Road verges are corridors and roads are barriers for the movements of flower-visiting insects
Written by Juliana Dániel-Ferreira, Åsa Berggren, Jörgen Wissman and Erik Öckinger
Swedish University of Agricultural Sciences, Sweden.
4th April 2023
What is the role road infrastructure for insect populations?
For the persistence of animal populations, it is essential that individuals can disperse among remaining habitat patches. The ability to disperse depends on the individual, their perception of the environment, and also the physical characteristics of the landscape. Roads and road verges can have an impact on dispersal movement. Flower-visiting insects may view road verges as habitat and use them as dispersal corridors. However, the same insects may perceive the road as a barrier due to the lack of vegetation and traffic can limit the success of individuals to cross. Thus, the net effect of road infrastructure on many species is unknown. In this study, we investigated how roads and road verges influence the movements of flower-visiting insects. Furthermore, we explored how these patterns are modified by traffic intensity and the diversity of flowering plants in the road verges.
Tracking insect movements in road environments can provide a clue to how roads and road verges affect landscape connectivity for flower-visiting insects
To explore the role of traffic intensity and flower diversity on insect movement patterns, we established a study design that consisted of ten matching pairs of sites that had a similar gradient in traffic intensity but with contrasting flowering plant diversity in the road verge (Figure 1).
Figure 1. Study design. The study consisted of four types of sites: low traffic intensity and low flowering plant diversity in the road verge (lower left), high traffic intensity and low flowering plant diversity in the road verge (lower right), low traffic intensity and high flowering plant diversity in the road verge (upper left), and high traffic intensity and high flowering plant diversity in the road verge (upper right). Sites with low flowering plant diversity were not completely devoid of flowers but were not as flower rich as the other type of sites. Traffic intensity varied between 100 and 6000 vehicles per day.
To isolate the effect of traffic intensity, all the selected roads had a similar road width (approximately 6 m) and speed limits (70 or 80 km/h). At all 20 sites, we placed 8 groups of potted flowers, of which two were marked with fluorescent dye (Figure 2, Figure 3). Using this technique, we were able to follow pollen transfer between the marked and unmarked flowers and estimated the frequency of three types of movements: 1) along the road verge, 2) across the road, and 3) across the road and into the adjacent habitat.
Figure 2. At one side of the road, fifteen flower heads of the species Scabiosa columbaria were marked with red fluorescent dye. As flower-visiting insects visited the marked flowers, some of the dye would attach to their body. If the same insects then visited the unmarked flowers placed in the vicinity, they would leave some of the dye behind which enabled us to track their movements.
Figure 3. On the other side of the road, fifteen flower heads were marked with blue fluorescent dye. Unmarked flower heads were examined for the presence of the dye using a UV-lamp in the laboratory.
Road verges are corridors
We found that movements along the road verges were more frequent than movements into the adjacent habitat. Insects tended to move along the road verge, even after successfully crossing the road. Interestingly, we found that this occurred in both types of sites (with low and high flowering plant diversity), meaning that road verges do not necessarily need to provide a high amount of flowering resources to act as a corridor, and promote the movements of flower-vising insects.
Roads are barriers
We found that the frequency of movements that did not require crossing the road was higher than the frequency of movements that did, and that this happened irrespective of the traffic intensity. This suggests that roads are influencing the movement behaviour of flower-visiting insects and thus represent behavioural barriers to movement. In other words, the barrier effect of the road to some species of flower-visiting insects is more likely to emerge from a behavioural response to the paved surface as opposed to traffic mortality. If this is the case, the negative effects of roads for flowering-visiting insects cannot be mitigated. Road surface avoidance is known to be dependent on the width of the road and we therefore expect that the barrier effect of roads increases with road width.
The barrier effect of the road was stronger when the road verge had more flower resources
We also found that flower-visiting insects were less likely to cross the road if the road verge had a higher diversity of flowers. This might be because a high density of flower resources promotes ‘routine movements’, as opposed to directed movements aimed at dispersal. Therefore, flower-visiting insects can be moving to the closest flower, and therefore end up staying in one side of the road. This movement pattern might have an overall positive effect on their populations by reducing the collision risk of the insects when crossing the roads. Because of this, we suggest that managing road verges to increase flowering plant diversity (e.g. sowing native plants, collecting the plant cuttings, and reducing cutting frequency of the road verges) will be beneficial for the populations of flower-visiting insects.
Author information
Juliana Dániel-Ferreira - juliana.daniel.f@slu.se - Postdoc in Ecology at the Swedish Biodiversity Centre, Swedish University of Agricultural Sciences, Sweden.
Åsa Berggren, Professor in Ecology at the Swedish University of Agricultural Sciences, Sweden.
Jörgen Wissman, Researcher at the Swedish Biodiversity Centre, Swedish University of Agricultural Sciences, Sweden.
Erik Öckinger, Associate professor and Senior lecturer at the Swedish University of Agricultural Sciences, Sweden.
Source citation
Dániel-Ferreira, J. Berggren, Å., Wissman, J., & Öckinger, E. (2021). Road verges are corridors and roads barriers for the movement of flower-visiting insects. Ecography, 2022(2). https://doi.org/10.1111/ecog.05847
Editor:
Manisha Bhardwaj
Cite this summary:
Dániel-Ferreira, J. Berggren, Å., Wissman, J., & Öckinger, E. (2023). Road verges are corridors and roads are barriers for the movements of flower-visiting insects. Edited by Bhardwaj, M. TransportEcology.info, Accessed at: https://transportecology.info/research/road-barriers-flower-visiting-insects