How Does Road Traffic Affect Squirrels?
Roadkill is a major concern when it comes to protecting animals, particularly as human development continues to encroach on wild habitats. Currently, 70% of roads (4.2 million km) in the U.S. run through rural areas. However, previous studies on the effects of roads reported either neutral or net positive effects on the populations of small mammals, despite the fact that small mammals typically account for the majority of roadkills.
The authors of this study claim that research on small mammal roadkill incidents may suffer from a lack of accurate data. Moreover, most studies have failed to take demographic data into account to calculate roadkill rates within a population of animals, rather than for a species in general. This study seeks to correct this gap by analyzing data from a 30-year population study on golden-mantled ground squirrels in Gunnison County, Colorado.
To carry out the research, the authors collected demographic data on golden-mantled ground squirrels in the area surrounding the Rocky Mountain Biological Laboratory and observed roadkills on the nearby road, Country Road (CR) 317. Researchers live-trapped squirrels in four waves from May through early September of each year, measuring body mass and reproductive status and giving them ear tags. The researchers conducted multiple walk-throughs and observations per day, enabling them to know very quickly when a squirrel had been hit by a car.
Results showed that over the entire study period, 66 of 1316 squirrels they tracked were killed on CR 317, and the percentage killed ranged from 0% to 9.5% of the population each year. The proportionate number of adult females killed (11%) nearly doubled that of adult males (6%), and increased when cohort size was larger. The authors explain this by noting that adult female “chases” over territory disputes tend to peak at the same time of year that traffic volume peaks. In conclusion, the researchers determined that roadkill events constituted up to 20% of the annual local population of golden-mantled ground squirrels, with an annual average of 5%.
The fact that data was collected daily rather than weekly yielded a noticeable difference in results: The researchers estimated that had they only collected weekly data, their detection rate of roadkills would have been just 29% of the actual mortality rate. Most other studies have used weekly or monthly collections, meaning that established studies may be underestimating the effect of roads and roadkills on different animal species.
As climate change warms global temperatures, including in Colorado, higher rates of tourism and potentially longer active seasons for ground squirrels mean an increased risk of roadkills. While traffic isn’t the only threat to squirrels’ lives, the authors note that their limited reproduction capabilities mean that increased roadkills may eventually impact population levels. To protect squirrels from traffic collisions, public officials should consider including speed bumps, warning systems, and better signage to caution drivers. Advocates can also campaign for animal-friendly road crossings to give animals a safe option away from traffic.
https://doi.org/10.1016/j.biocon.2022.109863