When an otherwise nocturnal wombat shows up in the daylight, acting lethargic and having trouble walking, you know that animal is in trouble.

When thousands of wombats turn up sick, emaciated, balding and dying, you know you have a crisis.

That's what's happening in Murraylands, South Australia, where up to 85 percent of the region's southern hairy-nosed wombats (Lasiorhinus latifrons) are sick or dying, apparently the victims of invasive plants that have taken over the local ecosystem.

Brigitte Stevens, founding director of Wombat Awareness Organization, was one of the first to observe the sick wombats three years ago. At first glance the animals appeared to be suffering from a balding disease such as sarcoptic mange, which has previously been observed in wombats. But the problem got worse each year and by this May Stevens had documented more than 2,800 dead or dying wombats. "Some of them just lying down on their sides, just eating dirt, [and] can't even lift their heads," she told the Australian Broadcasting Corp. She thinks many more wombats may be sick or dead in their underground burrows where they can't be counted.

Necropsies performed on several of the wombats by wildlife veterinarian Wayne Boardman and veterinary pathologist Lucy Woolford, both lecturers at the University of Adelaide, ruled out mange. Instead, the animals were found to be suffering from a liver disease, most likely caused by an innocuous-sounding plant called potato weed (aka common heliotrope or caterpillar weed, Heliotropium europaeum). Potato weed, which was introduced to southern Australia in the 19th century and has since spread across most of the continent, contains pyrrolizidine alkaloids (PAs), chemicals that protect them from insects but which can be fatal or dangerous to many animals, including livestock such as pigs, cattle and horses.

At least three other invasive plants were found in abundance in the region, none of which are palatable to wildlife, but potato weed is the only one containing the dangerous PAs. The native Mitchell grass (several species of the genus Astrebla) and spear grass (Heteropogon contortus) that historically formed the bulk of the wombat's diet appear to have been squeezed out of the region.

The preponderance of invasive plants and lack of edible vegetation has a twofold effect on the wombats, Boardman says. Some of the animals are malnourished, which is partially responsible for their fur loss (alopecia). "Some die and some will undoubtedly get better—given more nourishing food," he says.

But those who ingest the potato weed are much worse off. "Potato weed leads to hepatic necrosis [liver death]," Boardman says. This in turns leads to the production of chemicals that make the skin more sensitive to ultraviolet light. The animals then suffer severe sunburns, which lead to vasculitis as well as necrosis of the skin.

Research into the effect of potato weed and other invasive flora on the wombats is ongoing. Writing for the Australian site The Conversation, Boardman and Woolford suggest that it could be possible to "remediate" the wombats' habitat by killing off the invasive weeds and replanting native vegetation, but further study is also necessary to find out why the plants took over that region in the first place.

Southern hairy-nosed wombats are not currently an endangered species, but with up to 85 percent of the Murraylands's wombats affected by this liver disease, that's an assessment that may not last much longer.

Photo 1: A healthy southern hairy-nosed wombat by James Reed via Flickr, used under Creative Commons license. Photo 2: A dying wombat, courtesy of Wombat Awareness Organization. Photo 3: Potato weed in Spain by Ferran Turmo Gort via Flickr, used under Creative Commons license