A herd of bison roams on the Fort Peck Reservation near Poplar, Mont., in 2012. (Matthew Brown/AP)

Even for a park with a history of unhappy encounters between people and wildlife, 2015 is shaping up as an eventful year for Yellowstone and its bison. Since mid-May, five visitors have been hurt — gored, trampled or tossed into the air — in run-ins with the park’s most famous residents.

The tourists all came away with treatable wounds and memorable stories. For bison, however, the year’s brushes with humans didn’t always end as well.

Since January, more than 500 of the woolly beasts — the most in years — have been chased onto trucks by government workers and hauled to slaughterhouses. Some 200 others that wandered off park grounds were rounded up in a similar fashion or stalked by hunters and shot. Next year’s takings are expected to be still higher, a consequence of a surging population and strict rules that park officials themselves find difficult to carry out.

“It is hard to watch,” acknowledged Rick Wallen, Yellowstone’s lead wildlife biologist for bison, describing the methods used to capture and restrain the animals. “But we do it as quickly and efficiently as possible.”

The culling of the bison is a routine, if little-known, facet of managing Yellowstone’s roughly 4,900 bison, the country’s biggest and wildest repository of descendants of the great herds that once roamed the Western plains. Under a 15-year-old agreement signed by federal and state agencies, Yellowstone officials are required to keep a lid on the bison population, limiting the numbers of animals and controlling their wanderings to appease farmers and cattlemen who object to intrusions of hungry bison onto their fields and grazing lands.

To keep bison numbers in check, workers spend the winter months corralling bison that venture near the park’s northern entrance, keeping them in pens until they can be shipped to nearby meatpacking plants. Some animals that cross into state or private lands face a gantlet of hunters, winners of state-run raffles that award a small number of licenses to kill bison that stray from their preserve.

Yet, despite expensive control efforts — the annual cost to taxpayers is about $2 million — the herd continues to swell. A February report by Yellowstone officials found that the bison’s population “is prolific and has recovered rapidly” from culling, conceding that the park’s management plan “underestimated bison reproduction and survival rates.”

While the management plan calls for capping the number of bison at about 3,000, the park’s population could soon approach twice that level. Restricted from relocating the animals to private lands, the park and its bison face an acute shortage of options.

“As a result,” the report said, “more bison must be removed to regulate the population.”

Park officials are pushing for changes that could result in alternatives to slaughter for the bison, including possibly easing limits on moving disease-free animals to new locations. But changing the policies requires a buy-in from multiple bureaucracies as well as private interests that have long fought to keep Yellowstone’s bison walled off inside the park’s boundaries. The prospects for that remain uncertain, at best, park officials acknowledge.

“Everything we do with bison is complicated,” lamented Wallen, the park’s bison expert. “This is a conservation program that is steeped not only in ecology and sociology but also in politics.”

Bison, also known as American buffalo, and their calves forage for food at Yellowstone National Park in Wyoming in 2011. (Mark Ralston/AFP/Getty Images)

The park’s system for dealing with excess bison is itself a compromise, one that grew out of years of wrangling between well-organized factions with diametrically opposing goals. On one side are ranchers and state officials who adamantly oppose any further expansion of Yellowstone’s wild herds, mostly because of perceptions of bison as disease-carriers that could spread infections to domestic livestock.

Favoring more freedom for bison is a diverse coalition of groups, including wildlife enthusiasts opposed to any killing of bison and Native Americans who want to start their own herds for use in hunting and tribal ceremonies.

Almost no one is happy with the current system. Wallen, the Yellowstone expert, acknowledged that the “park service is doing things that aren’t very park-like” in its ship-to-slaughter program. Wildlife groups criticize the management plan as a tarnish on what is arguably one of the most successful species-recovery stories of modern times: the regeneration of a thriving bison herd from a depleted stock that a century ago numbered 23 animals.

“If we can do it in Yellowstone, we should be able to do it in many more places,” said Kit Fischer, an outreach coordinator in the National Wildlife Federation’s Missoula, Mont., office. “But we still haven’t figured out how to deal with bison in the one spot where they’ve always been.”

The Obama administration, the third to oversee the controversial management plan, has acknowledged the need for a major overhaul. Park officials agree, and some — with heavy support from wildlife advocates — are pushing for the adoption by next year of new rules that would include alternatives to converting wild bison into ground meat.

Some advocates perceive a shrinking window, with 17 months remaining in the Obama presidency. A new administration of either party would probably push aside the issue for months, and perhaps years, to conduct further studies, said Bart Melton, the Montana-based regional director for the National Parks Conservation Association, a nonprofit organization dedicated to protecting national parks and their wildlife.

“We’ve now had 15 years of the current management plan, and we have the science and the understanding on the ground of what it will take to see success in bison management,” Melton said. “Now all we need is the political courage to make it happen.”

Yellowstone’s lush valleys have been home to bison since long before the area was designated as the country’s first national park in 1872. By then, the legendary herds that once swarmed the Great Plains had shrunk from an estimated 30 million to a few thousand. The huge beasts — adult males grow as tall as 5 feet at the shoulder and weigh a ton or more — were all but wiped out in a few decades, thanks in part to government policies that deliberately sought to exterminate the animals to deprive Native American tribes of a primary source of food and hides.

In the early years, an Army garrison helped to protect the park’s remaining wild bison from poachers. Scientists now say that the 23 animals that were still alive here in 1920 were the last genetically pure descendants of the creatures commonly known as American buffalo.

Throughout the 20th century, more sophisticated management practices allowed the herd to grow rapidly. More than 400,000 bison exist today, the bulk of them raised for meat on private ranches, and contain a mix of bison and cattle DNA.

Yellowstone’s bison, meanwhile, remain the largest population of wild bison anywhere. While regularly exposed to people, they are allowed to migrate freely across the 3,500-square-mile park and sometimes beyond.

While usually peaceable, they are unpredictable animals that can display an aggressive streak when they perceive a threat. Despite a barrage of warnings to the park’s 3 million annual visitors, tourists are injured every year by bison, usually after venturing too close. In May, a 16-year-old Taiwanese girl was gored after posing for a photo less than six feet from a resting bison. In June, an Australian man attempted a close-up with his iPad camera and was rewarded by being hurled into the air several times. The five attacks this year are higher than normal but far from a record, park officials say.

For neighbors who live on the fringes of the park, the fear is germs, not horns. More than half of Yellowstone’s bison test positive for exposure to brucellosis, a bacterial disease that can be devastating to domestic livestock. Until a few years ago, discovery of a brucellosis outbreak on a single ranch could lead to quarantines of cattle sales across an entire state.

The quarantine rules have been relaxed, yet cattlemen’s groups and their political allies have continued to fight in state legislatures and the courts to ensure that Yellowstone’s bison are confined to park land.

“This isn’t about wildlife or the cattle industry — it’s about disease control and protecting our livestock from the awful, contagious disease brucellosis,” Bob Hanson, president of the Montana Farm Bureau Federation, said in testimony before a state committee that was considering relaxing state oversight of wild bison.

The park’s bison experts and local wildlife groups say that the fears are exaggerated. While brucellosis infection rates among Yellowstone’s wild herds are high, there have been no confirmed cases of cattle contracting the disease from bison wandering outside the park, scientists point out. Meanwhile, there is little public outcry about migrating herds of wild elk, which also carry the disease.

Fischer, the National Wildlife Federation official, said resistance to changing the rules is deeply ingrained among established ranching families whose forebears settled the territory, driving out wild animals that threatened their livestock.

“If you walk into a bar in Montana, there are two animals you don’t want to talk about: bison and wolves,” he said. “The notion of bringing back these animals — of re-wilding the West — is contrary to their personal beliefs. For some, it’s like trying to bring back the dinosaurs. Why would you do it?”

Yet, “re-wilding” parts of the West will probably be a key part of any future solution, Fischer said.

“It’s more about social tolerance than biological limits,” he said. “Ultimately, it’s a question of how much wildlife humans are willing to accept on private lands.”