Pandas face a new threat to their survival, U.S. researchers say -- livestock, particularly horses, are horning in on pandas' critical bamboo buffet. "Across the world, people are struggling to survive in the same areas as endangered animals, and often trouble surfaces in areas we aren't anticipating," Michigan State University panda expert Jianguo "Jack" Liu said. "Creating and maintaining successful conservation policy means constantly looking for breakdowns in the system. In this case, something as innocuous as a horse can be a big problem." MSU doctoral student Vanessa Hull, who's spent years tracking pandas in China's Wolong Nature Reserve, most recently tracking pandas she's equipped with GPS collars, said she started noticing it wasn't just pandas chowing on bamboo. "It didn't take particular panda expertise to know that something was amiss when we'd come upon horse-affected bamboo patches," she said." They were in the middle of nowhere and it looked like someone had been in there with a lawn mower." She said she learned some Wolong farmers, who traditionally hadn't kept horses, started to raise them for much needed cash. Horses were barred from designated grazing areas because they competed with cattle, so farmers would let them graze unattended in the forests -- where they developed a taste for bamboo, the researchers said. "Livestock affect most of the world's biodiversity hotspots," Liu said. "They make up 20 percent of all of the earth's land mammals and therefore monopolize key resources needed to maintain the earth's fragile ecosystems." Although the horse problem has been somewhat alleviated by Wolong's reserve managers, who have banned horses from the reserve, the study has shed light on how competitive livestock can be in sensitive habitat, the researchers said.