Dec 11

Home on the Range: Yellowstone’s Ungulates

by Jenny Golding

A few months from now, hints of green grass will emerge from the snow at lower elevations around Yellowstone. Those first shoots will blossom into a “green wave” that climbs the hills and valleys towards higher elevations as spring advances. Yellowstone’s dominant ungulate species—bison, elk, pronghorn, bighorn sheep, mule deer—having scraped by on the nutritional equivalent of a cereal box all winter, “surf” the wave, following green-up from their wintering grounds at lower elevations (often outside the park) to the verdant meadows and slopes  of Yellowstone’s northern range.

It’s part of an annual cycle of movement that is as old as the land itself. Tens of thousands of animals migrate, some as far as 150 miles, pulsing in and out of Yellowstone with the seasons, weaving an ecosystem-scale tapestry of life, death, survival, and predation, with Yellowstone’s critical summer habitat at the center. Now, scientists are concerned that there may not be enough habitat to sustain them all.

Bison are roaming the park in historically high numbers—more than 4,500 at last count and averaging nearly 5000 over the last five years. And they are changing the landscape. John Muir famously quoted: “When one tugs at a single thing in nature, he finds it attached to the rest of the world.” Yellowstone is no exception; scientists are beginning to wonder how bison are affecting the rest of the ungulate tapestry that depends on Yellowstone for survival.

“A logical question is what effect bison are having on the grasslands, and indirectly on all of the other animals who rely on the grasslands for food—the bighorn sheep, the pronghorn, the mule deer, the elk. Is there home on the range for all these animals in Northern Yellowstone?” asks Chris Geremia, wildlife biologist with the National Park Service (NPS). “How do they negotiate this landscape of predators, and available habitat, and find a place to live?” A new multi-layered research project called Home on the Range aims to find out.

Home on the Range coordinates data collected by park biologists and Yellowstone Forever citizen scientists to evaluate bison, elk, bighorn, mule deer, and pronghorn diets, nutrition, habitat use, migration patterns, birth rates, survival rates, and population growth rates. While similar studies have been done in Africa, this is the first comprehensive look at the abundant ungulate and carnivore community in Yellowstone.

There is a very intricate relationship between ungulates and grasslands—the number of animals and how they graze affects the amount and quality of grass for a host of other species. Geremia likens it to a lawn: If you fertilize, water, and cut a lawn, it grows thick and full. Bison, in the way they graze in large herds in a focused area, are basically cutting and fertilizing the “lawn.” The idea is to understand whether the grassland is more productive with the current amount of bison in the park, or if they are tipping the range towards overgrazing. The more productive a grassland, the more ungulates it can support, which in turn provides enough resources for a robust carnivore population. “We all know there’s a way to kill a lawn,” says Geremia. “Can you actually kill the lawn of the Lamar or change the plant composition so much that it’s no longer usable to animals? We’re trying to determine if that’s happening.”

At approximately 30 study sites scattered from the Gardiner basin to the Mirror Plateau on  the northern range, and from Horse Butte outside West Yellowstone to the Hayden Valley, researchers collect samples to analyze the nutrients and microbial populations in the soil. They also install small, temporary fences to track how much grass bison are eating and to see how fast grass grows back. Working alongside vegetation researchers, they monitor plant composition to see if the types of species are changing.

If the quality of the range is declining, or if the system is out of balance, it should be evident in the way bison and other ungulates behave—through changes in habitat use, migration patterns, survival rates, and reproductive rates. To understand these factors, researchers collect data on group composition, counting the number of young and adults throughout the year to track juvenile survival, one of the key indicators of degrading habitat. In addition, approximately 100 animals across the five ungulate species in the study are fitted with GPS collars to track how the animals move. Researchers locate a collared animal and collect fecal samples, which are used to analyze the composition and quality of the diet and to look at the movement and behavior of different species using the same range. If bison and mule deer are in the same area, are they competing for resources, or are they eating different plant species? Or, are bison in winter creating foraging routes through the snow that other species can follow? Scientists can also overlay ungulate movements with predator locations. Are these animals moving because of available food, or because of predators, or both?

There’s more at stake than simply available food. If animals change their behavior too much because of declining habitat, ancient migration routes could be lost. “Deer, pronghorn, elk, bison, and sheep use routes that seem to be learned and passed on through generations. They have effects on the entire ecosystem. You might not be able to recover a migration route once it’s lost,” says Geremia.

A unique aspect of this project is that much of the research is being done by citizen scientists. Students on Yellowstone Forever programs get to actively participate in the research by conducting herd counts, locating collared animals, and even collecting scat samples.  Their work provides the NPS with much-needed data. “We’ve got hundreds and hundreds of observations; it’s incredible,” says Geremia. “We’ve more than quadrupled the amount of work we can get done with NPS researchers alone.” In addition to a 3-year, $150,000 grant from Yellowstone Forever, more than 600 Yellowstone Forever participants have provided 335 volunteer research hours since the project’s inception this year. Not only do participants feel the satisfaction of giving back to the park, says Joshua Theurer, citizen science program manager for Yellowstone Forever, “they realize ungulates do a lot more than lie down in the grass chewing their cud. They really look at the animals from a different point of view.”

Both Theurer and Geremia say that the study provides a deeper level of engagement, giving citizens the opportunity to talk about this idea of bison, grasslands, and sustainability and form their own opinions. They’ve particularly focused on engaging youth and high school participants, in hopes the students join the conversation about stewardship of these animals into the future.

Each layer of data is another tool in teasing apart the threads of this complicated tapestry and forming a picture scientists can understand and use. “For 100 years we’ve argued over what’s the right way for ungulates to graze Yellowstone,” says Geremia. Home on the Range will help further our understanding of the ecosystem, and provide decision-makers with new information that will help them guide future management.


GPS—or Global Positioning System—collars placed on Yellowstone animals provide important movement and location data for scientists. The GPS technology embedded in the collars records the specific location of an animal on a customized time schedule. This data is transmitted via a private satellite system called “Iridium” directly to researchers’ computers, providing them with daily or hourly movements, depending on the project. This enables researchers to “see” wildlife movement on the landscape in a highly efficient way, allowing the study of migration routes, locating an animal for population counts, and seeing how it interacts with other animals (or people).


This article was originally published in the Winter 2018 issue of Yellowstone Quarterly.