Australia's quokkas have been called the "happiest animals in the world," but numbers of the friendly, contented marsupials have been declining in recent decades.

New research -- conducted by scientists at Vanderbilt University and published recently in the Journal of Zoology -- suggests invasive species, primarily foxes, are responsible for the quokka's decline.

"Australia has experienced catastrophic losses due to warming temperatures, drought, and the combination of these effects on resident animals," Larisa DeSantis, Vanderbilt biologist and co-author of the new study, said in a news release. "The iconic wildlife Australia is best known for, evolved largely in isolation and has been in decline since Europeans introduced foxes, rabbits, goats, and other animals that have preyed upon and/or competed with native animals for food and water."

The quokka is a cat-sized macropod and the sole member of the genus Setonix. The herbivores look like small kangaroos and are most active at night.

Previous efforts to explain the decline of the quokka have yielded mixed results. Some studies have put the blame on climate change and shifts in vegetation, while others have emphasized the harmful effects of invasive species and hunting.

For the new study, scientists analyzed the teeth of both fossil and modern quokka specimens. By studying the enamel layers of tooth samples, researchers were able to determine the kinds of plants mainland and island quokka populations have consumed through time.

"Piecing together the ecological history of the quokka helped us better understand why they are an isolated and vulnerable species today," said lead author Elinor Scholtz, an undergraduate student at Vanderbilt. "We learned that quokkas on mainland Australia today occupy denser forests than in the past, likely to avoid predation by foxes. In contrast, quokkas typically live in more open habitats and feed on tougher vegetation on islands that lack foxes."

While the latest research suggests invasive species have historically been the main driver of quokka losses, their declines have left them increasingly vulnerable to other environmental stressors.

Rottnest Island is one of the few places where quokkas have been able to avoid the fox, but every year, the species suffers large die-offs when prolonged droughts precipitate declines in available grasses and shrubs.

As global temperatures continue to rise and extreme weather becomes more frequent and severe, researchers suggest quokkas are likely to become more vulnerable to drought and wildfire.

"To put this all in perspective, the entire geographic range of quokkas is only a fraction of the size of the forests that were completely decimated from fires during one year in Australia," said DeSantis. "We are essentially playing roulette with native species in Australia, and the odds are stacked against quokkas and many other native animals in the face of invasive species, fires, and the current climate crisis."

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