The North American grasslands (prairie) make up about 1.4 million square miles or 15% of the continent. They cover about one thousand miles of the center of our country, from the Rocky Mountains to the state of Indiana, and can be sub-divided into three broad categories of grass types. These categories are determined by the amount of rainfall.
Shortgrass prairie
10 in. rain
200 miles wide
eastern Wyoming, western South Dakota, Nebraska, and Kansas
grasses; blue grama, buffalo grass
Mixedgrass prairie
20 in. rain
400 miles wide
Most of Kansas, Nebraska, the Dakotas, Oklahoma
little blue stem, side oats grama
Tallgrass prairie
40 in. rain
200 miles wide
eastern Iowa, Missouri, Illinois
big blue stem, golden rod, asters
The interior of the continent is affected by wide daily temperature swings and extremes of weather including drought, dust storms, hail, blizzards, and tornadoes. The frost-free season varies widely ranging from 100 to 300 days a year. Growth is affected by both temperature and rainfall. Trees are absent due to the lack of rain, lower humidity, extremes of temperature, and the hot dry winds that come during the summers. The soil can vary from a few centimeters in the west to two meters deep in the eastern regions. It can range from a brown to a dark brown to black as the organic composition increases, usually in a west to east pattern along with the increasing rainfall. The original native vegetation produced the best soils, and increased agricultural use has promoted serious soil erosion and declining soil quality. Despite this American grasslands annual crops average about $150 billion. About 70% of the harvested crops are classified as grasses, including wheat, corn, rice, barley, millet, sorghum, and sugar cane.
Native grasses vary in size from the seven foot tall grasses with roots extending down into the soil six feet to the shortgrasses growing to a height of only eight or ten inches. These short grasses can have roots that extend three feet down. The grasses die back to their roots annually and the soil and the sod insulate the roots and the new buds from the winter cold. The air temperature above ground can fall to Ð40°F in some areas. These deep roots also are protected from the temperature extremes of a prairie fire. At a height of three feet the air temperature can reach 400°F while an inch or two below the ground the temperature may rise only a few degrees. Some of the grasses survival techniques include the ability to curl their leaves to reduce moisture loss, and rapid early spring growth while moisture is available. The seeds will be produced later in the spring and then become semi-dormant to make it through the hot summer and the cold winter. In areas that excessive grazing takes place grasses that are very drought-tolerant are more successful and annual grasses are favored over perennials. The prairie also is home to some spectacular displays of flowering plants such as the daisylike composites and the sweet peas, legumes.
If we look at the animals of the grasslands from the bottom up we will first focus on the creatures beneath the grass, the soil-makers known as earthworms. These animals mix and aerate the soil and enrich it with their droppings. Earthworms on a single acre pass an amazing fifteen tons of soil and vegetation through their digestive systems in a single year! The organic matter they take in and pass out is broken down into nitrogen, phosphorus, and potash which are essential nutrients for plant growth. Other subterranean animals include flightless insects and grubs, and the many burrowing mammals; rodents, mice, gophers, prairie dogs and rabbits. Of these the prairie dog is an interesting representative species. It is actually a ground squirrel, not a dog at all. It communicates with a sharp yipping sound and its predators are rattlesnakes, hawks, ferrets, and coyotes. The prairie dog has a highly organized colony broken into family groups taking up about 7/10 of an acre with large numbers of these families congregating together. One fantastic colony was studied in 1900 in Texas. This prairie dog town took up 25,000 square miles of burrows and an estimated population of 400 million prairie dogs!
Grasshoppers are another animal found in abundance on the prairie. They are usually kept in check by birds and other predators on them. In humid areas parasites and diseases control their numbers. But sometimes their population increases rapidly to create a problem. In 1870 a warm Fall to increase egg production and a cool Spring which delayed the hatching until abundant food germinated was compounded by dryness which reduced disease and prompted the birds to migrate away for water. All these conditions worked together to create a locust plague of historic proportions in the United States. The eastward moving swarm travelled at five miles per hour and witnesses say it took about six hours for the swarm to pass over. The group was thirty miles deep, one hundred miles wide and one mile high. The population was estimated to be about 124 billion locusts! A group large enough to do more damage than a prairie fire. It is very unlikely that a plague of such proportions would occur today due to improved use of pesticides, better communication, and early intervention techniques.
Grassland animals that live above the surface of the ground include birds and mammals. These have developed some adaptations to increase their success in this biome. Many of the birds nest on the ground, and are strong-legged walkers. They drink less and have a seed diet which provides much of their fluid requirement. A number of the birds are predators on the abundant rodent population which serves the dual ecological purposes of providing a food source and preventing overpopulation and overgrazing. The mammals of the grasslands originally were the herbivores; bison and pronghorn. The bison was well adapted to the extreme climate of the prairie in winter with his large size, thick fur coat, and large herd size. When snow covered the ground the bison swept it away with his huge head to get at the buried vegetation. When threatened by a predator the herd would gather together around the juveniles with their heads, horns, and hooves facing outward presenting a daunting front. Less impressive in size but equally well adapted to their environment are the pronghorn antelope. Their diet consists of the flowering plants, shrubs, and cactus of the shortgrass prairie. They have extremely good vision and can spot a predator up to two miles away. They can run at speeds from 45 to 60 mph with their long legs, enlarged heart, and wide trachea. They run with their mouths wide open gulping air to increase their stamina.
Today the grassland biome is much changed from when Europeans first arrived. The bison are gone, and antelope are greatly reduced. These niches have been filled by domesticated cattle and sheep. The tallgrass prairie is gone, replaced by agriculture plants, and the shortgrass prairie is extensively grazed.