Hot bison in a warming world: consequences of climate change, heat load, and nutrition for managing a keystone species in the Great Plains
Intro: |
Bison are an ecological keystone species affecting interactions and processes in the environments they inhabit. Bison are both a wildlife and production species that is an ideal research species and system that represent large-bodied, long-lived ungulates such as cattle, and deer, elk. Bison, like other large ungulates, are vulnerable to environmental change, and thus their ecological footprint.
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Global temperature in the 21st century is expected to rise between 2 and 4°C above the 20th century average. Animals must physiologically adapt to local changes in environment if they are not able to shift their distribution to more favorable ones because of property/fence lines. Small mammals appear to be able to adapt morphology and life history to environmental shifts within one to three generations. The adaptive responses of large mammals to environmental change are poorly understood. Previous studies could not encompass the long lives, large bodies, and the ability to move long distances in bison which buffer responses to environmental change. These characteristics increase the logistical difficulty of collecting sufficient data on an adequate number of animals over years, regions, and generations. Consequently, we do not have enough knowledge to project the effects of climate change for decisions of managing large herbivores.
Our research examines thermal and nutritional drivers of body size in bison to predict responses of large animals to changing environments. We have documented 40,000 years of fossil data to show that body size of bison responds to changes in global temperature; as global temperature increases, average body size diminishes by 40 to 60 kg per Celsius degree (Martin et al., submitted). We aim to expose the mechanisms underpinning this phenomenon. To do this, we use thermal imagery and tissue sampling along a north-south transect through the Great Plains to answer two questions: 1) do high temperatures cause heat loads that slow growth and diminish body size, and 2) do high temperatures alter the food supply and growth of tissue?
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We are comparing the nutritional side of the argument to thermoregulation. These two mechanisms have been long-standing pillars in the literature, but not explicitly compared one-to-one within a species or system. Our research project is focused on measuring growth of tissues of bison in diverse environments using isotopes of carbon (δ13C) and nitrogen (δ15N). We are testing the comparing relative proportional dynamics of direct and indirect mechanisms limiting growth and body size in bison along a north-south temperature gradient in the Great Plains from Saskatchewan to Texas. The direct mechanism is thermoregulation through pathways of energetic costs, whereas the indirect mechanism is nutrition through pathways of limited tissue growth. Each impact body size by limiting growth, but each have respective nuances that influence how effectively conservation efforts and management strategies can be implemented. Understanding the interaction of these two mechanisms with and within bison is fundamental to creating sustainable conservation and production plans in a changing environment.
Our transect is used to compare bison across a gradient of mean annual temperature from 0°C in the north to 20°C in the south. The transect corresponds with a change in body mass of 30% shrinkage from north to south. Our goal is to measure the difference in tissue stable isotope values for nitrogen and carbon. We will focus on sampling at least 3 males and 3 females at 2 years of age from as many as 10 localities that have already been used for thermal imaging, totaling 60 individuals and 300 samples.
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The project will provide new information about body size of bison and their relationship to environmental change, which will be published in scientific journals. We hope to implement a monitoring system that will gauge environmental health and condition using bison tissues and thermal images. We will also provide this information to stakeholders through policy briefs, magazines, newsletters and on-line documents to inform decisions about managing bison as a wild resource and as production animal.
Details: |
Bison (Bison bison) are large-bodied hoofed animals (ungulates) that respond to environmental pressures, that is environmental pressures influence the phenotype and body size of bison.
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As a general trend, bison and other ungulates follow Bergmann’s Rule, where as temperatures increase, body size decreases. One direct and one indirect pathway contribute to two mechanisms explaining this phenomenon and they remain elusive: thermoregulation or nutrition. Both direct and indirect pathways negatively impact body size because of increased temperatures and need to be parsed out to determine the primary driving mechanism for conservation and management applications. We aim the elucidate the roles of these body size driving mechanisms forced by environmental change. Because bison are large bodied, accessible, long-lived ungulates, we feel that bison are representative of other large game and production ungulates.
Our thermoregulation portion of the project is underway, measuring thermal exchange between bison and ambient condition using a high-performance thermal imaging camera. We have visited 19 public and private bison localities ranging from central Saskatchewan to central Texas, to measure body size and heat exchange of bison with thermal imagery during this past summer (Figure 1).
To measure tissue integrated nutrition, we will analyze isotopic values of fast and slow replacement tissues to record the multi-seasonal and annual variation of carbon (δ13C) and nitrogen (δ15N) nutritional integration regulating growth within bison populations along a latitudinal transect. Stable isotopes are indicators of landscape and physiological processes. The mission is to quantify climatic variation impacting vegetation and thus nutrition for bison in the Great Plains region. Levels of the stable isotope of carbon (δ13C) can track energy and distinguish the consumption of cool and warm season grasses (C3 vs. C4). Levels of the stable isotope of nitrogen (δ15N) can be used to track protein and understand tissue and body growth by comparing tissues with different rates of replacement. For example, replacement rates decrease from fast to slow for liver, tail hair, heart, bone, and teeth.
Methods: |
Heart, bone, teeth, hair, and liver samples of bison collected from commercial bison will be frozen and stored at 0°C until processing.
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Freezing and storage does not affect δ13C and δ15N. For isotope analysis, tissue samples will be freeze-dried and ground using a pestle and mortar and 1-mg samples of tissue will be weighed into tin capsules. Isotopic content of tissues will be determined using Costech Elemental Analyzer coupled with a continuous flow Delta Advance Stable Isotope Mass Spectrometer in Texas A&M laboratories and equipment.
Summary: |
We will use the responses of growth and body size to increasing temperature to predict the likely effect of projected climate changes on the populations of wild and captive herds of bison and the options for managing this keystone species on public and private lands.
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We have identified North American Bison in the Great Plains as an ideal candidate species and system that fits the criteria needed for measuring the proportional responses of mechanisms limiting growth and body size in large-bodied, long-lived ungulates because they: 1) have an extensive fossil record for a deep time perspective, 2) have a wide latitudinal distribution that are locally adapted to exhibit phenotypic variation, 3) are accessible for thermoregulatory data and tissue sample collection, and 4) are an organism that is representative for applying broad physiological mechanistic principals to large wild game and production ungulates. The fundamentals of each mechanism’s proportional influence on limiting growth and body size in bison will be applicable to large wild game and production ungulates (deer, elk, bighorn sheep, cattle, and domestic sheep).
Expected Outcomes: |
Our expected benefits resulting from this project is that we will have a greater understanding of mechanisms limiting growth and body size in bison. These mechanisms are applicable to other large ungulates and their management, such as deer, elk, sheep, and cattle.
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Informed decisions can be resolved in creating policies, protocols, and procedures for sustainable conservation and management for these big game and production animals. The project will provide new information about body size of bison and their relationship to environmental change, which will be published in scientific journals. We will also provide this information to stakeholders through policy briefs, magazines, newsletters, and on-line documents to inform decisions about managing bison as a wild resource and as production animal.