Road verges (regionally known by many other names) are the strip of grass or plants along the side of a road, often between a road and a sidewalk or other developed strip. These road verges can act as corridors for pollinating insects.
However, this contrasts with the roads themselves, which often act as barriers to movement at the landscape scale. There is a lack of understanding as to how insect movement through road verges is impacted by plant community diversity and traffic intensity of roads, and how roads and road verges interact to help or hinder movement.
Dániel‐Ferreira, J., Berggren, Å., Wissman, J. and Öckinger, E. 2022. Road verges are corridors and roads barriers for the movement of flower‐visiting insects. Ecography 2022(2): e05847.
A study in Sweden aimed to answer these questions by considering both structural connectivity (the physical aspects of the landscape that impact movement) and functional connectivity (how an individual’s behavior and mortality risk change due to landscape structure). The study included 20 sites with matching traffic intensity, and varying flowering plant diversity and floral density as measures of habitat quality. Insect abundance of all butterflies, bees, and syrphid flies was also estimated.
To measure the movement of flower-visiting insects, Scabiosa columbaria (variety ‘Butterfly blue’; a native, but rare, plant in Sweden) was placed along the road verges on both sides of the road with each side marked with different color dye on the flower heads. After the dyed flower heads were left exposed for 12 hours, unmarked flowers were examined for dye color and amount.
The sites of road verges classified as high quality had an average of 3.6 more plant species and an average of 29% more flowers than low quality sites. A total of 3,770 flower heads of S. columbaria were analyzed for dye; 397 flower heads had blue dye and 652 flower heads had red dye.
The study results provided support to the idea that road verges act as movement corridors for flower-visiting insects. There was also evidence that roads are barriers to insect movements, and the effect of roads on the movement corridors can be mitigated by improving habitat quality. Specifically, the results showed that movement across the road was less frequent than movement within the road verge. The sheer presence of the road was enough to restrict insect movement, although the traffic intensity of the road was not found to impact the movement of the flower-visiting insects.
The results provide an important contribution to understanding road verges as movement corridors. Future research is needed to better understand the impacts of traffic speed and road surface, as well as how to design roads and road verges to reduce the movement barrier effect. Current management considerations should avoid constructing new roads (i.e. introducing new movement barriers) to maintain the landscape connectivity. Enhancing plant diversity and maintaining native plant communities in road verges can also increase the functional connectivity of these landscapes.
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