Roadkill reality check: Ground squirrel population takes a hit due to increasing tourist traffic

Caitlin Wells noticed a spike in roadkill along an unpaved stretch of County Road 317 every summer when thousands of people would flock to the picturesque Gunnison Valley to view wildflowers and experience the great outdoors. The Colorado State University assistant professor of fish, wildlife and conservation biology was curious whether the traffic casualties represented a meaningful percentage of the wildlife population.  

What she found surprised her. 

While it appeared that there were only a few roadkills each year, the data showed that more than 10% of the population of adult female golden-mantled ground squirrels along the rural road with a speed limit of 25 miles per hour were killed annually. For context, COVID-19 deaths represented roughly the same percentage of deaths relative to the U.S. population in 2020. 

The roadkill death toll could be even higher because small mammal carcasses often are scavenged before they can be counted, and if the ground squirrel is a nursing mother, her pups underground will die too.  

As recreation and tourism expand in the Rocky Mountains, so do the environmental impacts. Wells’ study is one example showing the repercussions of increased traffic. 

“It’s a case study: one species along one mile of rural road in one state,” Wells said. “But multiply that by all the species along the 2.5 million miles of rural road in the U.S., or 40 million miles of road around the globe, and we can expect much bigger impacts; we just rarely have the data.” 

Thanks to 32 years of data collection on golden-mantled ground squirrels led by University of California, Davis Professor Dirk Van Vuren at the Rocky Mountain Biological Laboratory, the research team knew exactly how many squirrels were in the area and had documentation of individual deaths over three decades.

Foundational species 

Why does it matter if a few ground squirrels get run over every year? 

“Ground squirrels are foundational to high-elevation ecosystems,” Wells said. “Everything eats them: hawks, badgers, foxes, coyotes, weasels, mink – you name it, they probably love a ground squirrel snack.” 

In addition to being a primary food source for other mountain critters, ground squirrels are tiny gardeners and ecosystem engineers.  

Ground squirrel burrows create habitat for other underground species, including reptiles, amphibians and other mammals. Their burrowing activity also aerates compacted soil, facilitating nutrient cycling that makes soil more fertile and encourages plants to grow.  

Ground squirrels love seeds – in fact, their Latin name, spermophilus, means “seed loving.” Their underground seed stores sometimes sprout into new plots. 

“If there is a species you care about at high elevations, it’s probably benefitting from ground squirrels in some way,” Wells said. 

Unexpected results

Wells’ study, led by Pomona College student Katherine Burgstahler and published in Biological Conservation, found that female ground squirrels are dying more frequently than their male counterparts. This surprised the researchers. 

“We thought that males would die more frequently,” Wells said. “Because they move around so much during the early spring to find mates, we thought they’d cross roads more often and hence be more vulnerable.”  

But mating season coincides with the road’s closure, when it’s too snowy for vehicles. Adult females, on the other hand, seem to move around a lot during mid-summer, when traffic to the region peaks. 

Scientists are unsure why the females are active during this time of year. Van Vuren suspects they might be venturing farther from the burrow to find high-quality food to support lactation or getting back to their normal movements as their young become less dependent. 

“Or, I have long suspected that some females move away from the natal burrow to enforce weaning – ‘no more free milk, it is time you started feeding yourself,’” Van Vuren said.   

One thing is certain: adult females drive population increases and declines, so a decrease in their numbers impacts the whole species on a local scale.  

If climate change reduces spring snowfall, allowing the road to open earlier, males might start dying at higher rates too. Wells fears road mortality – along with habitat loss and poisoning, lethal trapping and recreational shooting by people who think they are pests – could eventually endanger the entire species, risking collapse of the ecosystem that relies on them.  

Solutions

Wells said traffic impacts could be reduced by busing people to popular recreation sites. Limiting the number of cars on the road would also cut down on noise that can stress animals, dust that covers roadside plants and magnesium chloride that attracts animals to salty-tasting roads.

“Anything that can reduce the numbers of personal vehicles while also allowing recreational access seems like a win,” she said. 

Digital signs programmed with messages to protect wildlife also might help. Evidence suggests that drivers pay more attention to dynamic signs than permanent ones.  

Wells said using digital signs to alert drivers to slow down could save animals. Who wouldn’t brake for a message reading: “Caution, deer fawns on road,” or “Three research animals killed by cars this week, please slow down”? 

“Most folks driving in the mountains are there because they love the mountain ecosystems,” Wells said. “We don’t want to be part of the problem – degrading those ecosystems. If signs tell us when to be careful, we can be more careful.” 

Authors of “Daily roadkill monitoring and long-term population census reveal female-biased mortality for a small mammal along a wildland-urban interface” are Burgstahler, Jessica Isidro, Van Vuren, Amy C. Collins, Jaclyn R. Aliperti and Wells. The Department of Fish, Wildlife and Conservation Biology is part of the Warner College of Natural Resources.