The Big Role of Small Mammals at Nachusa Grasslands

“The anticipation is really exciting. You never know what you’re going to find,” said Holly Jones, assistant professor of Biology at Northern Illinois University.

Holly has been monitoring the small mammal community at the Conservancy’s Nachusa Grasslands preserve since 2013 to understand how the presence of bison affects these critically important creatures.

“Upon visiting Nachusa and learning of the bison reintroduction, I immediately began designing a small mammals study,” said Holly. “A healthy small mammal community indicates a healthy ecosystem; So, to monitor their population is to monitor Nachusa’s health.”

In our homes, rodents are typically considered pests. But on the prairie, they fulfill the critical roles of prey, predator, and architect. 

“Small mammals are a central link in the food chain. They are both prey and predator. Larger creatures such as snakes, birds, coyotes, and raccoons rely heavily on small mammals for sustenance while they themselves prey upon seeds and plants, thereby shaping the plant community,” explained Holly.

Holly hopes to quantify the impact of Nachusa’s bison herd upon the small mammal community to ensure its health and the health of the prairie.

Peanut Butter and Oats
To monitor small mammals, you must first catch them. Luckily, they’re easily lured with a bit of peanut butter and oats. 

“Every six weeks, between April and October, we place live traps baited with peanut butter and oats in a grid pattern throughout the prairie,” said Holly. “Our monitoring focuses on eight species: deer mice, prairie voles, white-footed mice, thirteen-lined ground squirrels, meadow voles, short-tailed shrews, western harvest mice, and meadow jumping mice.”

However, those eight species are not all Holly catches.

“You never quite know what you’re going to find”, said Holly, “There’s a discovery waiting to happen every time. I’ve opened traps to find weasel, mink, toads … I’ve even had birds fly at my face!”

When a small mammal is caught, it’s given a small tag and its species, sex, and reproductive status are recorded.

“Tagging a small mammal is like microchipping a dog”, said Holly. “Tagging them enables us to track survival, body condition, and reproductive patterns, all of which grant us insight to population trends and fluctuations.”

No News is Good News

Holly and her team hypothesized the presence of bison would increase Nachusa’s small mammal community’s abundance and diversity.

“Different species prefer different settings; for instance, mice like open ground while voles prefer areas with more plant cover. The lumbering and grazing of bison creates varied grass height and thickness and promotes the growth of wildflowers—many small mammals’ favorite food.  So, we hypothesized abundance and diversity would increase in the presence of bison,” explained Holly.

However, preliminary results surprisingly suggest that the presence of bison has no significant impact on either abundance or diversity of the small mammal community.

“This is actually a really good finding,” assured Holly. “The small mammal population at Nachusa is already thriving, so it’s reassuring to know the presence of bison is not producing any unexpected or unintended consequences upon the small mammal community.”

Restoration and Fire

Since Nachusa is an actively managed prairie, Holly was also able to measure small mammals’ responses to prairie restoration and prescribed burning.

Her results revealed that small mammals are most numerous, and of greatest variety, at restoration sites between three to 10 years of age (time since seeding). Typically, as a restoration site increases in age, abundance and diversity of small mammals decreases.
However, in years with prescribed fire, both abundance and diversity increased with site age.

“We believe patterns related to restoration age are a result of the grass-to-wildflower ratio; wildflowers are more plentiful in younger restorations while older sites tend to be grass-dominated,” explained Holly. “Fire-related trends are likely driven by deer mice, the most numerous small mammal species at Nachusa. Mice love newly burned sites for the open ground that’s created, but other species, such as voles, prefer areas with more plant cover; thus, we catch large quantities of mice at burned sites but fewer of other species.”

It’s the Little Things
Holly hopes to monitor small mammals long-term to increase understanding and appreciation for the big impact of these small creatures.

“Small mammals are an underappreciated and understudied group,” said Holly. “They shape the prairie plant community and are a vital link of the food chain. Understanding what affects the health of their community can help us manage the prairie for the well-being of all its inhabitants.”


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