What brought bison back from the brink of extinction?

Although their numbers shrank to a perilously small figure at the end of the 19th century due to overhunting, bison weren't federally classified as an endangered species in the United States. Instead, private and public conservation efforts gradually nudged their population upward, with the greatest increase occurring in the last 40 years.

Nevertheless, the U.S. wild bison population today is less than one percent of what it was in pre-colonial times, hovering at around 20,000 animals. North America is home to only five free-ranging plains bison herds and eight wood bison herds [source: World Conservation Union Bison Specialist Group]. But it isn't for lack of trying.

Public bison preservation efforts began in 1907 when 15 were relocated to New York's Bronx Zoo. Yellowstone National Park had only a few dozen bison roaming on its land at that time. Offspring from that initial group would later be relocated to protected areas in Oklahoma, Montana, South Dakota and Nebraska [source: Wildlife Conservation Society]. Native American tribes have also contributed to repopulation efforts in recent years. In 1990, tribes around South Dakota met to form the InterTribal Bison Cooperative, and today the group is comprised of 57 member tribes that oversee 15,000 heads of bison.

A handful of factors have inhibited more robust bison population growth. Perhaps most influential is the sheer lack of space. Bison are prairie grazers, and much of their native land has been developed. Because the existing bison herds have remained relatively small, that has also diminished the diversity of the bison gene pool. Once that happens, inbreeding can increase the rate of health problems in a herd.

Speaking of health problems, a virus called brucellosis plagues wild bison, particularly those in Yellowstone National Park. Brucellosis causes female bison to abort their babies and reduces fertility and milk production. In addition, people can catch it from eating tainted meat and develop prolonged flulike symptoms [source: USDA]. As a result of the virus and the animal's naturally unpredictable behavior, more than 3,700 bison that have wandered outside of Yellowstone in the past 20 years have been shot by state and federal wildlife officials [source: Martin]. Quarantine periods have delayed wild bison from being relocated to other areas in the West in order to expand the population, although the USDA plans to do so, possibly in Alaska, during winter 2008 [source: The Economist].

Ironically, commercial breeding has had a greater impact boosting bison numbers, and more than 95 percent of bison are privately owned. People began breeding herds of bison in captivity as early as 1870 [source: Lott and Greene]. Then, in the 1970s, ranchers started buying more bison to build up a niche meat market [source: Cloud]. From a financial standpoint, investing in bison is a thrifty move for ranchers since the grass grazers don't require costly feed and their meat is low in fat and cholesterol. According to the USDA, the bison market has gradually expanded in the United States, from less than 18,000 commercial bison slaughtered for sale in 2000 to around 50,000 in 2007. Thanks to the growing demand, there are around 400,000 commercial bison living in the United States.

Granted, commercial bison are raised in order to be killed later, so it doesn't exactly constitute a resurgence in pure conservationist terms. Nevertheless, it has raised the profile of the bison and triggered a newfound appreciation for the animal -- even if it is an appreciation for how it tastes.

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