“The Incredible Shrinking Bison” [1] discussed how the increase of CO2 emissions has contributed to the rise of the Great Plains’ average temperature, and how the resulting warming trend has affected the bison. The bison, on the other hand, as a key species to the survival of the plains, can also have an indirect, mitigating effect on the CO2 levels in the atmosphere.

The predominate conversation around atmospheric CO2 has centered on the elimination of CO2 production.  There is, however, another conversation underway—one involving the removal of CO2 from the atmosphere. The process of capturing and storing atmospheric carbon dioxide is known as carbon sequestration.  It is one method of reducing the amount of carbon dioxide in the atmosphere, and consists of two types: geologic and biologic.  Geologic carbon sequestration involves the storing of carbon dioxide in underground geologic formations.  The CO2 is usually pressurized until it becomes a liquid, and then it is injected into porous rock formations in geologic basins [2]. 

Biologic carbon sequestration refers to storage of atmospheric carbon in vegetation, soils, woody products, and aquatic environments. For example, by encouraging the growth of plants—particularly larger plants like trees—advocates of biologic sequestration hope to remove CO2 from the atmosphere.  Within biologic carbon sequestration there are several means by which CO2 is removed from the atmosphere.  These include peatland, wetland, forestry, agriculture, carbon farming, deep soil, and ocean-related. But of all the terrestrial (as opposed to aquatic) methods, the forests receive the lion’s share of the world’s attention.  Forgotten are the grasslands which also harbor much of the wetlands. For North America the grasslands of the Great Plains and prairies, which occupy approximately one-third of the continent, are critical to carbon sequestration.  And key to the grasslands’ vitality is the North American Bison [3].

Grasslands quickly process carbon from the atmosphere and store this carbon in the root structures, which extend 8 to 15 feet into the ground, which can store 22.5 million tons of carbon. These roots can hold the carbon for decades, and process 1.7 million tons of carbon per acre to the soil annually.  This storage accumulates over time and moves carbon from the atmosphere to the ground continuously creating massive carbon deposits over the course of centuries. Prairies have the ability to store as much carbon below the ground as forests can store above the ground. When carbon is stored below ground it remains locked there and unable to enter the atmosphere.  Compared to forests, grasslands are more reliable.  In times of drought and forest fires, the carbon stored in the wood and leaves returns to the atmosphere.  During a grass fire, however, carbon is not released since it is stored in the roots underground [4].

School of Environmental Sustainability-Colorado State University [5]

Though the Great Plains and prairies occupy a vast swath of the North American continent, this does not translate into a great CO2 scrubber.  The conversion of this ecosystem into cropland has significantly reduced the ability of this region to sequester carbon [6]. Compared to native or natural vegetation, cropland soils are depleted in soil organic carbon (SOC).  When soil is converted from its native state the SOC content in the soil is reduced by approximately 30 to 40% [7].  Further, the crops replacing the native grasses are annuals with comparatively shallow root structures which are less effective in storing carbon and holding soil.  With less carbon stored and moved to soil, and increased possibility of soil loss, the effectiveness of the plains and prairies in atmospheric CO2 removal is significantly decreased.  

Short of returning the croplands back to the natural state of the region, there are agricultural methods aimed at sequestering atmospheric carbon into the soil and in crop roots, wood and leaves.  These methods are collectively referred to as carbon farming.  Besides removing CO2 from the atmosphere, increasing the soil’s carbon content—whether by reverting to the natural condition, or by carbon farming—aids plant growth, increases soil organic matter which improves agricultural yield, improves soil water retention capacity and reduces fertilizer use which is a source of the greenhouse gas nitrous oxide (N2O) [8].

Carbon farming or recovering the native perennials, however, is not the complete answer.  The ecosystems of the plains and prairies were dependent on the large herds of bison moving over the grasslands.  The grazing, trampling and recovery patterns associated with the bison were key in building soil, maintaining biological diversity and deepening plant roots, which are crucial elements in permanent carbon sequestration [9].  The bison not only provided nutrients for plant life, but tilled the soil with their hooves, working up and trampling dung into the soil, enabling plant-life to take hold, flourish and consequently become a significant carbon sink.

School of Environmental Sustainability-Colorado State University [10]

Though sequestration, as used here, is a technical term, the concept is quite familiar. When we hear the word “sequester,” the common association is with juries as in jury trials.  When a jury is sequestered, it is removed and kept apart from contact with the public.  The purpose is to ensure undue influence on, or tampering with, the deliberations of the jury, and ensure a just verdict.  As the jurors file out of the courtroom at the end of the defense’s and prosecution’s final presentations, if the judge has ordered they be sequestered, we see the tangible form of a removal to protect the integrity of the trial by jury justice system considered critical to our legal well-being.  The notion of using an act of removal in the protection of our well-being is only part of the meaning of sequestration.  What is being removed and where it is being kept are equally important.  Originally, “to sequester” meant “…to put in the hands of a trustee…” [11]. In regard to carbon sequestration the trustee is the earth itself, or more specifically, in the context of the bison and the grasslands of North America, it is the Great Plains ecosystem.  When we think of ecosystems, we tend to think of the land, the flora and the fauna.  Often missing in our consideration is the air above.  The bison—a  keystone species in regard to the flora and fauna and the land—is a crucial element of the trustee,  instrumental in the process of CO2 removal and the mitigation of the warming trend plaguing the Great Plains.

End Notes:

[1] Schuette, Keith. “The Incredible Shrinking Bison.” November 17, 2020.Bison Witness.

[2] “What is Carbon Sequestration?” What is carbon sequestration? ( Retrieved 4/24/21

[3] Schuette. “Dung Cake and Feces Pie: Yum!” April 26, 2019. Bison Witness.

[4] Davidson, William, “The Great Plains: America’s Carbon Vault” (2016). Op-Eds from ENSC230 Energy and the Environment: Economics and Policies. 73.

[5] Lavelle, Jocelyn. Soil carbon sequestration to combat climate change—a real solution or just hype? – Sustainability ( Colorado State University—School of Environmental Sustainability. Retrieved 4/30/21.

[6] 42% of the Great Plains has been converted to cropland, leaving 53% intact.  The remaining 5% holds water or has been converted to human use.  Understanding Grassland Loss in the Northern Great Plains. 2018. World Wildlife Organization.

[7] Poeplau, Christopher; Don, Axel (February 1, 2015). “Carbon sequestration in agricultural soils via cultivation of cover crops – A meta-analysis”. Agriculture, Ecosystems & Environment. 200 (Supplement C): 33–41. doi:10.1016/j.agee.2014.10.024.

[8] “Carbon Farming | Carbon Cycle Institute”. Also, “Carbon Farming: Hope for a Hot Planet – Modern Farmer”. Modern Farmer. 2016-03-25.  And Velasquez-Manoff, Moises (2018-04-18). “Can Dirt Save the Earth?. The New York Times. Retrieved 4/30/21.

[9] Wright, Pam. Bison: The Latest in Carbon Capture Tech.12/24/2017/by Regeneration International. Retrieved 4/30/21.

[10] Lavelle.

[11] Webster’s Unabridged Dictionary of the English Language. 2001. Random House.

The Incredible Shrinking Bison

Toward the end of the 19th century the bison faced extinction by extermination.  Today, even after more than a hundred years of restoration efforts, the plains bison is faced with another threat of extinction—the accelerated warming of the Great Plains.

The Plains Bison (Photograph by Kailyn Komro, West Bend, WI.  (

Even before the immense public attention on climate change, there has been great scientific interest in climate processes and extinction events in the Earth’s natural history.    Evaluation of fossil evidence has shown an inverse correlation between warming trends and body size and mass of large mammals.  As temperatures rise, body size shrinks over large geological time scales. Along with this negative correlation a consequent, positive correlation has been established between shrinking body size and extinctions [1].  

The warming trend, which began at the end of the last Ice Age, has been accelerating in recent decades. [2].  Since the beginning of the 21st century the northern Great Plains’ average summer temperature increased by 0.8˚C while for the southern Great Plains the mean summer temperature rose by 0.4˚C with winters rising by 0.25˚C for both the southern and northern Great Plains [3]. Consequently, the IPCC (Intergovernmental Panel on Climate Change) Working Group 1 predicts a 4˚C increase in global temperatures by 2100 over the 20th century—a period of 100 years.   This rate of temperature change is much greater than for the Bolling-Allerod period [4]—a warming period 14,700 to 12,500 years ago with a mean temperature 6˚C cooler than that for the 20th century.

The evolutionary history of bison has shown an absolute increase of 4˚C is not unprecedented.  However, the time frame in which the bison has had to adapt needs to be considered. From the end of the Last Glacial Maximum (approx. 14,700 years ago) to the 20th century the earth warmed 6˚C. During the Last Glacial Maximum, bison mass was, on average, approximately 910 kg. (2006 lbs.). The greatest decline in body size of 26% occurred between 12,500 and 9250 years ago. Given a generation time between 3 and 10 years, the change in body size occurred in 325 to 1080 generations, producing an average rate of change of 0.2 to 0.7 kg per generation.   If the current warming trend continues as predicted for the 21st century, bison body mass will likely decline from 665kg (current average body mass) to 357kg.  It is unclear whether bison can adapt their body size to a 4˚C temperature increase within 10 generations [5].

Changes in body size and mass of animals have long been used to indicate large-scale environmental processes over geological time scales, and have become predictors of extinction risk in mammals [6].  In regard to bison B. antiquus and B. occidentalis, these species did go extinct, but through phenotypic [7] and morphologic [8] adaptation to changing climatic conditions, they evolved into what is known today as the North American bison (Bison bison) which has existed throughout the Holocene epoch-the current geological epoch. The importance of body size in dictating extinction proneness is likely due to the fundamental association between size and other key life history traits such as fecundity, longevity, mating system, trophic level (step in a nutritive series, or food chain), dispersal ability and energetic requirements [9].

Bison Size Comparison (from

Fossil bison shrank with global warming probably because large-bodied grazers are disadvantaged both by heat dissipation and by the phenological [10] shifts in plant quality and abundance in warming conditions [11].  Impacts of climate change, then, are two-fold: 1) direct effects of temperature on the animal, demanding energy to compensate for heat, and 2) indirect effects of temperature on the animal’s food supply [12].

Maximum body size of endotherms–an animal that is dependent on or capable of the internal generation of heat; a warm-blooded animal—depends on optimal maintenance for the efficient production of tissues.  This is especially true in seasonal environments when food availability and environmental demands constrain the annual windows for growth.   Optimal maintenance is dependent on thermal loads (amount of heat energy).  High thermal loads increase cost of body maintenance to balance internal and external loads through thermoregulation, which reduces energy for growth.

Thermoregulation is the mechanism by which heat balance is achieved.  It affects the use of energy, water and nutrients such as electrolytes and organic nitrogen which affect resting and foraging behaviors. Thermoregulatory processes usually increase energy use by increasing heart rate and blood flow.  In hot weather thermoregulation increases the flow of body water because water is used for evaporative cooling (e.g., panting, perspiration).  In cold weather thermoregulation generates body heat through such efforts as shivering, increased metabolic heat production, and muscular activity in an effort to conserve core body heat through control of blood flow to the periphery [13].

The negative climate-body size correlation, then, reinforce feedbacks that may increase extinction rates [14].  Both excessive heat (> 40˚C) and excessive cold (< -30˚C) directly increase demands for energy, water and nutrients because thermoregulation outputs increase, whereas indirect effects of rising temperature decrease forage quantity and quality—ultimately affecting the supply of energy, water and nutrients [15].  Smaller body size, then, is more efficient in regulating increased thermal loads due to rising temperatures.

Conceptual model of the direct and indirect effects of elevated ambient temperature on body size of Bison bison from Martin, et. al., 2018.

In regard to food supply, climatic warming tends to exacerbate nutritional stress and reduce weight gain in large mammalian herbivores by reducing plant nutritional quality.  Warming trends have the potential to not only reduce the nutritional quality of plant species, but also by decreasing the relative abundance of nutritionally critical plant species.  For the North American plains bison this is likely to result in an increase protein stress, reducing bison growth and reproduction [16].

Compounding the issue of decreased nutritional quality of grasses, the warming trends have resulted in an increase of droughts in the Great Plains.  The lack of water availability reduces the availability of critical plants necessary for bison growth.  Consequently, droughts cause declines in the number and body size of bison [17].

One of the driving factors in the rising temperatures may be the increasing CO2 concentrations which reduce plant protein concentrations in grasslands [18].  Increasing atmospheric CO2 concentrations have been causing Nitrogen to become progressively more limiting to ecosystem productivity.  Nitrogen is a crucial element for many structures and metabolic processes in plants. Plants are required to manufacture the complex molecules by use of minerals from the soil that contain nitrogen such as nitrate ions. Plants too, like animals, need some important macro and micro nutrient elements including nitrogen, oxygen, hydrogen and carbon to keep them healthy. The wellness of plant parts (leaves, roots, trunks, etc.) depends on the availability of essential nutrients like nitrogen to enhance the plant’s biological processes including growth, absorption, transportation, and excretion [19].

Science has offered information and theories concerning the effect of warming trends on the size and survival of bison.  The question for us is: how do we respond?   The Great Plains are predicted to warm, resulting in longer, hotter summers accompanied by more severe droughts.   The anticipated warming and drying along the Great Plains will shift the distribution and protein efficacy of vegetation types by mid-and-late century, altering the supply of digestible energy and digestible nitrogen to bison, native wildlife and domestic livestock [20].  Bison are very good at adapting to shifts in environmental processes given the rates of change in the past.  But with the acceleration of warming rates, their adaptive ability comes into question.

With decreasing body mass life history traits that are dependent on body mass will also shift. Age of maturity, reproduction rates and growth rates will be reduced.  Preliminary data already indicate a decrease in the life span of female bison, reducing reproductive rates [21].

In response, there are ways to mitigate the observed effects of the climate shifts on bison according to Dr. Jeff Martin— an integrative conservation ecologist.  Prescribed burns to the land to boost available energy and protein in grasses are one example.   More generally work is needed to determine how best to create landscape heterogeneity for bison to select the best available forage [22].

To achieve such a goal, management questions arise.  For instance, bison diet remains poorly understood which limits the ability to determine the plant species most critical, and consequently prohibits a full understanding of the required management of dietary needs.  Plains bison are considered strict grazers.  This implies they primarily consume grasses and grass-like flowering plants—such as sedges—as opposed to browsing on forbs, shrubs or trees (woody species).  Being strict grazers would suggest that climatic warming may reduce bison performance by altering the productivity and nutritional quality of different grass species. However, earlier analyses may have overemphasized the contribution of grasses and underemphasized the amount of herbaceous and woody species in their diet.  Recent studies have suggested that bison utilize eudicot species to some degree.  If eudicot species constitute a critical component of bison diet, then managers will need to take into account the relative abundance of these and their nutritional quality when considering mitigation strategies [23].

Bison have been wonderfully adaptive to environmental and climatic changes over the course of their history. Until recent times, though, they have had great expanses of time to acclimate to new conditions.  The recent accelerated warming trends have placed another hurdle in their evolutionary path—a shortened time frame in which the species has to respond.  It is unclear whether the species will be able to offset the induced biological stress with a shift in body mass in the allotted time. It is highly unlikely the climate shift underway can be halted or reversed.  Mitigation efforts, then, need to focus on land management to provide the requisite forage.  This will, however, require additional studies and the implementation of known effective practices. 

Simply restoring bison numbers is not enough. To ensure the survival of this keystone species, land and vegetation management practices which will mitigate the current climate effects need to be developed.  

End Notes:

[1] Isaac, Joanne L. 22-May-2008. Effects of climate change on life history: Implications for extinction risk in mammals.  Endangered Species Research. Vol. 7:115-123, 2009.

[2] See IPCC-AR5, 2013; USGCRP, 2018.

[3] Martin, Jeff M. Perry S. Barboza. 06-Dec- 2019. Decadal heat and drought drive body size of North American bison (Bison bison) along the Great Plains. Wiley. Retrieved 19 Oct 2020.

[4] The transition of from the last Glacial Maximum (12,500 years ago) to the Holocene

[5] Martin, Jeff M., Jim I. Mead., & Perry S. Barboza. 10-Apr-2018. Bison body size and climate change. Wiley.

[6] Isaac.

[7] Pertaining to the appearance of an organism resulting from the interaction of the genotype and the environment—Webster’s

[8] Pertaining to the form and structure of an organism considered as a whole—Webster’s

[9] Isaac

[10] Pertaining to the influence of climate on the recurrence of annual phenomena of animal and plant life—Webster’s Unabridged Dictionary of the English Language. 2001. Random House.

[11] Craine, J. M., Towne, E. G., Joern, A., & Hamilton, R. G. (2009). Consequences of climate variability for the performance of bison in tallgrass prairie. Global Change Biology15(3), 772– 779.  See also

Martin, Jeff M., & Perry S. Barboza. 06-Dec-2019. Decadal heat and drought drive body size of North American bison (Bison bison) along the Great Plains. Wiley.

[12] Martin, Jeff M., Jim I. Mead., & Perry S. Barboza. 10-Apr-2018. Bison body size and climate change. Wiley.

[13] Martin, Jeff M. Perry S. Barboza. 08-Jul-2020. Thermal biology and growth of bison (Bison bison) along the Great Plains: examining four theories of endotherm body size. ESA Journals.

[14] Isaac.

[15] Martin 2019.

[16] Craine, Joseph M. E. Gene Towne, Mary Miller & Noah Fierar. 16-Nov-2015. Climatic warming and the future of bison as grazers. Nature.

[17] Craine, J. M., Nippert, J. B., Elmore, A. J., Skibbe, A. M., Hutchinson, S. L., & Brunsell, N. A. (2012). Timing of climate variability and grassland productivity. Proceedings of the National Academy of Sciences109(9), 3401– 3405. Retrieved 19 Oct 2020. Also Martin, Jeff M., Perry S. Barboza. 06-Dec-2019.

[18] McKauchlan, K.K., Ferguson, C.J., I. E. Ocheltree, T. W. & Craine, J.M. 2010. Thirteen decades of foliar isotopes indicate declining nitrogen availability in central North American grasslands. New Phytol, 187, 1135-1145.

[19] Tajir, Amir. 02-Nov-2016. What’s the function of Nitrogen (N) in plants? Greenway Biotech.  Retrieved 19-Oct-2020.

[20] Martin, 2020.

[21] Martin,, 2019.

[22] Kobilinsky, D. 16-Dec-2019. Droughts and high temperatures are shrinking bison. The Wildlife Society.  See also Jeff Martin’s website

[23] Craine, et al., 2015.