In 1924, a film crew descended on California’s Catalina Island to shoot a silent Western called The Vanishing American. When the filming was finished, the crew members packed up and left. Legend has it, however, that they abandoned several of their extras. The castoffs—14 American bison—took up residence in the arid hills of this rocky island.

In the decades that followed, the bison did what bison do: The animals grazed and bred. Each spring a new batch of calves was born. By the late 1980s, some records suggest the herd had swelled to more than 500 animals (pdf). On this small island where no bison had ever lived before, these unlikely ungulates thrived.

The buffalo’s success, however, came at the expense of Catalina’s vegetation. The island, covering just 75 square miles, is home to more than 400 native plants, several of which are found nowhere else in the world. The free-roaming bison’s voracious appetite, sharp hooves, and penchant for scuffing out wallows—dusty depressions where the animals roll—took a toll on the grasslands. The bison and their shaggy coats also helped spread the seeds of non-native plants.

The Catalina Island Conservancy, a land trust charged with managing most of the island and its wildlife, took charge of the bison in the 1970s. To shrink the herd and prevent overcrowding, the agency began shipping bison to the mainland to sell at auction. Some were eventually slaughtered.

In 2003, the Conservancy found another way to thin the herd. The organization shipped more than 100 animals back to their ancestral home, the Great Plains. The bison, some weighing more than half a ton, first had to be ferried to the mainland and then trucked to South Dakota, where they now live on the Cheyenne River and Standing Rock Lakota Indian reservations. More bison made a similar journey in 2004.

But trucking bison cross-country has its downsides. It’s expensive, and it’s stressful for the animals. In 2009, the Conservancy tried another tactic: birth control. The females receive a yearly injection of a wildlife contraceptive called PZP, short for porcine zona pellucida. The vaccine consists of protein from the membrane surrounding unfertilized pig eggs. When injected into a bison, this protein triggers the production of antibodies, which then bind to the membrane surrounding the buffalo’s own unfertilized eggs and prevent sperm from fertilizing the egg.

The birth control project seems to be working. The number of newborn calves fell from 29 in 2010 to just five last spring. “The bison do what they enjoy. They just don’t have babies,” says Bob Rhein, a spokesperson for the Catalina Island Conservancy. And it’s far cheaper than trucking the bison back to the Great Plains.

Of course, there is another, perhaps simpler solution to the bison problem. The Conservancy could ship them off the island permanently. That option may make sense from a conservation perspective, but it has little support among Catalina’s residents. The herd is a part of the island, says Patricia Maxwell, marketing director at the Catalina Island Conservancy. “The bison are beloved.”