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Bison Roundup



For thousands of years, bison (the North American buffalo) traveled widely over the western plains of North America, grazing on prairie grasses and finding drinking water with their acute sense of smell. These enormous animals can stand as tall as six feet and weigh up to 2,200 pounds. Plains Indians hunted bison, using almost every part of the animal in some way. They ate the meat, fashioned weapons from bones, ground hoofs into glue, and made hides into ropes, blankets, clothing, and shelters. Even bison droppings served as a source of fuel. The Native Americans adapted their way of life to follow the bison herd as it randomly moved across its large home range. Less than 200 years ago, over 60 million bison roamed the western grasslands. Settlers described herds so dense and covering such a large area that the ground shook and the prairie appeared to move as they ranged the grasslands. By the late 1800s, however, less than 1,000 bison survived. Experts link the decline directly to the movement of cattle onto the plains. Cattlemen recognized the uniqueness of the plains country. Unlike eastern grasses, wild western grasses of the plains resisted drought and dried out on the open ground, providing nourishment for cattle even during winter. The cattlemen felt they faced one problem, however. Buffalo and Native Americans already occupied the grasslands. As long as they remained, the land couldn't be converted to pasture for cattle. Because people thought the bison population was limitless, few laws protected the animals. Their size and gregarious (social) behavior made bison easy targets. From 1872 to 1874 alone, over three million were killed. Railroads and the army contracted with hunters like Buffalo Bill to supply buffalo meat for employees and soldiers. Professional hides men moved into the area, hoping to make big profits off the valuable hides, and homesteaders came to settle the new land. By the late 1800s, the animal was virtually eliminated from the western plains. This dramatically affected Native Americans. Robbed of their livelihood, entire nations surrendered to the U.S. government. Most were driven onto reservations where they became totally dependent on the government. Luckily, bison were never completely eradicated. Today, about 120,000 survive. Gone are the days when huge herds roamed the prairie. Instead, constrained by boundaries and surrounded by people, bison live a life quite different from that of earlier days.


Often we cannot determine the exact number in an animal population; instead, an estimate will do. How do scientists estimate animal populations? One method is by tagging a sample. Imagine that in a region, scientists can capture 90 bison and tag them to identify them later. The next year, in the same region, scientists capture 60 bison and find that 45 are tagged. Thus, three out of every four bison captured were tagged. Scientists then use a proportion to estimate the entire bison population for the region: number of bison tagged (1st count)/population = 90/y number of bison tagged (2nd count)/population = 45/60 90/y = 45/60 To solve for y: 60 Yen 90 = 45y [Cross multiply] 5400 = 45y 5400/45 = 45y/45 [Divide both sides by 45] 120 = y Scientists could estimate that the area's bison population is 120. Now, it's your turn to predict a bison population! Materials
  • pencils
  • crayons/markers
  • polystyrene packing pieces ("peanuts" or "macaroni")
  • calculators
  • paper
  • large paper grocery bags (1 per group)
  1. Divide into groups. Ask a person not in the groups to choose (and not tell!) a number over 100 to represent the size of the bison population. Then have that person place this same number of polystyrene pieces into each group's grocery bag.
  2. Each group will tag the sample using the following method to predict the size of the population. (No fair counting the entire bag!)
    • (a) Scoop out some polystyrene pieces and mark them with one color. Count them and place this number in the proportion.
    • (b) Place these tagged pieces back into the grocery bag and shake it. Scoop out some polystyrene pieces again and count the tagged and untagged pieces. Place these numbers in the proportion.
      total number tagged   number tagged in 2nd sample ___________________ = ___________________________ predicted number of   total number in 2nd sample population (y)  
    • (c) Cross multiply and divide to find the predicted size of the population. Record this prediction.
    • (d) Try tagging and sampling several times, using a different color each time you tag. Record the results.
  3. Each group shares the predictions. What was the actual number? Questions
    1. How did the predictions compare with the actual number of bison (polystyrene pieces)? What was the result of tagging more than once?


  • Dary, D.A. (1974) The buffalo book. Chicago: Sage Books.
  • Lott, D.F. (1991) American bison socioecology. Amsterdam: Elsevier Science Publishers.
  • Luoma, J.R. (1993, Mar/Apr) Back home on the range? Audubon, pp. 46-52.
  • Macdonald, D. (Ed.) (1985) Encyclopedia of mammals. New York: Facts on File.
  • McHugh, T. (1972) The time of the buffalo. New York: Alfred Knopf.
  • Mitchell, J. (1993, Jan/Feb) The way we shuffled off the buffalo.