Due to gross similarities in size, food preference, and appearance, it is often asserted that bison and domestic cattle are ecological analogues. However, a review of their evolutionary history demonstrates that they have significant differences in evolutionary pressures that manifest themselves in strikingly different modes of resource exploitation. Compared to domestic cattle, bison wander more, are less apt to regraze a site during a single growing season, will use steeper terrain, select and consume drier, rougher forage, and spend less time in riparian areas and wetlands.
In addition, bison were unable to colonize vast portions of the West due to a variety of factors. Evidence suggests these major rangeland ecosystems evolved in the virtual absence of large herding animals. The introduction of domestic livestock into these areas has had particularly negative impacts upon these landscapes.
This paper will discuss the differences in habitat utilization, and behavior between cattle and bison, and the implications for management of western rangelands and preservation of native biodiversity.
Bison (Bison bison) once ranged across much of North America from the eastern seaboard states to southeast Washington, eastern Oregon and northeastern California. They also roamed the high parks of the Colorado Rockies and were known from higher elevations of the Northern Rockies in Glacier National Park, and the mountains surrounding Yellowstone (Meagher, 1973; McDonald, 1981; Reynolds et al. 1982). However, the greatest numbers were found on the shortgrass plains east of the Rocky Mountains that stretched from Alberta to Texas (Reynolds et al. 1982)–sometimes referred to as the “bison belt”. Some authorities estimated that 75 million bison roamed North America in the pre-Columbian era, while a more conservative estimate by McHugh suggested the maximum number should be pegged at 30 million (Reynolds et al. 1982). Due to hide hunting, sport hunting and perhaps also as a consequence of the introduction of the horse which increased Native American hunting efficiency (Reynolds et al. 1982, Urness, 1989) bison numbers plummeted nearly to the point of extinction by the late 1800’s (Meagher, 1973).
Whatever the exact number in pre-Columbian times, there is no doubt from historic accounts that bison were abundant within their preferred habitat on the plains. The following quote from John Kirk Townsend (1978) is representative of the kind of abundance that early sojourners recorded. Traveling along the Platte River of Wyoming in 1834, Townsend noted that the “whole plain, as far as the eye could discern, was covered by one enormous mass of buffalo. Our vision, at the very least computation, would certainly extend ten miles, and in the whole of this great space, including about eight miles in width from the bluffs to the river bank, there was apparently no vista in the incalculable multitude.”
That bison were a major ecological force that shaped western rangelands is seldom disputed. Many plants in the “bison belt” have adopted flexible growth strategies that enable them to tolerate bison herbivory pressures as well as other ungulates, rodents like prairie dogs and even invertebrates (Delting and Painter 1983, Painter et. al 1989). This suggests a long-standing relationship with herbivory pressure. However, Belsky (1986) suggests cautions that this should not be interpreted as suggesting plants “benefit” from herbivory since shoot and leaf production to replace lost photosynthetic area does not itself constitute greater production.
Because of their current limited distribution, bison no longer function as a major disturbance factor, nor influence ecosystem function in most of their former habitat. Within the last hundred to hundred fifty years, bison were replaced across most of their natural range by domestic cattle. Due to gross similarities in shape, size, and foraging habits between bison and cattle, many have argued that cattle are merely filling the vacant niche left by the virtual extinction of bison. For example, in an article (Thomas, 1991) which appeared in Rangelands published by the Society for Range Management, the author states, “Long before the American pioneers laid eyes on the mountains and plains of North America, there were ‘cattle’ on our western ranges. Bison roamed the hills, migrating from winter to summer grazing areas, making seasonal use of these lands much as our domestic livestock do today. The bison and the domestic cow belong to the same family (Bovidae) and are genetically similar. They are also very similar in their grazing habits and preferences”.
Not surprisingly, because of the assumed similarities between the two animals, it is claimed that cattle are nothing more than domestic bison. Since bison herbivory was an important ecological influence upon many grassland ecosystems, many people assume that properly managed cattle have no negative impacts upon western rangelands, in part because they are thought to mimic bison herbivory pressures (Savory, 1983, Lauenroth et al. 1994, Goetz, H. 1994, Thomas, 1991). The manager for the Nature Conservancy’s Pine Butte Swamp in Montana uses these presumed similarities to explain the use of cattle grazing on the Conservancy’s preserve, stating “because bison historically graze in the area, native grasses actually thrive with limited grazing” (Cheater, 1993). A livestock promotional piece published by the U.S.F.S., BLM, and Public Lands Council (USDA, 1989) says “for range grasses to thrive, they must be cropped to promote vigor. Often, the lack of grazing results in unhealthy, less dense stands that overwhelm good grasses, wildflowers and other plants.” However, others argue that plant response to herbivory is merely a defensive mechanism that should not be interpreted as a benefit or promoting rangeland “health”, but rather as generalized plant responses to damage (Belsky et al. 1993).
Some even assert that western rangelands, particularly the most arid or “brittle” landscapes require disturbance impacts from domestic cattle to replace the impact it is claimed resulted from vast herds of bison, antelope or other wildlife in these areas. For example, Thomas (1991) concludes that extirpation of the bison “would have put thousands of acres of rangeland into a stagnant and very unnatural situation except for one saving grace: we substituted domestic livestock for the buffalo.” Others suggest that herding animals are necessary to break up soil crusts, trample seeds, remove “decadent” plant material, and are purported to increase rainwater penetration (Savory, 1983, 1989, USDA, 1989).
Unfortunately, there have been few attempts to verify whether bison or cattle are ecological analogues, and furthermore if rangelands “require” herbivory for ecosystem health, or merely tolerate grazing pressures.
There are important implications for both of these arguments with regards to public lands management. The argument that cattle are ecological equivalents to bison is used as an “scientific” justification for continuing domestic livestock grazing on some public lands.
Furthermore, public agencies are supposed to manage lands for ecosystem stability and health. Those who oppose removal or reduction of livestock from public lands frequently argue that grazing and herding impacts are necessary to avoid ecosystem decay, and that plants “benefit” from herbivory by removing “decadent” plant growth. Thus removal of domestic animals is reasoned to be counter-productive to good resource management. This argument has already been used by land managers in a number of instances (Tohill and Dollerschell, 1990). So ingrained is the idea that livestock and grazing are synonymous that some livestock supporters assume removal of domestic cattle and sheep equal “no grazing” (Laycock, 1994) and caution against the presumed deleterious effects upon rangeland “health”.
It must be pointed out that the absence of livestock is not the same as no grazing since most native rangelands experience a wide variety of herbivory pressures, from nematodes in the soil to invertebrates like grasshoppers, to larger mammals and birds like prairie dog, antelope, elk and bison where they are still found (Archer and Smeins, 1991). Furthermore, under natural conditions, the populations of these animals fluctuate seasonally as well as annually due to changing conditions, such as variation in predation pressure, competitive interactions between and within species, and availability of forage. Due to these factors, some have questioned whether the ecological effects upon rangelands of near constant numbers of cattle can be equated with naturally fluctuating numbers of wildlife (Heitschmidt, 1990).
Despite the strong advocacy for the idea that cattle are merely tame bison, few studies have attempted to quantify the ecological-evolutionary differences between these two animals and their interactions with native ecosystems.
However, there is evidence that, taken together, can shed some light on the issue, and provide the way for further research and better management decisions. These include a review of bison, cattle, and plant community evolutionary histories along with studies of their effects upon habitat selection, nutrition and behavior.
Bison (Bison bison) and cattle (Bos tarus) both evolved from a common ancestor in Asia. This animal, named Leptobos, appeared in the Pliocene, and became widely distributed throughout Eurasia. Both Bison and Bos branched from Leptobos prior to its extinction in the late Pleistocene. Early in its evolutionary history, bison resembled cattle in many features including horns that pointed forward, a straight back and few seasonal secondary sexual characteristics. Both cattle and the early bison forms inhabited woodlands and forested areas. During the Pleistocene, bison gradually evolved adaptations that enabled them to exploit the steppe tundra ecosystem, and eventually spread into the available habitat across Asia and Europe. This form became known as Bison priscus (McDonald, 1981).
Bison priscus colonized North America via the Bering land bridge during the early to middle Pleistocene (Guthrie, 1980). Bison evolutionary history in North America is complex and interpretation of the finer points has been controversial. Some authorities believe several successive waves of bison immigration from Asia are responsible for the great variation in bison archeological remains, while others maintain bison responded rapidly to changing climatic and habitat conditions, producing numerous variations on the same theme (McDonald, 1981). Bison latifrons , for example, had very large horns almost resembling Texas Longhorn cattle. This form became extinct in the late Pleistocene. Another form, Bison antiquus survived into the Holocene where it evolved into two recognized subspecies Bison bison bison , the common plains bison and Bison bison athabascae Bison bison athabascae, the woodland or mountain bison that makes up the animals found in various wild herds in the Northwest Territories and northern Alberta. Bison bison bison reached maximum range and density approximately 2,500 BP (Guthrie, 1980; McDonald, 1981).
Guthrie (1980) summarizes the basic characteristics expected of animals evolving under different habitats. Savanna-steppe adaptations include non-lethal fighting apparatus, large groups, class hierarchy, elaborate gaudy social organs, migratory-nomadic behavior, less selective feeding and the ability to digest coarse fiber, and marked seasonal adaptations. By contrast, species evolving in woodlands tend to display the following adaptations: lethal fighting apparatus, small groups, linear or modified linear hierarchy, conservative social organs, territorial fidelity, selective feeding strategies, and reduced seasonality. A comparison between cattle and bison clearly shows that American bison fit the savanna-steppe adaptations while cattle, despite centuries of domestication and breeding, are defined more by the woodland attributes.
Although bison became an inhabitant of open landscapes such as plains, or grassy savannas, most of the evolutionary precursors of domestic cattle are inhabitants of subtropical lowland regions where they reside in swampy humid forests. Bison are the only members of the entire Bovini line that does not have at least a partial tropical distribution (McDonald, 1981). Today, several species of wild cattle are confined to southeast and central Asia where they occupy open areas in rainforests and uplands, feeding by grazing and browsing (McDonald, 1981). Their niche somewhat similar to whitetail deer in North America, which favor the edges along forest borders and the lush cover of riparian and other shrubby vegetation.
Residual herds of bison are still found in Eurasia where they are restricted to forest and mountain areas. However, these animals are said to resemble “cattle” in appearance and behavior (Guthrie, 1980). In North America, bison occupy primarily grasslands or parklands and have evolved into an animal adapted to “open” landscapes exhibiting migratory behavior, and a tolerance for arid environments and a shifting mosaic of resources (Renyolds et al. 1982).
During the Pleistocene, vegetation across wide swaths of North America changed substantially from broadleaf evergreen vegetation towards a greater abundance of coniferous species, resulting in a deterioration in herbivore forage opportunities. However, at the same time, there was an expansion of steppe areas with fibrous and abrasive foods (McDonald, 1981). The adaptive response of bison was to enlarge body size, while expanding its ruminant digestive system to permit greater intake of low quality forage (McDonald, 1981).
In woodlands where food resources are patchy, small isolated groups of animals are all that can be supported within any parcel of the landscape. As a consequence, one would expect animals in these situations to take on certain characteristics. For instance, living in small groups or in isolation increases vulnerability to predation. Larger body size can discourage predators, and not surprisingly, early forms of bison that lived in forested landscapes tended to be larger overall. However, larger size results in a slower maturation process, with animals living longer, but producing fewer young. Social behavior is less highly developed (Geist, 1971) as well.
Evolution of Bison bison under a grassland regime favored an animal with small body size (compared to woodland and earlier glacial versions of bison-though bison are still the largest terrestrial mammal in North America), a high degree of social behavior that manifests itself in strong herding characteristics, high biotic potential and rapid maturation rates. These are all considered adaptations to an open grassland living situation (McDonald, 1981). The modern day bison evolved into its present form only 5,000 BP (Hudson and Frank 1987).
Bison tend to have decreased aggressive encounters that are ritualistic rather than lethal. During the rut, bull bison develop exaggerated hair tuffs on the head and front legs that is used as a social display of size and rank. They lose these features outside of the rut, taking on the gross features of females. This defuses antagonism between bulls outside of the rutting season and is an adaptation to herding that permits living together in open spaces. (Geist, 1971; Guthrie, 1980; McDonald, 1981)
Morphological adaptations of the bison to facilitate existence in a grassland environment include the downward rotation of the head relative to the vertebral column, along with the lateral placement of the eye orbits which permits maintenance of visual contact with the herd as well as predator detection while grazing. Short limbs also permit easier access to short grasses (McDonald, 1981).
Also, bison horns have rotated towards the side of the head where they are less effective as lethal weapons (Guthrie, 1980). This is an adaptation to the intense daily social encounters that characterize herd animals. Most aggressive encounters involve head shoving, with encounters typically decided by threats (Meagher, 1973). Again this is considered an adaptation to mixed sex herd conditions (Guthrie, 1980; McDonald, 1981).
Another adaption to the mobility and the requisite movement necessary in open prairie landscapes is the high shoulder hump and lower hind quarters of the plains bison. According to Guthrie (1980) this rotation of forequarters, with the shoulders functioning as a fulcrum permitted a cantering gait–a rolling, energy efficient movement. Guthrie theorizes this was an adaptation to frequent long distance movements, in which wild bison seek areas with adequate forage, attempt to capture phenological peaks in plant growth, or obtain water. As we shall see later, behaviorists have all noted the mobility of bison compared to domestic cattle as well as many other ungulates.
Although this rocking gait is efficient, it is not particularly rapid. However, since the termination of the last Ice Age, most of the larger predators that posed a threat to bison went extinct, freeing bison of the need to outrun predators as a predator avoidance tactic. Wolves were the only predator other than humans that posed any threat to bison. The large size of bison is an effective defense against predation by other predator species. Recent predator-prey studies between wolves and bison have shown that calves and older adults make up the majority of prey for wolves, while healthy adults are relatively safe from attack (Carbyn, et. al. 1993). Furthermore, where wolves have a choice of alternative smaller prey such as elk or deer, bison are seldom prey upon.
Prior to the adoption of the horse by Native Americans, most human bison predation was opportunistic relying upon bison jumps or occasional vulnerability created by deep snow. This relationship changed upon the introduction of the horse, destroying the bison’s major predator avoidance strategy of size and herding. As a consequence, they were easily run down on horses (Guthrie, 1980; Urness, 1989). There is evidence that once Native Americans obtained the horse, they were able to exterminate small, marginal populations in some of the peripheral portions of their range, particularly in southeast Idaho and northern Utah (Urness, 1989).
By contrast, cattle, which evolved in moister woodland environments more characteristic of early woodland forms of bison, lack the well-developed features of bison that are adaptations to short grass plains environments, including the downward rotation of the head relative to the vertebral column. Plus, cattle tend to have longer legs (this varies among breeds), and a straight back which may permit short term burst of speed, but are not useful for long distance movement. Not only are cattle less mobile by nature, domestic breeding programs have resulted in animals that are even more obese and less fit for long distance movement.
Many studies have documented the more persistent movement of bison compared to cattle. Van Vuren (1979, 1983) studying bison in the Henry’s Mountains of Utah reported that an introduced herd of wild bison differed from cattle using the same ranges in several notable ways. Bison tended to stray further from water sources, used steeper terrain and higher elevations than cattle. Van Vuren also noted that bison seldom stayed in one location more than 3 days.
Even where habitat variation is low, bison seem to wander widely and Lott and Minta (1983) characterized bison as “highly mobile” animals. According to Lott (1991) who has studied free roaming bison introduced to Catalina Island in California, bison had much larger home ranges than nearly all species of African ungulates, except for during migration periods. For example, water buffalo which are approximately the same size as American bison, have home ranges about 5% of the bison on Catalina Island, even in drought periods (Lott, 1991).
Carbyn and his colleagues (Carbyn et al. 1993) commented that wood bison in Canada often moved up to 32 km over a short period of time “for no apparent reason”.
Norland, (1984) studying bison in Theodore Roosevelt National Park, noted that animals seldom stayed in the same location for more than 48 hours and characterized them as being “highly mobile, moving to new localities and habitats almost daily.” Norland concluded that due to the constant movement and random nature of these movements that plants were “potentially grazed only once, if at all, in a 3-4 week period”.
Meagher (1989) noted that though bison may have a strong fidelity to a home territory, they do make sudden movements of considerable distance.
Such sudden movements were noted by many early travelers on the plains as well. Along Wyoming’s North Platte River in 1834 John Kirk Townsend (1978) commented that “buffalo still continue immensely numerous in every direction, and our men kill great numbers…”. But the next day he wrote, “When we rose this morning, not a single buffalo, of the many thousands that yesterday strewed the plain, was to be seen. It seemed like magic. Where could they have gone? I asked myself this question again and again, but in vain.”
Not only do bison move more frequently than cattle, but their selection of habitat within the landscape is also different. In northern Colorado, Peden et.al. (1974) found that bison spent less time near water and only watered once a day. Similarly, Norland (1984) reported that bison would go to water once a day. The length of stay at watering areas was “short duration–one hour or less for even the largest herds”. In both studies it was noted that bison appeared to prefer drier forage, spent less time in swales and depressions where soil moisture was higher than might be expected.
Cattle, on the other hand, are less efficient water users and display a marked preference for moister forage. Pinchak and colleagues reported that 77% of the observations of cattle grazing foothill ranges in Wyoming were within 366 meters of water (Pinchak et al. 1991) and noted that the majority use was on wetlands or subirrigated, level sites. Similarly, Smith (Smith et. al. 1992) found that cattle selected a higher percentage of floodplain habitat and a lower percentage of upland habitat than these habitat types represented in their study area, as did Goodman (Goodman et. al. 1989). The negative effects of domestic cattle on riparian ecosystems are well documented (GAO, 1988, Kauffman, and Krueger, 1984) as is the ecological value of riparian areas to wildlife (Chaney, et al. 1990; GAO, 1988; Beschta, et al. 1991). Riparian areas make up approximately 1% of the land area of the West, yet 60-80% of the native vertebrate species are associated or require this habitat for their continued survival (GAO, 1988; Chaney, et. al. 1990).
Because of their natural propensity to linger in riparian areas or wetlands, domestic cattle pose a far greater threat to arid land biodiversity than native species like bison. Preventing damage by livestock to riparian areas requires capital investments in upland water development, fencing, salting, and riding–all of which increase the costs per unit of production, quickly exceeding the financial return upon investment in many arid western rangelands, unless costs are subsidized (Holechek, 1992).
Peden (Peden et al. 1974) also noted that bison selected rougher, less digestible forage. This gives them a competitive advantage on native grasslands where forage quality varies seasonally. Plus the ability to utilize lower quality forage results in better distribution of herbivory pressure on rangelands grazed by bison than under livestock usage.
A comparison of digestion between domestic cattle, bison and Tibetan yak (Schaefer, et al. 1978) found that bison retained forage in its digestive tract longer, hence had a greater ability to digest fibrous feed material, and resulted in higher nitrogen intakes. This may be one reason that bison can survive and persist on ranges where cattle perish without supplemental feed.
Furthermore, bison are able to forage in deep snow (Meagher, 1973, Carbyn et.al. 1993) which at least in northern regions is a distinct advantage over domestic livestock which require supplemental feed. Bison also have a hide of higher insulative value than cattle (Peters and Slen, 1964), another adaptation to harsh winters and seasonal food limitations.
The factors listed above explain some of the differences between cattle and bison utilization of the landscape. Bison naturally wander widely, far more than cattle, even under essentially open range conditions (Pinchak et. al. 1991).
Although there are historical accounts documenting that bison occasionally heavily grazed an area, it would be incorrect to assume that bison carpeted the plains as one great mowing machine. Many early travelers on the plains noted both the abundance and the absence of bison and other large ungulates due to seasonal movements and other factors. For example, in 1806 while returning from the Pacific, Captain Clark (1964) of the Lewis and Clark Expedition traversed nearly 200 miles of Montana from the Continental Divide in the Big Hole Valley near the Idaho border, down to the Missouri headwaters confluence, across the Gallatin Valley and down the Upper Yellowstone to where the town of Big Timber, Montana now sits before they were successful in seeing and killing a single bison. Their Indian guide, Sacajawea informed Clark that the bison were once numerous in the Upper Missouri headwaters, but had been driven away by heavy Indian hunting. Nevertheless, further down the Yellowstone below what is now Billings, Montana, Clark (1964) noted “the entire country is enlivened by herds of buffalo, elk and wolves.” And in 1834, John Kirk Townsend (1978) crossing the region near South Pass, Wyoming complained that his party was suffering from a what of food, commenting that “buffalo are rarely seen.” Thus the shifting mosaic of grazing pressure resulting from the near constant movement, along with periodic mass die-offs due to disease, harsh weather, or predation, almost ensured that heavily grazed areas would be rested, often for months or even years.
Finally, bison interact with other native species in ways not typically observed with domestic livestock, that may result in reciprocal ecological relationships between different native species. For example, it was noted that bison tended to graze areas around prairie dog towns, thus enjoying succulent new regrowth of plants previously cropped by prairie dogs while at the same time reducing the grass cover which benefited the rodents by making it easier to spot predators (Coppock et. al. 1983).
In addition to the above morphological and behavioral characteristics, a good portion of the West, particularly the majority of public rangelands that are located in the Intermountain West and Southwest consist of ecosystems that historically did not support large herds of bison, if at all. These include most of the sagebrush steppe of the Intermountain West, the Southwest desert regions and the palouse grasslands of Washington and eastern Idaho (Miller et al. 1994; Mack and Thompson, 1982).
A number of researchers have commented upon the historic absence or small numbers of bison in the sagebrush-steppe zone west of South Pass, Wyoming and total absence from the Great Basin Sagebrush deserts. When the first Europeans entered the western United States during the fur trade era in the early 1800’s, bison were reported for southeast Idaho (Work, 1913, Russell, 1955), northern Utah, and eastern Oregon (Ogden, 1910, Bailey, 1936). However, they were absent from many other portions of the Intermountain West such as the Great Basin Sagebrush deserts of Nevada, the desert grasslands of Arizona and elsewhere west of the Rockies (Reynolds et. al. 1982). Other large ungulates such as elk, deer, antelope, and bighorn sheep were also patchily distributed or even absent from large areas of these semi-desert and desert regions according to early historic accounts, with bighorn sheep and antelope being the most numerous large animals (Davis, 1982; Ogden, 1910; Simpson, 1983).
There is evidence that suggests that even where bison were found, their numbers were small, and distribution was patchy. Periods of favorable climatic and forage conditions probably enabled intermittent recolonization of suitable habitat by herds moving in from the plains. However, deformities among the skulls and teeth of bison remains from eastern Oregon suggest such periodic recolonizations were infrequent occurrences and these populations were isolated, locally inbred populations (McDonald, 1981).
A number of people have addressed this issue. Daubenmire (1985) argued that protein deficiencies of native bunchgrass rangelands, along with occasional deep snowfalls limited bison populations along their western margins–although as seen earlier, bison are more efficient at extracting nutrients from forage than other ruminants (Peden et al. 1974)
Mack and Thompson (1982) suggested that grass phenology may have limited bison reproduction compared to the plains. While cool season grasses provide plenty of protein early in the spring, early onset of dormancy in summer, with a consequent loss of nutritive value, may have stressed lactating female bison. On the plains, a mixture of cool season and warm season grasses extend the season of high nutrient food resources, permitting greater exploitation by bison.
Van Vuren (1987) has postulated that bison living on rangelands in the Intermountain West found forage inadequate to sustain large numbers of bison except for a few locations. Frequent local extinctions due to weather, human hunting, or the effects of inbreeding, with slow recolonization, rates may have kept bison numbers exceedingly low over this region and may account for the relative lack of historic sightings and limited distribution.
Further evidence to the absence or limited distribution of bison throughout the Intermountain West comes from the native vegetation itself. The native vegetation of these regions has no previous evolutionary experience or adaptation (Mack and Thompson, 1982) to persistent, heavy grazing and trampling.
Although Savory (1983, 1988) and others contend that arid landscapes suffer as a consequence of “undergrazing” or from the absence of herd trampling effects, there is growing evidence that soil disturbance by exotic livestock has damaged microphytic crusts and lichen cover in many parts of the Intermountain West. These crusts are important for nitrogen fixation, reduced overland flow, and increased moisture infiltration and often enhance native vegetation establishment and growth (Harper and Pendleton, 1993). Most researchers agree that livestock destroy such crusts through trampling (Anderson, et. al 1982; Miller et al. 1994).
Unlike the rhizomatous Great Plains grass species dominated by blue grama (Bueteloua gracilis) and buffalo grass (Buchloe dactyloides) that seem to tolerate grazing pressure, the native vegetation in this region including dominants like bluebunch wheatgrass (Agropyron spicatum), Idaho fescue (Festuca idahoensis), and Indian ricegrass (Oryzopsis hymenodies), are caespitose or bunchgrasses, and thus less tolerant of both grazing and trampling (Mack and Thompson, 1982, Mack, 1986).
The net effect of livestock introduction into regions where bison numbers were restricted or absent is a significant loss of native biodiversity and major shifts in ecosystem function. For example, for the 10,000 years prior to European settlement the flora of the sagebrush steppe remained essentially unchanged, although migrations up and down mountains due to shifting climatic conditions did occur (Miller, et. al. 1994).
Due to their respective evolutionary histories, a variety of behavioral, biological and ecological differences exist between bison and cattle. Cattle are poorly adapted for a dry, arid landscape with rugged terrain, and the consequences of their evolutionary heritage may lead to degraded rangelands (Jacobs, 1990). Wild free-roaming bison, on the other hand, are more favorably adjusted to their environment, and were sustained for thousands of years without contributing to serious degradation of rangeland ecosystems.
Since substantial differences in behavior and habitat use and selection exist between bison and cattle, it may be erroneous to suggest that domestic livestock fill a vacant niche left by the extirpation of the bison. Rather cattle should more properly be viewed as a new ecological force that differs significantly from the native species.
Furthermore, some native ecosystems apparently did not support bison in any numbers or they were completely absent. In particular, much of the Great Basin, Palouse Prairie, Southwest deserts, and California annual grasslands evolved without the presence of bison. These native rangeland ecosystems display limited tolerance to grazing pressure of any kind (Jones et. al 1991; Miller et al, 1994). Indeed, in many of these areas large groups of any kind of herding animal were only of local abundance or completely absent (Holechek et al. 1989; Jacobs, 1990; Mack, 1982). Under grazing pressure from domestic animals these arid landscapes have not adapted or thrived in the presence of domestic livestock as some suggest (Savory, 1988), rather they have shown substantial degradation.
Careful attention to the evolutionary histories of both native species and ecosystem development may prevent degradation of ecosystem function and loss of native biodiversity. The assertion that exotic animals are a replacement for native species should be scrutinized closely before it is assumed gross outward similarities really translate into similar use of the landscape.
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