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 Species Spotlight

American Bison, photo courtesy of life.nbii.gov
[Photo: life.nbii.gov]

American Bison
Bison bison

Description: The American bison, commonly referred to as buffalo, are large ungulate mammals with males ranging in size from 3.6 to 3.8 meters (11.8-12.5 feet) long and 1.67-1.86 meters (5.5- 6 ft) tall. Females are smaller, reaching 2.1-3.2 m (7-10.5 ft) long and 1.52-1.57 m (~5 ft) tall. Bison, depending on their sex, weigh between 318-900 kilograms (700-2000 pounds). Bison have brown fur with hair longer in the front than rear. Bison have a predominant head and shoulder hump, and black curving horns are present on both males and females.

Life History: Bison graze year round, primarily on prairie grass species. Bison live, on average, 15-20 years in the wild and up to 40 years in captivity. Both males and females become sexually mature at 2-3 years of age, however, males (bulls) will not breed until they reach the size where they can compete with other bulls, usually around six years of age. Bison breed once a year from late June through September and give birth to one offspring from mid April through May. Calves are 15-25 kg (30-40 lbs) at birth and become independent after one year. Bison live in groups, with females and young males (under three years of age) living in herds and males of breeding age either living alone or in their own herds.

Habitat: Bison live predominately in grassland and savanna habitats, but can also live in montane wooded and semi-desert habitats, if enough food is available.

Distribution: Bison bison were once found throughout North America from northwest Canada down to Mexico and east to the Appalachian Mountains. Wild populations of bison now exist only in national parks and refuges in the west.

Status: Populations of bison were once estimated at 60 million in pre-settlement North America. Today, there is an estimated 500,000 bison in North America with 350,000 bison in the US. There are approximately 30,000 free-roaming bison found in national parks and refuges, with the remaining bison commercially raised for meat. Bison are a keystone species of the prairie ecosystem and thus, are beginning to be re-introduced on prairie preserves and prairie restoration projects. There are only about 12,000 genetically pure bison left, with the remainder existing as cattle-bison crossbreeds.

Resources:

University of Michigan, Animal Diversity Web

IUCN Redlist of Threatened Species

The Nature Conservancy, Tallgrass Prairie Preserve


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 Tall-grass Prairies: A Habitat in Peril

Prairies once covered a vast area in North America (400 million acres), stretching from Saskatchewan in Canada, south to Texas, and from just east of the Mississippi River to the Rocky Mountains in the west.

Prairies are categorized as short-grass, mid-grass and tall-grass; precipitation amounts determine prairie community categories. Tall-grass prairie is native to the wetter eastern and southern portions of the Great Plains, while short-grass prairie is found in the drier West. Mid-grass prairies lie between these two types. The historic extent of the North American prairie ecosystem can be seen in the figure below.

Historic extent of short, mid and tallgrass prairie in North America [Figure: Houston Advanced Research Center, adapted from the USGS Northern Prairie Wildlife Research Center and the USGS National Wetlands Research Center].
Historic extent of short, mid and tallgrass prairie in North America [Figure: Houston Advanced Research Center, adapted from the USGS Northern Prairie Wildlife Research Center and the USGS National Wetlands Research Center].


All prairies have experienced significant declines in total acreage, but the tall-grass prairie has experienced the most substantial decline and as a result, is listed as an "imperiled habitat" by The Nature Conservancy. Only 4% of the almost 68 million acres of tall-grass prairie habitat remain today. Of even greater concern are coastal tall-grass prairies, found along the Texas and Louisiana coasts, which have retained only 1% of the original 9 million acres. Only 0.1% of this habitat remains in an undisturbed state.

In general, tall-grass prairie species can grow up to eight feet tall and are dominated by grasses such as big bluestem, indiangrass, little bluestem and switch grass. The major factors that contribute to the maintenance of prairie habitat are rainfall, soil type, fire, and grazing pressure. Historically, bison and elk provided grazing pressure, and drought and fire assisted in preventing successional woody species from taking over the habitat. Native prairie grass and forb species, which were adapted to drought and fire, thrived in these natural conditions due to the large amount of their biomass (75-80%) that is stored underground.

Over one hundred fifty years ago settlers discovered the rich soil of the tall-grass prairie and began to till it under for use as farmland. Since this time, conversion of prairie to cropland has been the major causal factor in the decline of prairie habitat. In coastal areas, not only conversion to croplands, but also the rapid pace of urban development, has caused a major decline in coastal prairies. In areas where prairie habitat remains, suppression of fire and overgrazing has allowed prairie lands to be overrun by invasive, woody species, converting once native prairie lands into woodlands.

Resources:

NPS, Tallgrass Prairie

TNC, Tallgrass Prairie Preserve

USGS NPWRC, Regional Trends of Biological Resources

USGS NWRC, Coastal Prairie Fact Sheet

USGS NWRC, Coastal Prairie Region


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 Prairie Plant Life

Tallgrass prairies are dominated by grasses, which comprise 80% of the prairie's vegetative biomass.  There are approximately 40-60 species of grasses (Poaceae) present in typical tallgrass prairie communities.

Click on the image below to see more prairie grass species.

[Photo: Yellow Indiangrass (Sorghastrum nutans), Andy and Sally Wakowski, Lady Bird Johnson Wildflower Center]
[Photo: Yellow Indiangrass (Sorghastrum nutans), Andy and Sally Wakowski, Lady Bird Johnson Wildflower Center]


The other 20% of the vegetation is made up of over 300 species of forbs, or wildflowers.

Click on the image below to see more prairie wildflowers.

[Photo: Indian blanket (Gaillardia pulchella), Joseph A. Marcus, Lady Bird Johnson Wildflower Center]
[Photo: Indian blanket (Gaillardia pulchella), Joseph A. Marcus, Lady Bird Johnson Wildflower Center]


There are also over 100 species of lichens or liverworts present in tall grass prairies.

[Photo: Reindeer lichen (Cladonia subtenius), Ed Uebel, WikiCommons]
[Photo: Reindeer lichen (Cladonia subtenius), Ed Uebel, WikiCommons]


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