Tracking Bison Across the Grasslands of Montana

My name is Hila Shamon, and I’m a landscape ecologist with the Smithsonian Conservation Biology Institute. My job is to study how different ecological processes effect ecosystems (like how a bison’s behavior changes its habitat). I’m also part conservation biologist, which means I research ways to better protect Earth’s biodiversity.

I spent the first two weeks of April working with colleagues at American Prairie Reserve to place GPS collars on plains bison in the grasslands of the Reserve. Before I talk about how we accomplished this, I should explain why we’re tracking this subspecies of American bison — and why it’s important to study how they move.

Imagine a grassland from a bird’s-eye view. Most would picture an ocean of grass, the same bland landscape as far as the eye can see. But this is far from accurate. A closer look would reveal a diverse mosaic of habitat patches that support hundreds of plant species, each adapted to its own niche within the whole.

This complex grassland system is created and maintained by variations in soil, topography (the shape of the land), climate, fire and by animals that ecologists call “ecosystem engineers.” Ecosystem engineers are organisms that physically modify their environment in a way that provides new habitat for other species — and bison are a great example. These grassland engineers transform the land in many ways.

What bison bring to the prairie

Bison create landscape heterogeneity — or landscape diversity — through grazing, trampling and wallowing (rolling on the ground). In turn, this heterogeneity supports hundreds of prairie species, including grassland birds. The McCown’s longspur, for example, prefers habitats with short patches of grass or bare ground, which the bisons’ grazing and wallowing provides.

Bison were once a major ecological force on the North American Great Plains; however, they were forced to the brink of extinction by European settlers in the 1800s. Developing restoration efforts that mimic the bison’s natural influence on grasslands is extremely difficult.

Any prairie restoration plan that aims to maximize biodiversity needs mega herbivores — or large, plant-eating animals — to engineer the habitat for native wildlife. Some current efforts include the use of cattle in a rotational system. Moving cattle between fenced pastures, for example, mimics large-scale bison movement and prevents overgrazing. But how effectively this system replicates the relationship between bison and grasslands remains largely unknown.

Bison are adapted to the Northern Great Plains, and the other grassland species that occur here evolved alongside them. So, it makes sense that one strategy to restore grasslands is to bring back bison.

American Prairie Reserve first reintroduced bison more than a decade ago. They changed the usage of the Reserve’s pastures from seasonal cattle grazing to year-round bison grazing in hopes that bison can carry out their ecological role. Bison are migratory herbivores that need to move across large landscapes, and these movements are thought to have a key impact on grassland biodiversity.

The bison at American Prairie Reserve don’t currently roam free; they live in large, fenced pastures that are between 6,000 and 22,000 acres. Can they still fulfill their ecological role in a place this size? This is one of the questions we hope to answer by tracking their movement.

We also want to understand exactly how bison movement patterns relate to biodiversity. Following individual bison around day and night for an entire year will help us answer these questions. That’s where GPS collars come in.   

GPS-tagging a bison

Putting a collar on a bison is no small task. They are large, social animals that live in herds. When one of them is perceived to be at risk, others will come to the rescue. This makes bison handling dangerous, so every step must be planned to ensure the safety of the animal and of the handling crew.

First, we worked with APR’s bison management team to create a detailed protocol for bison handling. The protocols were reviewed by an animal care committee at the Smithsonian, and we received the appropriate permits to capture and GPS-tag bison. The APR team has years of experience, and they are the only people certified to dart the animals and handle the immobilization drugs.

The next step was timing. We can’t work in the rain, because muddy conditions in the field make it hard to follow the all-weather bison. We can’t work in too-warm temperatures either, because bison could overheat. And we can’t interfere during calving season, because over-protective new moms are prone to aggression and need stress-free time with their newborns.

When the stars finally aligned, providing the perfect field conditions within the narrow time window available to get the job done, our team was ready.

Each morning started with a safety talk. Every team member had a specific task, and we all needed to work in coordination. APR’s professional handlers would approach the herd and dart a bison. They would immediately radio the second vehicle, which was waiting at a safe distance. Then, the clock started ticking.

The entire procedure needed to be done within 20 minutes. It took just a few minutes for the bison to lay down, effectively asleep. The bison handlers then approached the sleeping animal and drove away the curious and protective bison concerned for their companion.

The second team, myself included, was then permitted to approach. My role was to quickly place the collar, while other team members collected hair and blood samples for health and genetics testing.

Another team member recorded everything on paper and watched the herd to alert the team if the other bison approached. The collaring and sampling took just a few minutes, and then everyone returned to the vehicles. Finally, an APR handler injected the reversal drug, and the bison woke up almost immediately and reunited with its herd.

We repeated this process until we successfully collared 16 bison, and I’m happy to report that everything went smoothly.

I spent the next few days monitoring the bison remotely via laptop, as well as with in-person trips to the field, to make sure all were adapting to their new collars. Thankfully, the bison continued their usual movements with the herd.

These collars will collect data over the next year and eventually fall off on their own. Next year, I’ll return to collect the collars, so we can refurbish them and redeploy them on different bison.

This is an exciting first step in a project that I hope will go a long way not only toward understanding how bison move, but also how they fulfill an important role at the center of the prairie ecosystem.