More than half of all U.S. National Parks are overrun with invasive animal species, such as rats, pythons and feral hogs, according to a new study published in the journal Biological Invasions. These invasive species severely threaten native plants and animals and pose a “deep and immediate threat” to the mission of the National Park Service (NPS), and yet there is no comprehensive program in place to deal with the problem, the research found.

The National Park Service currently manages 419 park units covering more than 84 million acres of land in the U.S. Scientists found there are 311 invasive animal species threatening these parks, ranging from cats to aquatic species such as lake trout and the quagga mussel. But while the Park Service has had a broad invasive plant management program in place for almost two decades, there is no equivalent program for invasive animal species. In fact, NPS has management plans for just 23 percent of the 1,409 reported invasive animal populations in U.S. national parks. And only 11 percent of those populations are designated as “under control” by the parks.

Invasive animal species significantly impact native wildlife populations, introduce new diseases, impair visitors’ enjoyment of parks and require substantial economic investments for control efforts. In Virginia, for example, an infestation of hemlock woolly adelgid has caused “extensive mortality” in hemlock stands along the Blue Ridge Parkway and in Shenandoah National Park, killing trees in as little as three years. And an outbreak of Burmese pythons, some as long as 23 feet, in the Florida Everglades since 2000 has led to sharp declines in native mammals like raccoons and opossums.

“If we don’t take action, native species will continue to struggle due to the invasives,” Ashley Dayer, a wildlife biologist at Virginia Tech and lead author of the new study, said in a statement. “But taking action is no small feat; it requires the commitment and resources of the National Park Service, neighboring lands and the public.”

The findings are the result of a three-year project led by the National Park Service to assess the risks caused by invasive animals in parks and to determine potential solutions. “The NPS is very concerned about nonnative and invasive species across the landscape within and outside of national park units and their impacts on native biodiversity, especially at-risk species and their habitats,” Elaine Leslie, the former chief of the NPS Biological Resource Management Division, said in a statement. “Nationally and internationally, the world is losing native biodiversity at an alarming rate. Threats from invasive species play a critical part in this loss.”

This article was originally published by Yale Environment 360. Read the original story here.