The Grasslands of the US

Grasslands once extended aroung the earth once covering more than a quarter of the earth's surface: from the veldt and savannas of Africa, to the steppes of Eurasia, the pampas of SA and more closely the prairies of NA.

The term prairie itself derives from the word French trappers and explorers gave to the sea of grass they encountered in the center of North America.

The prairie once spread across 1.5 million square kilometers of the Great Plains (that's seven times the size of Minnesota!)—but less than 2% of native prairie remains today.

Flowing east of the Rocky Mountains, from the badlands of North Dakota to north-central Texas and West of the Rockies, in the Great Basin states of Oregon, California and Idaho, these remanents of the Dust Bowl now total almost four million acres under federal control.

Prairies are characterized by:

The Great Plains grassland evolved in the rain shadow of the Rocky Mountains, where seasonal precipitation occurs mostly in spring and summer. From the Rocky Mountains east to the Mississippi River, the amount of precipitation increases and the frequency of droughts decreases (Simms 1988). Along a north-south gradient from central Texas to south-central Canada, the growing season becomes shorter, the average temperature decreases, and a greater proportion of annual precipitation occurs as snow. These broad-scale environmental gradients significantly influenced the evolutionary composition and distribution of prairie communities.

Types of Grasslands. 

In the past, grassland dominated central North America and, during the warm, dry interglacial times, reached--as the prairie peninsula--into parts of Wisconsin, Illinois, Indiana, and eastern Ohio.


Tall-grass Prairie  
Tall-grass prairie is the wettest of the grassland provinces and is predominantly composed of sod-forming bunch grasses.

.Since 1830 declines in the area of tall-grass prairie within specific states and provinces are estimated to be 82.6 to 99.9% and exceed those reported for any other major ecological community in North America.It is this soil formation that is associated with the darkest of the mollisols, the chernozems. Much of it today has been converted to corn.

These humid grasslands represent a fire or fire and grazing subclimax is generally accepted today. With the extermination of the bison and the suppression of prairie fires in the late 1800s, that part of the prairie that had not been put under the plow reverted to oak-hickory forest in the US and to aspen woodland in the north, in Canada. Today prairie preserves are managed with controlled burns.

The tall grass prairie gets its name from the upright bluestems (Andropogon spp. shown left ) that reach heights of 6 feet or more by late summer. Their roots may extend to depths of 9 feet or more, binding the soil and enriching it with organic matter as roots die back at the end of the growing season. The above-ground parts of the plants also die down in winter and are converted to abundant humus when bacterial action begins in the spring.


Three herb layers are apparent in the tall grass prairie, each characterized by relatively high species diversity. . Bluestems comprise the uppermost herb layer.

Other, shorter upright grasses and forbs form an intermediate layer. Recumbent species such as the grama grasses (Bouteloua spp.) make up the lowest, ground-hugging layer.

Mixed-grass Prairie  
One can envision the short-grass and tall-grass prairies intergrading just east of an irregular line that runs from northern Texas through Oklahoma, Kansas, and Nebraska, northwestward into west-central North Dakota and South Dakota

In general, the mixed-grass prairie is characterized by the warm-season grasses of the short-grass prairie to the west and the cool- and warm-season grasses, which grow much taller, to the east . Because of this ecotonal mixing, the number of plant species found in mixed-grass prairies exceeds that in other prairie types.

It has two layers of grasses and, one reaching about 12 inches above the ground surface, the other, more open, about 48 inches. Both bunch and sod-forming grasses are present, as well as many forbs. Roots extend to depths of about 5 feet. The soils are dark brown, lighter than chernozem.

Short grass prairie. The westernmost and driest part of the North American grasslands, the short grass prairie occupies regions with 10 to 12 inches of precipitation a year. A single herb layer exists and consists primarily of bunch grasses some 12 to 18 inches high (blue grama and buffalo grass)
They have relatively shallow root systems. Soils are lighter brown than those under the other two formations and the calcium carbonate layer is closer to the surface. These grasslands are used today as rangeland for cattle. Many parts, including the national grasslands, are severely overgrazed and have been invaded by prickly pear cactus and other thorny plants. Some states with short grass prairie still have large populations of pronghorn. Elk, feral horses, and mule deer also share the range with cattle. Bison still occur but are privately owned or are maintained on special federal reserves.

Unlike the more eastern species, short-grass prairie species remain digestible and retain their protein content when dormant.
In the mid-1800's the numbers of individuals of native mammal species--bison, prairie dogs, pronghorn, elk, grizzly bears, and gray wolves--rivaled or exceeded those now in the African Serengeti . Major antigrazing structures evolved in plants: thorns and spikes; thick or hard tissues difficult to bite, chew, or digest; and secondary compounds difficult to digest. These structures have arisen through the long coevolutionary association between plants and animals with grazing on grasslands.

At present, extensive areas of short-grass prairie are dominated by invasive perennial and annual species, whose presence is attributed to overgrazing by domestic livestock and dryland farming.


Frequent fire is essential to maintaining native species diversity, and it affects other components, including nutrient cycling and productivity. On tall-grass prairie, the relationship between total plant species richness and the number of times a site is burned is important and positive.

After a burn the soil warms more rapidly in the spring. Removal of the litter permits soil temperatures to average as much as 52 F (11 C) higher than on unburned sites in early spring ). Soil temperatures in early spring are inversely related to the amount of litter and duff .
Early rising soil temperatures stimulate the increase of certain bacteria that decompose organic matter. This allows warm-season grasses to grow at an optimum rate if moisture is adequate. Most of the fertilizing effects after a fire result from nitrates released by bacteria consuming organic matter, not from nutrients in the ash

Native annuals are usually encouraged by burning if the fires occur at the appropriate time. Many annuals, as well as shortlived perennials, are opportunistic or pioneer species which require the open soil, reduced competition, and full sunlight characteristic of many post-burn sites.soil temperature in many grasslands can reach 140 F (60 C) for several hours after a fire due to solar radiation. Fire is not detrimental to the seeds of native annuals; soil surface temperatures of even 194 F (90 C) for a few seconds were not harmful to most seeds.

Fire adaptations of plants include their deep root systems, underground rhizomes which can sprout new leaves after fires, and flammable dry above ground leaf material.

Two basic grass types are:
* Turf- or sod-forming grasses, with rhizomes or underground stems from which new plants spring forth; associated with the more humid grasslands
* Bunch grasses, without rhizomes, that reproduce by seed; associated with the drier parts of the biome.


Although it is thought that grazing is critical to maintain the grasslands, it may be less important in controlling total growth relative to species diversity. Biomass may remain constant,but species composition and diversity are strongly influenced by grazing.

Defenses against grazing and the dry climate include the evolution of Kranz anatomy. Vascular bundles are sheathed by silica which act to grind down the teeth of grazers, and make it more difficut for sucking insects to tap into the phloem.The advanced C4 photosynthesis pathway which allows the plants to store CO2 earlier in the day and later in the afternoon, permits them to close their stomata during the heat of the miday sun.

How grasslands made the soils so rich:

Calcification is the dominant soil-forming process in semiarid regions. Mild leaching, high organic content, and concentration of calcium carbonate in the B horizon typifies the dark brown mollisols developed under the temperate grasslands. When this process works on a loess that itself is rich in calcium, the world's most fertile soils are created, the chernozems. What observations can you make based on these photos?

The central concept of Ultisols is that of soils that have a horizon that contains an appreciable amount of translocated silicate clay and few bases (base saturation less than 35 percent). The central concept of Mollisols is that of soils that have a dark colored surface horizon and are base rich.

Destruction of the Praires 
Managed agriculture began on the easternmost grasslands in the 1850's. Surface cover is reduced by agriculture, and soil structure is destabilized by reducing aggregate size. Organic carbon loss is accelerated by agriculture, and cultivated crops (particularly in dryland areas) return little carbon to the soil. Few agricultural practices of the early settlers captured or retained moisture. The black-dust storms of the 1930's resulted from exposing vast areas of cultivated prairie soil to wind action and drought .
Soil productivity (indexed by corn grain yield) declined 71% and soil nitrogen 49% during a 28-year interval after cultivation began . Retention of organic matter--and thus the level of productivity--in grassland soils is only possible if the correct proportions of carbon, nitrogen, and phosphorus are present .
Soil formation is a slow, continuous process. About 2.5 centimeters of new topsoil is formed every 100 to 1,000 years, depending on climate, vegetation and other living organisms, topography, and the nature of the soil's parent material. Some soils, particularly where moisture is a limiting factor and growing seasons are short, may take 10,000 years to produce 2.5 centimeters of soil.

On average, annual loss of topsoil in the United States is nearly three times greater than that being formed.

Dust Bowl
The United States acquired most of the Great Plains and Great Basin from France with the Louisiana Purchase of 1803. Until the late 1860s, the Great Plains region was perhaps America’s last frontier.
The Homestead Act of 1862 brought almost six million settlers by 1890 who tried to replace grass with crops more beneficial to economic aspirations. The settlers soon discovered, however, that while these vast grasslands were productive in wet years, they were also subject to serious drought and bitter winters. Land that should never have been plowed yielded its topsoil to incessant dry winds. Above parts of Oklahoma, Texas, Wyoming, Nebraska, Kansas, Colorado and the Dakotas, dust clouds rose to over 20,000 feet. Ten-foot drifts of fine soil particles piled up like snow in a blizzard, burying fences and closing roads.

By the early 1930s, the broad midsection of America was in trouble. Not only because of the Dust Bowls, but the Great Depression was reaching its economic depths.
Emergency measures were taken to save the farmers and settlers. The National Industrial Recovery Act of 1933 and the Emergency Relief Appropriations Act of 1935 allowed the federal government to purchase and restore damaged lands and to resettle destitute families.

Partial redemption via the Ogallala:

The development of the High Plains aquifer for irrigation (1940-1980) is evident in an average area-weighted, water-level decline of 3 meters (0.07 meters per year). Declines vary with locale, exceeding 30 meters in some parts of the central and southern High Plains; 6 meters in southwestern Kansas, east-central New Mexico, and the Oklahoma and Texas panhandles; and 3 to 6 meters in northeastern Colorado, northwestern Kansas, and southwestern Nebraska. Since 1980, water levels in such areas have continued to decline but at a slower rate, the result of greater than normal precipitation (10 to 15 centimeters annually), water conservation practices (particularly minimum tillage), and reduction in irrigated land area (about 540,000 hectares,.The Ogallala Formation was deposited about 10 million years ago, during the geologic time periods of late Miocene and early Pliocene by eastwardly flowing braided streams, which originated in the Rocky Mountains

Animals of the Prairie:

The grasslands have a fertile history of plant- and seed-eating mammals. A rich array of large herbivores, including camels, rhinoceroses, mammoths, mastodons, and bison, evolved on the North American grasslands only to disappear during the last Ice Age .

The surviving large grassland herbivore, the plains bison, evokes a mystique not shared by any other North American mammal and which is largely derived from Native American and frontier heritages. In the past, bison numbered from about 60 to 70 million and roamed in large herds that, in the 1860's, often required horseback riders several days to successfully penetrate and cross.
As many as 5 billion prairie dogs may have been in North America before European settlement. An estimated 98% decline in prairie dog numbers has occurred since European settlement . The black-tailed prairie dog may occupy less than 0.5% of its original range, the short-grass and mixed-grass prairies. As a result, a variety of species closely associated with the prairie dog are either federally listed as endangered or are being considered for listing as threatened or endangered. These include the black-footed ferret, the swift fox, and the mountain plover.
The ranges of more than 100 native mammals extend into the prairie; nearly half of these occur in the forest-grassland ecotone and others in diverse habitat types . Nevertheless, surveys of eastern and western species whose ranges stop short of the Great Plains support the effectiveness of the grassland as an evolutionary barrier to dispersal .

Estimates of Great Plains-restricted mammals range from as few as 10 (Risser et al. 1981) to as many as 18 (Jones et al. 1985). These species include one lagomorph (the white-tailed jack rabbit), eight rodents (thirteen-lined ground squirrel, Franklin's ground squirrel, black-tailed prairie dog, plains pocket gopher, olive-backed pocket mouse, plains pocket mouse, plains harvest mouse, and prairie vole), and two carnivores (swift fox and black-footed ferret).Invasions of nonindigenous plant species after fire suppression in the eastern, central, and southern prairies, as well as water developments in the western plains, have drastically altered grassland landscapes. Establishment of woodlots, shelterbelts, and tree-lined river and stream corridors within the prairie has contributed to a significant and ongoing loss of genetic diversity in North American grasslands

Badger --Black-footed Ferret --Bison ----Black-tailed Jackrabbit ----Coyote----Eastern Cottontail
Meadow Vole---Northern Grasshopper Mouse ---Prairie Dog---Pronghorn---Red Fox--Richardson's Ground Squirrel
13-lined Ground Squirrel---White-tailed Jackrabbit

Prairie Dogs

At the turn of the century, as many as 5 billion prairie dogs occupied millions of acres of grass prairies across the West.

Today, after decades of eradication by federal, state, and local governments, devastation from disease, poisoning, recreational shooting and habitat destruction , prairie dogs are rapidly disappearing

Recent studies suggest that prairie dogs possess the most sophisticated of all natural animal languages. They apparently issue different sounds identifying various predators, which include hawks, owls, eagles, ravens, coyotes, badgers, ferrets and snakes.

The various native plants of the Great Plains make up the prairie dog's primary diet, comprising all kinds of grasses, roots and blossoms. They acquire all of their water from the food they eat.

Because they eat as much as 7 percent of a ranch's forage, prairie dog eradication programs have been underway for decades in the American West. But a growing number of experts argue that prairie dogs may actually be beneficial, that they are natural fertilizers who also increase the protein content and digestibility of rangeland grasses.

The presence of prairie dog colonies creates habitat favorable for such wildlife as burrowing owls, which use the abandoned burrows. The almost extinct black footed ferret used to prey on the prairie dogs and use their burrows, as well. Rattlesnakes are the only poisonous snakes found in the grassland and feed on the pdogs.

During the same time, bison were largely eliminated by westward expansion. Ranchers filled the large open ranges of the plains and the Great Basin with cattle and sheep.

Bison, with wider mouths and bigger fermentation stomachs, evolved to consume an enormous quantity of high fiber low protein forage. Without their large numbers in giant herds to harvest or trample the prairie grasses the forbs and grasses suffer from the abundant growth if the grass is not consumed or mulched each year.

Bison came to North America during the Pleistocene Epoch via the Bering land bridge. Eventually they ranged from Canada's Great Slave Lake to Mexico and from eastern Oregon almost to the Atlantic. . Two races of bison are recognized: Plains Buffalo and Wood Buffalo of Canada. Their number was reduced to 750 in 1890. They were then protected and now number about 80,000. Bison live only in parks and reserves.

Pronghorn antelope and deer have a different digestive system, designed for higher dietary protein with lower fiber content bison. These smaller animals would starve on a diet of Big Bluestem.

Birds of the Prairie:

Of the 435 bird species that breed in the United States, 330 breed on the Great Plains (Knopf and Samson 1995). Nevertheless, few North American bird species are believed to have evolved within the Great Plains. Mengel suggested that only 12 bird species are endemic to the grasslands. An additional 25 species are believed to have evolved on the grassland, though they range widely into adjoining vegetation provinces..Most of the endemic bird species of the short-grass and mixed-grass prairies are associated with large grazing animals ; others, such as the ferruginous hawk, prairie falcon, and burrowing owl, are either somewhat or strongly associated with prairie dog colonies

Burrowing Owl:

Burrowing Owl
Athene cunicularia
Size: 23 cm (9 in.)
Habitat/Range: The burrowing owl lives on the dry, short-grass prairie. They can be found west of the Mississippi River from southern Canada throughout the western U.S. south through Mexico and into South America.
Eating Habits: Burrowing owls feed on a wide variety of insects, small rodents, lizards, and birds, depending on location and time of year.
This small owl, about eight inches tall, has long bare legs, no ear tufts and a small facial disc. The owls are dusty brown with white markings on the belly and a prominent white chin stripe.

Greater Prairie Chicken
Tympanuchus cupido
Size: 40-46 cm (16-18 in.)
Habitat/Range: Tallgrass prairie
Eating Habits: Greater prairie chickens eat insects like grasshoppers, ants, and leafhoppers.
The greater prairie-chicken is a medium-sized grouse with barred plumage; it has a short rounded tail, feathered toes and narrow elongated feathers that cover inflatable orange sacs on both sides of the neck; these narrow elongated feathers are poorly developed on the females. Males fill orange sacs and raise side feathers into "horns" during courtship displays.


I have a little on the forbs ( herbaceous) species here only as a reminder that prairie species are wonderful gardening plants that survive droughts like we have had these last few years. The following pictures I took are of 3 species found in my very neglected garden.

Black-eyed Susan;Black-eyed Susan
Rudbeckia hirta

Habitat/Range: Widely distributed in high-quality dry to mesic prairies, more common in grazed prairies, dry pastures, hay meadows, old fields, and along roadsides throughout the tallgrass region
Purple Coneflower
Echinacea purpurea
Aster Family (Asteraceae)
Habitat/Range: Occasional in prairies and open woodlands; scattered throughout the tallgrass region west to southeastern Kansas.
Plants to 5 feet tall with branched stems. The rough leaves are coarsely toothed and alternate. Flower heads are on individual stalks near the tops of stems, with each head 2.5-5 inches wide and consisting of up to 20 purple, petal-like ray flowers surrounding a cone-shaped head.
Prairie Blazing Star Prairie Blazing Star
Liatris pycnostachya
Aster Family (Asteraceae)
Habitat/Range: Common throughout the dry and mesic prairie region
Slender, spikelike plants up to five feet tall with abundant grass-like leaves and usually hairy stems. The flower heads are in a dense spike at the top of the plant. They are mostly 5-10 small, 5 lobed, purple disk flowers per head, with 2 prominent thread-like style branches protruding form each flower.
Others include: Common Spiderwort
Purple prairie clover
Western Prairie-fringed Orchid
Wild Bergamot ( I have the red species)


More Problems on the Prairie

The human-caused breakdown of barriers to dispersal that has permitted invasion of nonindigenous species has caused the extinction of more grassland species than any factor except habitat loss (D'Antinio and Vitousek 1992). Nonindigenous species include exotics, which are transported beyond their natural range, and aliens, those that colonize an altered landscape. Introducing nonindigenous species may increase the number of local species, but it reduces integrity, above- and belowground, and also the number of native species, both aquatic and terrestrial.
A more subtle threat to integrity is loss of genetic diversity. Species hybridize along forested corridors that now fragment the Great Plains . Human activity, either accidental or deliberate, moves species from one place to another at ever-increasing rates. As a result, species that evolved in isolation from one another are forced into contact. In terms of conservation of biological diversity, the loss of six bird subspecies due to the hybridization arising from these forested stepping stones and artificial corridors (Knopf 1986) rivals the loss of three species attributed to fragmentation of the eastern deciduous forest.
These recent forest patches and woody corridors that border rivers on the Great Plains also favor movement of reptiles and mammals from east to west, which contributes to the degradation of the biological diversity and integrity of the Great Plains. In 1842, in eastern Colorado, the explorer John C. Fremont observed that "antelope were tolerably abundant, wolves were seen in great numbers, and buffalo absolutely covered the plains on both sides of the (South Platte) river" but reported no deer. In recent years, deer abundance has increased markedly, particularly that of the eastern white-tailed deer, which may replace the mule deer, known to have occurred on the western plains since before European settlement. Hybridization between the two deer is known to occur.    

Nonindigenous species now account for 13% to 30% of prairie species .

What appears to be was not for long: the story of how our pristine west was long disturbed before the first colonists

Fifteen thousand years ago giant animals roamed the North American continent. These extremely large animals, known as megafauna, included several species of elephant, such as the well known woolly mammoth, and sabertooth tigers which were lion-sized cats with knife-like teeth. There also were larger cousins of animals still present today, such as the giant short-faced bear (Actodus simus) which was 30 percent larger than today's grizzly bear, and the massive dire wolf (Canis dirus). There were camels, llamas, horses, cheetahs, sloths, and other mammals, both large and small, which no longer exist on this continent.

Dire Wolf ran in packs similiar to todays dogs and wolves
An average Columbian mammoth stood over 12 feet tall at the shoulder and weighed over 10,000 pounds. The mammoth's primary diet consisted of grasses, which was quite different from the diets of their modern relatives, the Indian and African elephants. Yet, like these elephants, the Columbian mammoth had a set of four teeth that was replaced by new sets as the older teeth eventually became worn. This type of tooth replacement continued to produce six sets over the course of about 60 years and then, like all elephants, the mammoth would starve as the final set wore out.
Evolving from the tree sloths in South America, ground sloths are very distantly related to anteaters and armadillos. As this animal adapted from a tree dweller to being ground-based, its limbs still showed a relationship to its ancestors. Typically, ground sloths walked on the sides of their hind feet and the backs of their forefeet


The western camel had a very similar build to the living bactrian (two-humped) camel, but was slightly taller (standing seven feet at the shoulder) and may have lacked humps. Although its teeth suggest a diet of grasses, plant remains extracted from its teeth show very little grass and would suggest that the camel was an opportunistic herbivore (eating any plants that were around) like its modern day relatives. The western camel was more closely related to the llama than to living camels.

Closely related to the modern African lion, the American lion was the largest member of the cat family. Its enormous size just begins to shed light on the nature of this extinct predator. Like modern felines, the American lion had a tail that was about as long as its torso-roughly four feet. The tail was used to help maintain balance and direction while chasing down its prey

The sabertoothed cat had strong limbs and a heavy muscular build, making it much more bulky than other cats, which tend to be agile and nimble. The sabertoothed cat had a short tail (similar to a bobcat) and research suggests that it relied on its powerful muscles to ambush and pounce upon prey, instead of chasing it in a manner similar to lions and other cats.

For more on these animals see: guide/flora/wolf.htm

From: Interior Columbia Basin Ecosystem Management Project,

Flora and Fauna of the Intermountain West
The origins of current Intermountain flora dates back to the late Miocene, 12-20 million years before present (B.P.)
Prior to the uplift of the Cascade-Sierra Cordillera the Great Basin and Columbia Plateau were vegetated by
hardwood-deciduous and conifer forests . Such temperate flora probably flourished in a mild climate of 35-50
inches of rainfall with little seasonality.
By late Miocene as the Cascade-Sierra uplift began to block the Pacific storm track, the landscape
to the east became progressively more xeric and seasonal . The temperate forests were slowly being replaced
by shrub land and deserts. Regional pollen records indicate a distinct increase in herbaceous angiosperms during
the Miocene These include species from such families as Chenopodiaceae, Gramineae and Compositae
all important plant families in
the deserts and shrub lands of the Intermountain region today. Gray reports the earliest fossil pollen
record of Artemesia (sagebrush) to be in late Miocene deposits in northeastern Nevada. By the end of the
Miocene (about 12 million years B.P.) much of the Intermountain West had become distinctly more arid and was
vegetated by xeric woodlands .
During the Pliocene (2-10 million years B.P.) the CascadeSierra underwent the greatest uplift rising
as much as 5,000-6,000 feet in the Cascades and more in the Sierra
. This active
mountain building also accelerated desertification by intensifying the rain shadow on the leeward side of the
mountains. Precipitation decreased to levels similar to historic times and with a similar seasonality . With substantially
less growing season moisture the Intermountain flora increasingly shifted toward
shrub lands at the lower elevations and coniferous forests in the mountains.

The fossil record indicates that by the beginning of the Pleistocene Ice Ages (2 million years B.P.) the flora of the Intermountain Region was essentially the same as our modern flora
Evolution of the flora most certainly was not the only biological event occurring during the past 20
million years.

Concurrently with this floral evolution was the appearance of the myrid of new animal species
The neo-tropical forest dwelling creatures of the early to mid Cenozoic era slowly evolved into the rich faunal assemblage. This fauna has come to be known by
scientists as the Pleistocene mega fauna. The fossil record indicates that grazing herds of elephants, mammoths, rhinos, camels, horses, burros, ground sloths, and many other grazers and browsers roamed throughout western North America for several million years. Prehistoric cattle were also part of this faunal assemblage. Several genera from the Bovidae family including Bos (cattle) have been found in the North American Pleistocene fossil record .

The Pleistocene mega fauna resulted from the coevolution of flora and fauna over several million years. This biotic complex successfully existed throughout North America despite numerous major climatic
fluctuations. "The Pleistocene game-carrying capacity of western North America must have equaled and very likely exceeded, the 40 million units of livestock which it now supports."

Pleistocene Extinction
Just as the fossil record reveals the coevolution of the Pleistocene flora and fauna and the existence
of these widespread natural herbivories on each continent; the fossils also record the demise of the mega fauna . In western North America the fossil record indicates that the majority of large herbivores and their associated predators became extinct between 10,500 and 7,000 B.P. This massive extinction over an extremely short time period removed over 70% of the Pleistocene mega fauna in North America (Martin 1986). Similar extinction occurred in other continents but at somewhat different times. North America lost 33 out of 45
genera of large fauna during this late Pleistocene extinction (Martin 1986 and 1990). From 7,000 year B.P. to the present the depauperate remnants of the Pleistocene mega fauna include bison, elk, moose, deer, antelope, and bighorns.

To date neither evolutionary substitution (for which there has been far too little time) nor immigration have filled the empty niches in this natural herbivory (Martin 1970). The implications of the Pleistocene extinctions on current efforts to comprehend our western ecosystems is tremendous, even if not yet recognized. Underlying nearly all aspects of land management is the assumption that the fauna and flora of North America-at the time of European contact was in a pristine
natural state of balance.
The demise of the Pleistocene mega fauna has perplexed scientists for many years.

I. Climatic change
during the last major deglaciation period which would have caused environmental stress for the "ice-age"
fauna has commonly been advanced as the driving force behind the Pleistocene extinctions (Martin 1986
and Grayson 1987 and 1991). However, certain features of the extinction are not well explained by the
climatic theory. Differential timing of the extinction between continents and the apparent lack of effects on
small fauna and flora are difficult to explain under the climatic theory. Equally troublesome are some of the
most recent interpretations of past climatic fluctuations which suggest that the Pleistocene mega fauna
survived several early periods of glacial and interglacial climatic pulses which were more severe than that of
10,000 years ago
2. More recently the theory that the Pleistocene extinction were primarily driven by human predation
is gaining scientific proponents . It now appears that the first humans immigrated to North
America from Asia crossing the Bearing Straits land bridge during a glacial period at least 12,000-15,000
years B.P. Apparently it took about 1500 to a few thousand years for this new predator, hunter
man, to populate the new lands and begin to dramatically impact the mega fauna. An interesting aspect of
this extinction theory is that the chronology of Pleistocene extinctions on each of the world
continents and major islands occurs shortly after the arrival of man (Martin 1989; Fleharty and Hulett
1977). Whatever the cause, the extinction by 7,000 years B.P. of most large herbivores and predators left
a natural rangeland grazing ecosystem, which had existed several million years, with many vacant large
fauna niches.

3. Disease: From:Pleistocene Megafauna Extinctions: Source: Stevens, William K. 1997. New Suspect in Ancient Extinctions of the Pleistocene Megafauna: \
"A. third hypothesis has emerged. Scientists who find neither the climatic nor the blitzkrieg theory convincing argue that rampant disease was the main villain. In this view, the megafauna were betrayed not by the naivete of the big animals themselves but that of their immune systems. And it was not the spears carried by people that wiped them out, but the pathogens carried by dogs, rats, birds, parasites and other living baggage that accompanied the continent's first human arrivals from Siberia." "The disease hypothesis, worked out by Dr. MacPhee and Dr. Preston A. Marx, a virologist at the Aaron Diamond AIDS Research Center in New York, holds that the animals were infected by lethal pathogens unknown to their immune systems. (Similarly, diseases introduced later by Europeans ravaged populations of immunologically defenseless Indians.) As envisioned by Marx, disease would have advanced across the continent in the wake of humans, either reducing animal populations to levels from which they could not recover or springing up again and again to infect new generations.
For this to have happened, Marx said, several conditions would have been necessary: The killer pathogen would have had to kill rapidly. It would have had to affect all age groups in a given animal population (something hunters do not do). It would have had to have an independent host, either people or creatures that arrived with people, to provide a reservoir of immune carriers from which the disease could spread. And it would have had to affect a broad array of species without causing epidemics in humans.
A number of diseases meet some of the criteria; canine distemper and rinderpest, for instance, have all caused severe reductions in populations of several animals. But they have not caused any known extinctions. MacPhee and Marx have so far identified only two existing pathogens that fulfill all the requirements: leptospirosis, a bacterium spread in rat urine, and the rabies virus. The point is not that either of these diseases were necessarily the actual agents of extinction, the two scientists say, but rather that there exist some pathogens that conceivably could have done the job."

Bison was one of the few really large herbivores to survive the Pleistocene extinctions and vast
herds of these animals roamed the American prairies at the time of European contact
. It is
ironic that within slightly less than 400 years after Columbus landed in the vicinity of the America's,
European descendants all but hunted the North American bison to extinction.
At the time of European man's arrival in the Intermountain West (ca 1800), he found a vast region
vegetated largely by open shrub stands with an abundant perennial grass understory. Climatically, the
shrubs and junipers could out compete the herbaceous species creating dense shrub or woodland stands
with meager understory. Periodic lightning and Indian-set fires shifted the vegetation back to a perennial
grassland and kept the adjacent juniper woodland largely restricted to the more rocky, fire-safe sites. The landscape of the early 1800s supported scattered herds of bighorn
sheep, antelope and some deer and elk . In parts of the Intermountain Region game animals were spare enough that early explorers sometimes had difficulty acquiring sufficient food