LOS ANGELES (Reuters) - Three groups of California’s rare island fox were removed from the U.S. endangered species list on Thursday, and a fourth was downgraded to threatened, marking the fastest recovery yet for an American mammal once deemed to be on the brink of extinction.

An Island fox is surrounded by vegetation in this undated National Park Service photo in California. Courtesy National Park Service/Handout via REUTERS

The population of the four subspecies in question on California’s Channel Islands, which had plunged to fewer than 200 animals during the late 1990s, has bounced back to nearly 6,000 as of 2015, the U.S. Fish and Wildlife Service reported.

The agency said the island fox rebound was hastened by an intense recovery program that included captive breeding of the animals, removal of feral pigs from the islands and reducing an influx of golden eagles from the mainland that had become an invasive predator.

Wildlife officials also began vaccinating the foxes against canine distemper, which remains a particular threat on Santa Catalina Island, the only island in the archipelago with a significant year-round civilian population.

Hunting will not be an issue because the Catalina Island fox remains listed as threatened, and the de-listed subspecies inhabit the three largest of five other islands that comprise the Channel Islands National Park, where hunting is prohibited.

At the time they were formally listed as endangered in 2004, the fox populations on Santa Catalina, Santa Rosa, San Miguel and Santa Cruz islands had plunged by 90 percent from historic levels, and the four subspecies were given a 50 percent chance of going extinct within a decade.

As of last year, the population of all four groups had been restored to, or exceeded, historic levels, the Fish and Wildlife Service said.

The island fox, one of America’s rarest mammals, are diminutive cousins to the mainland gray fox. Roughly the size of a house cat, they weigh 3 to 5 pounds (1.4 to 2.3 kg) and stand about a foot (30.5 cm) tall. They exist nowhere on Earth but on the six largest of the eight Channel Islands off the coast of Southern California.

The subspecies on two of those islands, San Clemente and San Nicolas, were never listed. The two smallest islands, Santa Barbara and Anacapa, have no foxes.

The de-listing of the island fox, 12 years after it was classified as endangered, now ranks as the quickest recovery for any mammal so protected. The next fastest was the Stellar sea lion, de-listed after 23 years in 2013, according to the Fish and Wildlife Service.

More than 99 percent of all plant and animal species listed under the Endangered Species Act have been prevented from going extinct since the law was enacted in 1973, the agency said.