Analysis by moonlight: Examine exhibits bison alter ‘panorama of concern’ for rodents in grasslands

DeKalb, IL – When it comes to when and where rodents eat, no decision is subtle.

The consequences of these decisions can spread through food webs. This is what makes a recently published study so important.

The study looked at how bison reintroduction to Nachusa Grasslands — a 3,800-acre wildlife sanctuary in Franklin Grove, Illinois — has affected the way small mammals respond to moonlight.

A deer mouse in the Nachusa Grasslands. Copyright: Stephanie Kong

“What happens when bison are reintroduced is that they change the ‘landscape of fear’ for small mammals,” said Professor Holly Jones, a co-author of the study conducted by her Evidence-based Restoration Lab. Jones holds a joint appointment at NIU in Life Sciences and Environmental Studies.

What is the “Landscape of Fear”? Put simply, this means that some geographic areas are more frightening than others for small mammals.

While eating by moonlight may be tempting to some, many rodents would prefer to avoid it. For them, high moonlight means a higher chance of being spotted as prey.

So what happens when a bunch of 1,000-pound bison roll around and eat plants? Well, the living space becomes more open than it would be without her. Goodbye beautiful thick plant canopy. Hello bright moonlight.

“From a mouse’s perspective, your habitat looks very different when these large animals are reintroduced,” said Pete Guiden, lead author of the study. “So naturally we wanted to see if there was a pattern here.”

A former researcher in Jones’ lab, Pete Guiden, then a postdoctoral fellow at NIU and now an assistant professor of biology at Hamilton College in Clinton, New York, set out to figure out how changing habitat was affecting the routines of rodents like mice and voles affects and thus the entire food chain in the grassland.

“From a mouse’s perspective, your habitat looks very different when these large animals are reintroduced,” Guiden said. “So naturally we wanted to see if there was a pattern here.”

It might not seem like much to the casual observer, but any change in behavior can lead to a better understanding of how ecosystems work, say the stakeholders.

And the science behind it can directly help people who are actively involved in restoring biodiversity, the variety of life on earth.

“We hope that people will pay a little more attention to the behavioral changes that the bison are causing,” Jones said. “We have a good sense of ‘yes, they affect the plants and nutrients,’ but any change can have significant impacts on the entire ecosystem.”

“A matter of life or death”

Virtually extinct by the late 1800s, bison populations have been reintroduced to the North American grasslands in recent years. Since 2014 they have roamed 1,500 acres of rolling land in the Nachusa Grasslands.

Jones’ research team has studied their effects on the abundance of small mammals, but have yet to address potential behavioral changes influenced by the animals and the moonlight.

“Rodents like mice and voles are great subjects for this. They eat many different plants and insects, but trying to eat something in the wrong place or wrong time can be vital if one of your predators is hanging around,” said Guiden, lead author of the study published in Ecology, a journal of the Ecological Society of America.

In addition to Professor Jones, co-authors include Restoration Lab researcher Erin G. Rowland-Schaefer, a Ph.D. Candidate; and Labor alumni Angela Burke, conservation/volunteer coordinator at The Nature Conservancy; Jessica Fliginger, restoration technician at NIU; and former graduate student Kirstie Savage.

As a PhD student, Guiden studied the impact of moonlight on rodents in local forests.

“We saw that the mice didn’t care what the moon was doing under a nice dense canopy of plants, but I was honestly shocked at how strong the moonlight’s influence was in the more open habitat,” he said. “Since then I’ve been looking for more ways to test this rodent/moonlight connection and Nachusa seemed like a great place to ask this question with some cool new twists.”

What’s that supposed to mean?

Guiden used eight years of data collected by Jones’ research teams on small mammal populations. To determine when small mammals were active, he used temperature loggers on traps. And he calculated the moonlight that reached the ground, the time of the highest moonlight, and the mean moonlight.

To determine when small mammals were active, Guiden used temperature loggers on traps.

The researchers found that in places with bison, 20 percent more light reached the ground than in places without.

“So these megaherbivores are causing a big change from the perspective of small mammals,” Jones said.

The most common species in the traps – deer mice – were four times more active outside the bison unit (where plants are denser) on nights with maximum moonlight, but they showed no differences in activity when there was no moonlight.

Conversely, prairie voles were twice as active during full moon compared to new moon, but only in the bison unit. Although more active, most of their activities in the bison unit took place before moonrise or after moonset. In other words, they avoided periods of bright moonlight.

“So what does it all mean? The introduction of megaherbivores may have some currently underestimated impacts on the prairie by altering the behavioral patterns of other animals,” Jones said.

And this can change the food chain, she said.

From Guiden’s perspective, the research shows subtle and perhaps unexpected effects.

NIU Professor Holly Jones

“Bison has been reintroduced to Nachusa in large part to increase plant biodiversity,” he said. “These gains have not yet been realized because plant communities can change very slowly, but in the meantime the bison could reestablish many other connections that we didn’t expect.”

He wrote credit to Professor Jones and the eight years of data provided by their research teams, as well as the hundreds of volunteers who helped. Working with Professor Jones means tackling cutting-edge ecological problems and directly helping those actively working to restore biodiversity, he said.

“Apart from science, Holly is also an incredible mentor who cares deeply about her students,” he said. “I am always grateful for the lessons I learned from her on how to run my own lab.”

Media contact: Jami Künzer

About NIU

Northern Illinois University is a student-centered, nationally recognized public research university with expertise that benefits its region and spans the globe in a variety of fields including science, humanities, arts, business, engineering, education, health and law. Through its main campus in DeKalb, Illinois, and undergraduate and professional learning centers in Chicago, Naperville, Oregon, and Rockford, NIU offers more than 100 areas of study while serving a diverse and international student body.