British colonizers in the late 18th century—gnaw through about $115 million in crops every year. And the rabbits are not the only problem. For more than a century Australians have battled waves of invasive species with many desperate measures—including introducing nonnative predators—to limited avail.

Australia is not the only country with invasive creatures. But because it is an isolated continent, most of its wildlife is endemic—and its top predators are long extinct. This gives alien species a greater opportunity to thrive. “In other places, you'll see a much bigger predator community,” says Euan Ritchie, one of the directors of the Ecological Society of Australia. But the Tasmanian tiger, the marsupial lion and Megalania (a 1,300-pound lizard) are gone. The only top predator left, the Australian wild dog, or dingo (photograph), is under threat from humans because of its predilection for eating sheep.

Along with rabbits, Australia is trying to fend off red foxes (imported for hunting), feral cats (once kept as pets), carp (brought in for fish farms) and even camels (used for traversing the desert). Wildlife officials have attempted to fight these invaders by releasing viruses, spreading poisons, building thousands of miles of fences, and sometimes hunting from helicopters. In one famous case, the attempted solution became its own problem: the cane toad was introduced in 1935 to prey on beetles that devour sugarcane. But the toads could not climb cane plants to reach the insects and are now a thriving pest species themselves.

Despite scientists' protestations, the government plans to introduce another virus later this year to try reducing the out-of-control carp population. “We can't go back to the past,” Ritchie says. But “we have a lot of native mammals and other species that are holding on.”