Small animals crossing roads put the animals at risk and present a significant safety hazard to road users. Motorists who suddenly stop or slow to avoid small animals crossing the road can cause significant safety concerns. Motorcyclists and bicyclists risk serious injury if they swerve or hit an animal, as do pedestrians in the road trying to assist the animals. A new, cost-effective fence design was tested and shown to be effective in preventing small animal crossings, benefiting both public safety and conservation.
Turtles are the world’s most imperiled group of vertebrates, and roads are a primary threat. Two of Minnesota’s native turtles—Blanding’s and wood turtles—are currently listed as threatened by the State of Minnesota and are undergoing assessments to determine if they should be protected under the federal Endangered Species Act.
Starting in late spring each year, turtles begin actively looking for mates or nesting sites. They may move across roads since their habitats often extend on both sides.
“The results of this study confirm that we have a cost-effective design for fencing that keeps small animals off busy roadways and redirects them to safe crossing locations. Benefits include both motorist safety and improved conservation of one of the most imperiled groups of vertebrates in the world,” said Christopher Smith, wildlife ecologist, MnDOT Office of Environmental Stewardship.
Wildlife crossing signs have not proven effective at reducing wildlife-vehicle collisions. Mitigation measures to address road crossings of larger mammals have been implemented but few have focused on small wildlife, and their effectiveness has not been evaluated. Barriers that prevent small animal access to roadways vary in design, can be expensive and are not always successful.
Recognizing that it’s not cost-effective to rely on project-specific barrier designs, MnDOT, in partnership with the Department of Natural Resources, drafted standard plans and specifications for small animal exclusion fencing using off-the-shelf materials.Before the design could be finalized, however, MnDOT needed to ensure its effectiveness.
What Was Our Goal?
The goal of this project was to evaluate the efficacy of barriers designed with simple chain-link fencing for reducing small wildlife mortality on roadways, using turtles as an indicator species.
What Did We Do?
Researchers conducted a before-after-control-impact study in the Twin Cities area over four years. Eleven sites were chosen for the study with three groupings to represent different regions. Other criteria included the potential for turtle mortality, logistical feasibility of installing and monitoring fencing, wetlands on both sides of the roadway and multiple sites along a single stretch of road.
Baseline data was collected at all sites in 2018, and researchers assessed the mortality of turtles and other wildlife. Four sites were chosen in collaboration with MnDOT to receive fencing treatments, and each site was paired with one or two comparable control sites along the same road.
In early spring of 2019, standard 6-foot chain-link fencing was installed at three of the four sites. Installation at the fourth site was delayed until fall due to high water. The fencing was buried approximately 1 foot into the ground to prevent animals from digging underneath. The fence ends were designed to direct the turtles back to the wetlands and away from the road. Researchers monitored the installations and found that the fencing did not keep hatchling and juvenile turtles from crossing the road, so in 2021 they retrofitted the fences with 0.5-inch wire mesh hardware cloth.
From 2019 to 2021, the sites were monitored with weekly visits and trail cameras, which captured time-lapse images. Mortality data was collected at the four sites with fencing and 11 control sites. Researchers also deployed traffic counters to summarize traffic volume at all sites and monitored habitat connectivity among wetlands linked with standard culverts under the road at two sites.
Finally, researchers applied a statistical model to four years of data to examine the effectiveness of the fencing in decreasing turtle mortality.
What Did We Learn?
Researchers found that standard 6-foot chain-link fence with curved end treatments reduced the mortality of adult turtles by approximately 83%, but did not reduce mortality of younger turtles. Adding 0.5-inch hardware cloth to the fencing significantly reduced mortality of all turtles—adults, juveniles and hatchlings—by up to 91% over pretreatment levels.
“Our statistical analysis of multiyear data indicates a significant reduction in adult turtle mortality after fencing was installed. The retrofit further acted to protect juvenile and hatchling turtles,” said Tricia Markle, wildlife conservation specialist, Minnesota Zoo
Both the buried base treatment and the turnaround treatments at the fence ends were essential to prevent animals from digging under the fence and for redirecting the turtles away from the road. Culverts were effective in providing a habitat corridor under the road, as shown in the collected photographs. Also, the fence installations and materials, which were cost-effective and readily available, held up to the elements over the three-year period they were observed.
The fencing design explored in this project is effective and can now be used as a standard best management practice with confidence. MnDOT plans to finalize the plan specifications into a standard plate—an approved design drawing—and consider guidelines for when and where to install the fences, including the identification of high-priority sites.
By avoiding additional engineering or design work for individual projects and by using low-cost, readily available fencing material, MnDOT will enhance public safety and wildlife conservation efficiently and cost-effectively.