Bos bison (American bison)

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Title Adult Caption Bos bison (American bison); bull, on American plains. USA. Copyright Public Domain image: released by USDA-ARS/Jack Dykinga	Adult	Bos bison (American bison); bull, on American plains. USA.	Public Domain image: released by USDA-ARS/Jack Dykinga

Identity

Preferred Scientific Name

• Bos bison Linnaeus, 1758

Preferred Common Name

• American bison

Other Scientific Names

• Bison bison Hamilton-Smith, 1827
• Bison bison athabascae Rhoads, 1898

International Common Names

• English: American buffalo; bison, American

Local Common Names

• Canada: wood bison
• USA: buffalo, American

DADIS main name

• Bison

Summary of Invasiveness

The American bison, Bos bison, is a large mammalian herbivore that was once numbered in the tens of millions and was distributed throughout most of North America, with numbers greatest in the grasslands of the Great Plains.  Overhunting during the 19^th century resulted in its near-extinction; bison were recovered largely through captive propagation and restoration of herds in the original range, but a few herds were established outside the original range. Today there are more than 700,000 bison; most are kept on fenced ranges for commercial purposes, and the rest are managed for their conservation value. Two of the herds outside the original range, in Grand Canyon National Park, Arizona, and on Santa Catalina island, California, are considered to be non-native and are doing damage to natural resources by consuming and trampling native vegetation, and potentially to cultural resources by trampling archaeological sites.  Bison are not on any alert list or considered a regulated pest.  One subspecies of bison, the wood bison (B. bison athabascae) of northern Canada and Alaska, is listed in Appendix II by CITES (CITES, 2015; IUCN, 2015), and as “Threatened” by Canada (IUCN, 2015) and the United States (NatureServe, 2015); the species as a whole is classified as “Near Threatened” in the IUCN Red List (IUCN, 2015).

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Chordata
•             Subphylum: Vertebrata
•                 Class: Mammalia
•                     Order: Artiodactyla
•                         Suborder: Ruminantia
•                             Family: Bovidae
•                                 Genus: Bison
•                                     Species: Bos bison

Notes on Taxonomy and Nomenclature

Bison taxonomy has long been a matter of debate (Groves and Grubb, 2011).  Bison were long placed in the genus Bison based on morphological features, but genetic data have revealed that a shift to the genus Bos is warranted, although this revision is not universally accepted (Reynolds et al., 2003; Boyd et al., 2010).  The European bison, or wisent (Bos bonasus), is sufficiently closely related to the American bison that the two have sometimes been considered as subspecies of the same species; however, recent work suggests that they should be separate species (Boyd et al., 2010; Groves and Grubb, 2011).  Two subspecies of American bison are generally recognized, although not without dissent (Reynolds et al., 2003; Boyd et al., 2010):  the wood bison (B. bison athabascae), which ranged from central Canada to Alaska, and the plains bison (B. b. bison), which ranged from central Canada south to northern Mexico.

Description

B. bison is a large ungulate with brown pelage, characterized by a hump at the shoulders, a large head and short neck, and short, curved horns.  Calves weigh 14-18 kg at birth and grow steadily until reaching adult mass at about 8-10 years of age for males and 5-6 years for females (Reynolds et al., 2003).  Adult males weigh about 600-850 kg and stand 160-190 cm tall at the shoulder, and adult females weigh about 350-550 kg and stand 150-160 cm tall (Reynolds et al., 2003).

Distribution

Before the near-extinction of the species in the 19^th century, its range covered much of North America. More than 700,000 American bison exist today, but 97% of these are privately owned and kept on fenced ranges for commercial purposes or as pets (Reynolds et al., 2003).  These privately-owned bison occur in every state of the United States and every province of Canada, and in other countries as well (National Bison Association, 2015).  The Distribution table in this datasheet lists only “conservation herds”, those herds of bison managed for their conservation value (Gates et al., 2010).

Bison on Santa Catalina Island, California, were introduced in 1924 and are known to be non-native.  The population is managed and so is not spreading or increasing.

Bison at the House Rock State Wildlife Area, Arizona, originated from an attempt to hybridize bison with cattle that began in 1906.  Eventually the herd was sold to the state of Arizona, which managed it for wildlife value.  During the late 1990s the herd began expanding its range into the adjacent Grand Canyon National Park.  There is no historical evidence that bison occurred in the park or surrounding areas, and the National Park Service considers bison to be introduced (Reimondo et al., 2015).  However, there is some disagreement with this classification, based on archaeological evidence, dating to at least 650 years ago, of bovid remains in the general vicinity of the Grand Canyon that have been identified as likely being bison (Martin, 2014).

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

Introductions of bison generally occurred as part of efforts to restore species numbers after near-extinction occurred in the late 19th century.  Numerous bison herds were initiated as a part of this effort; most were established within the original range of the species, but a few were established at locations that were outside the original range, or close enough to the original range that it is debatable whether the species is native or non-native.

Risk of Introduction

Bison are large animals that are difficult to handle and transport, so the risk of an intentional, surreptitious introduction is low.  However, they have relatively generalized habitat requirements and are highly mobile, so there is a possibility of the introduction of private bison herds that escape captivity outside their original range, or the spread of unconfined, introduced herds.

Habitat

Historically, bison are known for their large herds occupying the grasslands of the Great Plains of North America, but in smaller numbers they also frequented other habitats that included a substantial component of grass or grass-like species, such as mountain meadows, shrub-steppe habitats west of the Rocky Mountains, and aspen parklands and sedge meadows in Canada.

Habitat List

Biology and Ecology

Genetics

B. bison have 2n=60 chromosomes, the same as domestic cattle (Bos taurus), to which they are sufficiently related that they can be hybridized, though only with considerable difficulty; numerous attempts to cross the two species were made during the early 20^th century in order to obtain favourable characteristics of both (Hedrick, 2009).  Cross-breeding attempts were eventually abandoned, but the survivors of these attempts were sometimes used to initiate herds during efforts to restore bison numbers.   Recent genetic assays have revealed that most conservation herds of bison show evidence of cattle ancestry (Hedrick, 2009).  Genetic contamination by cattle, along with suspected low levels of genetic variation due to the extreme population bottleneck experienced during the late 19th century, are current concerns (Hedrick, 2009). 

Reproductive Biology

Mating occurs once per year, during late summer or early fall, with breeding occurring later with increasing latitude (Vuren, 1987).  Males defend one female at a time against rival males in a pairing known as a “tending bond” (McHugh, 1958).  Most breeding is done by “prime bulls” from 6 to 9 years old (Reynolds et al., 2003), even though they may not yet have stopped growing at this age.  The gestation period is 9-9.5 months, and calves are born between April and June.  One calf is typical, and twinning is rare.  Not all females reproduce each year; the calving rate (number of calves per adult female) ranges from 62-88% among bison herds within the original range (Reynolds et al., 2003).  In more arid areas, adequate precipitation and subsequent forage growth appear to affect calving rate (Vuren and Bray, 1986); density of the introduced population on Santa Catalina Island, California, is limited by the Mediterranean climate whose dry summers limit availbility of forage in the bison calving season (B. Coblentz, Oregon State University, Corvallis, Oregon, USA, personal communication, 2015).

Physiology and Phenology

Bison are highly adaptable and are able to tolerate a wide variety of climates that range from the arid grasslands of northern Mexico to sedge meadows near the Arctic Circle.  Although known as swift runners on the Great Plains, where they have been clocked exceeding 50 km/hour (McHugh, 1958), they are also readily traverse steep slopes exceeding 30 degrees in montane habitats (Vuren, 2001).

Longevity

The maximum lifespan of the species is unknown.  They are thought to have reached old age by 15 years, although there are reports of bison in captivity living much longer (Reynolds et al., 2003). 

Activity Patterns

B. bison show a diurnal activity pattern of grazing in the morning and afternoon, and resting during the middle of the day.  Night-time activity is not well known.  Historically, some authors believed that bison on the Great Plains exhibited a pattern of latitudinal migration, moving north in the summer and south in the winter, but there is little evidence in support of this belief (Roe, 1970).  There is, however, some support for bison historically showing a regular migration between seasonal ranges (Roe, 1970), and some extant herds show such a pattern.

Population Size and Density

B. bison as a species totals over 700,000 animals, but very few of these bison live in what might be considered a “wild” state. 

Nutrition

B. bison are grazers and primarily eat grasses and sedges.

Climate

Latitude/Altitude

Notes on Natural Enemies

Wolves (Canis lupus) are the only important predator of bison and can be a significant regulating factor for some populations (Reynolds et al., 2003).  Wolves do not occur in localities where bison are considered to be introduced.  Numerous parasites and diseases have been identified in bison (Reynolds et al., 2003), but disease-caused mortality, and resultant population effects, are poorly known.

Means of Movement and Dispersal

Natural Dispersal

Bison are large, mobile animals that can cover long distances, so dispersal outside their original range is a possibility. 

Accidental Introduction

This is unlikely because bison are large and conspicuous.

Intentional Introduction

B. bison have been introduced to numerous locations outside their original range, in the past as part of efforts to restore numbers after near-extinction, and more recently for meat production, as pets, or for other reasons.

Pathway Causes

Impact Summary

Economic Impact

B. bison living outside their original range that come into conflict with humans have a negligible economic impact, aside from the cost of control.  Some bison within their original range have caused or might cause economic impacts.  Those in Yellowstone National Park (USA) and Wood Buffalo National Park (Canada) carry either or both of two diseases, brucellosis and tuberculosis (Reynolds et al., 2003); these diseases have the potential to be transmitted to domestic cattle, with important economic impacts if that were to occur.  Bison in Alaska have caused substantial economic losses to grain farmers by consuming and trampling crops (Gipson and McKendrick, 1982).  Bison in in some portions of their range have the potential to be competitors of domestic cattle for forage (Vuren, 2001).

Environmental Impact

Bison are large herbivores that have a major impact wherever they occur, by consuming and trampling herbaceous vegetation, by their wallowing behavior, by horning or rubbing against woody vegetation, by urination and defaecation, and by seed dispersal (Reynolds et al., 2003).  In general, these impacts are considered desirable in locations managed as natural areas, and bison were a “keystone species” in tallgrass prairie (Knapp et al., 1999) and perhaps other portions of their original range as well. 

However, bison impacts are considered undesirable for herds introduced outside the original range.  On Santa Catalina Island, California, bison impacts include a major reduction in plant cover and plant species diversity, trampling of woody species (Knapp, 2014), and spread of non-native plants (Constible et al., 2005); the population is managed to limit these effects (US Fish and Wildlife Service, 2006). Crushing injuries to garter snakes (Thamnophis hammondii), presumably caused by bison, have also been observed on the island (B. Coblentz, Oregon State University, Corvallis, Oregon, USA, personal communication, 2015).

In Grand Canyon National Park, Arizona, bison grazing in wetlands resulted in a 70-90% decrease in vegetative cover, a 25% decrease in vegetation height, and a 40-50% increase in bare soil (Reimondo et al., 2015).

Threatened Species

Social Impact

In Grand Canyon National Park, Arizona, bison are suspected of damaging archaeological sites (Minard, 2003).

Risk and Impact Factors

Impact mechanisms

• Herbivory/grazing/browsing
• Interaction with other invasive species
• Pest and disease transmission
• Trampling

Impact outcomes

• Ecosystem change/ habitat alteration
• Infrastructure damage
• Modification of nutrient regime
• Modification of successional patterns
• Negatively impacts forestry
• Reduced native biodiversity
• Threat to/ loss of endangered species
• Threat to/ loss of native species

Invasiveness

• Gregarious
• Has a broad native range
• Highly adaptable to different environments
• Highly mobile locally
• Is a habitat generalist
• Proved invasive outside its native range
• Tolerant of shade

Uses

Economic Value

Most bison live in fenced, privately owned herds that are maintained because of the economic value of bison meat; byproducts such as skulls, horns, skins, and wool also have value (Reynolds et al., 2003).  Several herds are open to public hunting, which brings considerable economic value (Reynolds et al., 2003).

Social Benefit

Because bison are iconic of the American West, bison herds that are accessible to the public are highly valued.  This social value can restrict management actions for exotic herds (Duncan et al., 2013). Bison also have great cultural and spiritual significance for Native American tribes that coexisted with them historically (Reynolds et al., 2003).

Environmental Services

Historically, bison were a keystone component of the Great Plains (Reynolds et al., 2003), but that role is minimal to nonexistent today.

Prevention and Control

Options available for controlling non-native bison are limited because of the high sociological value of bison and legal restrictions.  Three main methods have been employed for non-native bison: fencing, live removal, and fertility control.  Fencing can be effective in limiting spread but the approach is costly and can affect non-target species (Minard, 2003); also bison can easily break fences that are not substantial enough (B. Coblentz, Oregon State University, Corvallis, Oregon, USA, personal communication, 2015).  Live removal to reduce numbers requires expensive facilities for capturing, handling, and transporting bison, and is stressful for the bison (Duncan et al., 2013).  Fertility control via immunocontraceptive injection shows promise for reducing bison numbers (Duncan et al., 2013). All three of these methods have been used on Santa Catalina island (Duncan et al., 2013). Hunting has been attempted as a control method in Arizona, but with minimal success; in any case it is prohibited inside the Grand Canyon National Park (D. Van Vuren, University of California Davis, Davis, California, USA, personal communication, 2015; Bullock, 2015).

References

Boyd DP, Wilson GA, Gates CC, 2010. Taxonomy and nomenclature. In: American bison: status survey and conservation guidelines 2010 [ed. by Gates, C. C. \Freese, C. H. \Gogan, P. J. P. \Kotzman, M.]. Gland, Switzerland: International Union for the Conservation of Nature and Natural Resources, 13-18.

Bullock AM, 2015. How do you solve a problem like the 'Beefalo'? London, UK: British Broadcasting Corporation. http://www.bbc.co.uk/news/science-environment-31661920

CITES, 2015. Appendices. Geneva, Switzerland: CITES. https://cites.org/eng/app/appendices.php

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Gates CC, Freese CH, Gogan PJP, Kotzman M, 2010. American bison: status survey and conservation guidelines 2010. Gland, Switzerland: International Union for the Conservation of Nature and Natural Resources.

Gipson PS, McKendrick JD, 1982. Bison depredation on grain fields in interior Alaska. In: Proceedings of the Fifth Great Plains Wildlife Damage Control Workshop [ed. by Timm, R. M. \Johnson, R. J.]. Lincoln, Nebraska, USA: Institute of Agriculture and Natural Resources, University of Nebraska, 116-121.

Groves C, Grubb P, 2011. Ungulate taxonomy. Baltimore, Maryland, USA: Johns Hopkins University Press.

Hedrick PW, 2009. Conservation genetics and North American bison (Bison bison). Journal of Heredity, 100(4):411-420. http://jhered.oupjournals.org/

IUCN, 2015. IUCN Red List of Threatened Species. http://www.iucnredlist.org/

Knapp AK, Blair JM, Briggs JM, Collins SL, Hartnett DC, Johnson LC, Towne EG, 1999. The keystone role of bison in North American tallgrass prairie. Bison increase habitat heterogeneity and alter a broad array of plant, community, and ecosystem processes. BioScience, 49(1):39-50.

Knapp DA, 2014. Ecosystem restoration on Santa Catalina Island: a review of potential approaches and the promise of bottom-up invader management. Monographs of the Western North American Naturalist, 7:421-434.

List R, Ceballos G, Curtin C, Gogan PJP, Pacheco J, Truett J, 2007. Historic distribution and challenges to bison recovery in the Northern Chihuahuan Desert. Conservation Biology, 21(6):1487-1494. http://www.blackwell-synergy.com/loi/cbi

Martin JM, 2014. Late Pleistocene and Holocene bison of Grand Canyon and Colorado Plateau: implications from the use of paleobiology for natural resource management policy. Johnson City, Tennessee, USA: East Tennessee State University. [MS Thesis]

McHugh T, 1958. Social behavior of the American buffalo (Bison bison bison). Zoologica, 43:1-40.

Minard A, 2003. Unwelcome bison may face a DNA test. Science, 299:1835.

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US Fish and Wildlife Service, 2006. Sibara filifolia (Santa Cruz Island Rockcress). Five-Year Review: Summary and Evaluation. USA: US Fish and Wildlife Service, 24 pp.

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Vuren D van, Bray MP, 1986. Population dynamics of bison in the Henry Mountains, Utah. Journal of Mammalogy, 67:503-511.

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Contributors

22/08/2015: Original text by:

Dirk H. Van Vuren, Department of Wildlife, Fish, and Conservation Biology, University of California, Davis, CA 95616, USA

Distribution Maps

• = Present, no further details
• = Evidence of pathogen
• = Widespread
• = Last reported
• = Localised
• = Presence unconfirmed
• = Confined and subject to quarantine
• = See regional map for distribution within the country
• = Occasional or few reports

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