Carnivores living near people get as much as half their calories from human food, according to a new study published Monday in the journal PNAS.

For many wild animals, human food is the equivalent of junk food -- and researchers worry the lure of junk food could disrupt North America's carnivore-dominated ecosystems.

When scientists conducted chemical analysis of bone and fur samples from more 700 different carnivores across the Midwest, they found animals living closer to human population centers were more likely to include human food in their diet.

Animals evolve to win competition, but they also evolve to avoid competition, if possible. As a result, many predators target niche prey.

But as humanity's footprint expands and more and more predators develop a taste for human food -- out of desperation, convenience or both -- scientists worry many carnivores will find themselves increasingly in conflict with one another.

The incursion of human food could threaten the relationships between predators and prey, researchers warn, undermining ecological balance achieved over thousands of years. As well, changes in diet could increase the odds of human-animal conflict.

The latest analysis showed the carnivores living in human-altered habitats got an average of 25 percent of their calories from human food. And some species were more likely to eat human food than others.

"What you see is that the sort of generalist species that you might expect -- coyotes, foxes, fishers, martens -- in human-dominated landscapes, they're getting upwards of 50 percent of their diet from human foods," lead study author Phil Manlick said in a news release.

"That's a relatively shocking number, I think," said Manlick, a postdoctoral researcher at the University of Mexico.

Manlick conducted the research while working as a graduate student at the University of Wisconsin, under the tutelage of Jon Pauli, a professor of forest and wildlife ecology.

The researchers were able to identify the diets of different carnivores by analyzing the ratios of carbon isotopes in their bones and fur.

"Isotopes are relatively intuitive: You are what you eat," said Manlick. "If you look at humans, we look like corn."

The carbon signatures of wild foods are markedly different than the carbon signatures of human foods, which are typically heavy in corn and sugar.

Moving ahead, scientists hope to identify and more precisely characterize the ways human foods are altering local food chains and reshaping ecosystems.

"When you change the landscape so dramatically in terms of one of the most important attributes of a species -- their food -- that has unknown consequences for the overall community structure," said Pauli. "And so I think the onus is now on us as ecologists and conservation biologists to begin to understand these novel ecosystems and begin to predict who are the winners and who are the losers."

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