Plains Bison

Plains Bison

Bison bison bison

Plains bison once roamed freely across the North American Great Plains with an estimated population of 30-60 million individuals. The species was forced to the brink of extinction by European settlers in the 1800’s. By 1900, the entire North American bison population is estimated to have been less that 1000 individuals (a 99.99% decline in the population). Bison are considered ecosystem engineers, supporting landscape heterogeneity through grazing, trampling, and wallowing activities, which influence the diversity and abundance of hundreds of grassland species.

Facts

Length: 2.2-3.8 m (7 to 12.5 ft)

Weight: 815-1,100 kg (1,800 to 2,400 lbs)

Conservation Status: Near Threatened

Tracking

The Northern Great Plains are one of the most threatened ecosystems on earth. Over 40% of these grasslands have been altered to other landuses and the majority of remaining unplowed lands are grazed by cattle. American bison are large grazers that roamed in tens of millions throughout the Northern Great Plains.

Smithsonian Institution has partnered with the American Prairie Reserve (APR), a private not-for-profit organization, on a grassland restoration project in eastern Montana. Part of the restoration efforts are supporting or restoring key functions and species in this ecosystem. Plains bison were first reintroduced to APR lands over a decade ago.

Bison are migratory herbivores that can and need to move across large landscapes. These movements are thought to have a key impact on grassland heterogeneity. Today, however, bison are not free to roam. As a result, bison are unable to move large distances and are confined to fenced pastures that are between 6,000 and 22,000 acres in size.

We seek to understand the scale at which bison carry out their role as ecosystem engineers, related to grassland heterogeneity and biodiversity patterns. We are doing so by sampling the fine scale movement patterns of bison in three pastures that differ in size and stocking rates by fitting individual bison with GPS tracking collars. We will relate the observed patterns in movement to vegetation and other landscape attributes, such as water availability. We will also assess bison resource selection and include behaviour analyses to these evaluations. Adding behaviour will allow us to better understand how and why bison use certain patches and whether this changes between seasons or years. We hope that this work can help the direct management of bison herds and further support native heterogeneity.

In April 2019, we fit 16 bison with GPS tracking collars across three management units in the American Prairie Reserve. Shown above are the movements of 7 of these animals in the Sun Prairie unit. Each of these animals is a young female (1-2 years of age). The movement data shows that animals are moving across the entirety of this ~27,000 acre management unit, at times moving independently of each other and at other times, joining together and moving collectively. We aim to use these data to provide a better assessment of the areas that bison frequently revisit, while also providing concrete results to inform proper management of the herd.

Bison movement patterns

The main objectives and goals of our study are to:

1) Investigate fine-scale movement patterns of individual bison in relation to pasture size, herd size and other landcover gradients. Understanding bison grazing patterns will inform land managers how best to preserve and restore historical heterogeneity.

2) Determine bison habitat use and resource selection using GPS collar data.. We will characterize bison behavior using accelerometer data. Relating behaviour and location will help us determine how bison use different patch types across the landscape. Behaviour and movement data will be related to pastures size and resources, including water and vegetation structure/cover.

3) Assess seasonal changes in movement patterns in relation to landscape attributes obtained by using remote sensing technologies (mainly satellite imagery), climate, and pasture size.

In addition to monitoring bison with GPS collars, we also are evaluating emerging technologies to better understand how collective decisions influence the movements of bison. Here, video collected from an unmanned aerial system (UAS or drone) is providing detailed movement information on calves moving with their mothers, juveniles moving in relation to adults, and how adult males influence the movements of the entire group. Moving beyond more traditional analyses that focus solely on the individual, we aim to provide a more holistic understanding of movement that focus on the important social bonds that influence bison movement, factors that have both aided bison avoid predators over thousands of years but which may also make bison more susceptible to 21st century challenges.

Meet the Team

Bill McShea, Ph.D.

Wildlife Ecologist
Smithsonian Conservation Biology Institute (SCBI)
Conservation Ecology Center

Hila Shamoon, Ph.D.

Landscape Ecologist
Smithsonian Conservation Biology Institute (SCBI)
Conservation Ecology Center

Jared Stabach, Ph.D.

Ecologist, MoL Program Coordinator
Smithsonian Conservation Biology Institute (SCBI)
Conservation Ecology Center
National Zoological Park

Peter Leimgruber, Ph.D.

Conservation Biologist
Smithsonian Conservation Biology Institute (SCBI)
Conservation Ecology Center