Research Article |
Corresponding author: Petter Andersson ( petter.andersson@calluna.se ) Academic editor: Klaus Henle
© 2017 Petter Andersson, Anna Koffman, N. Erik Sjödin, Victor Johansson.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Andersson P, Koffman A, Sjödin NE, Johansson V (2017) Roads may act as barriers to flying insects: species composition of bees and wasps differs on two sides of a large highway. Nature Conservation 18: 47-59. https://doi.org/10.3897/natureconservation.18.12314
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Roads may act as barriers to animal movements, but direct barrier effects on insects have rarely been studied. In this study we collected data on bees and wasps along two sides of a large road in Sweden using yellow pan traps. We then analyzed if the species composition differed between the two sides of the road; first for the whole community, and then only for the smallest species (which typically are poorer dispersers). As a complement, we analyzed if different vegetation variables differed between the two sides of the road, as this may also affect differences in species composition. Finally, we analyzed if species richness and abundance in general differed between the two sides and how these two response variables were explained by the vegetation variables. There was a significant difference in species composition between the eastern and the western side of the road when analyzing the whole community, and this relationship became even stronger when the largest species were excluded. The vegetation variables did not strongly differ between the two sides, and there was no difference in species richness and abundance of bees and wasps either. Abundance was, however, explained by the number of flowering plants in the surroundings of the trap. Even though using a rather limited data set, our results indicate that large roads may act as barriers on the movement of bees and wasps, especially for small species with poor dispersal ability. On the other hand, road verges may be important habitat for many species, which leads to a potential conflict that is important to consider in the planning of green infrastructure.
Infrastructure, barrier effects, habitat fragmentation, Aculeata , wild bees
The increasing amount of infrastructure and road networks in the landscape is a major cause of habitat fragmentation (e.g.
Some studies on the effects of roads on insects exist, indicating that large roads can cause disruption of movements between habitats in butterflies (
One additional reason that barrier effects of roads on insects deserve attention is that road verges have been highlighted as important habitat for many species (
Aculeata (i.e. bees and wasps) is a species rich group that may provide several ecosystem services (e.g.
The aim of this study was to investigate if large roads may act as barriers on the movements of bees and wasps (Aculeata). Specifically, we compared the species composition of bees and wasps on different sides of a large road, both for the whole community and for only the smallest species. We expected the community composition to differ between the two sides, given that there was a barrier effect. As a complement, we also analyzed general differences in foraging and nesting variables, and the abundance and number of species of bees and wasps, between the two sides of the road.
We performed an inventory of bees and wasps (Aculeata) using yellow pan traps at five sites in Sollentuna municipality, north of Stockholm, during late July-early August 2015. The sites were situated along the highway E4, which runs approximately in a north-south direction through Sollentuna municipality (Figure
Overview of the study area along the E4 in Sollentuna, showing the five sites where communities of bees and wasps were sampled with two traps on each side of the road.
At each site, we placed four yellow pan traps, two on each side of the road, i.e. 20 traps in total. At four sites the distance between the traps across the road was approximately 50 m and at one site the distance was approximately 200 m. The distance between traps at the same side of the road varied somewhat between the sites, but the distance between the traps on the same side of the road were at each site approximately the same as the distance to the nearest trap on the opposite side of the road (with a square-shaped setup, were each trap was a corner). The vegetation at the sites had previously been mowed during the summer. The exact dates for mowing are not known, but the vegetation height was between 15–40 cm at the sites.
The traps consisted of 0.8 l aluminum boxes (with the lid removed) sprayed with yellow paint. The traps were filled with water and a drop of detergent with the purpose to reduce surface tension of the water surface. The traps were placed at the sites during early morning at days with suitable weather conditions (sunny weather, 20–25°C), and the traps were emptied and removed from the sites four days later, at late afternoon/evening. The collected material was stored in ethanol. All bees and wasps where identified to species-level.
At each collection plot (i.e. four plots per site), we made a rough estimation of three variables of importance for foraging and nesting of bees and wasps. These variables were based on four photographs of the vegetation in a 1x1 m wooden frame on the ground placed at four different positions at each collection plot; one at the position of the trap, two positions ten meters from the trap in each direction parallel to the road, and one position ten meters from the trap perpendicular to the road. These photographs where then used to estimate three variables: 1) the number of flowering plant species within the test square (as a qualitative measure of the foraging habitat), 2) the proportion of vegetation (in %) covered by flowering plants within the test square (as a quantitative measure of foraging habitat), and 3) the proportion of the area (in %) covered by bare soil within the test square (as a measure of nesting habitat availability). Based on the estimations from the four photographs, we then calculated an average of the three variables for each collection plot, which was used in the analysis. At each site we also subjectively assessed the inclination of the road verges on the eastern and western side of the road based on photographs taken at each plot. This was done to detect possible patterns in inclination between the two sides of the road that potentially could affect our results.
To test whether the species composition of the bee and wasp community differed between the eastern and the western side of the E4 highway, we used Canonical Correspondence Analysis (CCA;
As a complementary analysis, we investigated if the road verges on both sides of the E4 differed in vegetation characteristics, as these variables may have consequences for the interpretation of the results from the CCAs. For this purpose, we analyzed the number of flowering plant species, cover of flowering plants (in %), and cover of bare soil (in %) with the side of E4 (eastern/western) as the explanatory variable, using generalized linear mixed models (GLMMs) with site as a random factor.
Furthermore, we also examined if there were any general differences in abundance and number of species in bees and wasps between the eastern and the western side of the E4, and if the three vegetation variables could explain these two response variables. For this purpose, we used GLMMs with Poisson distributions and site identity as a random factor. However, initial analyses indicated that the number of flowering plant species was tightly correlated with the cover of flowering plants (Pearson correlation: r=0.73, p<0.001), and as the first provided a slightly better model fit (when tested separately) we decided to only include the number of flowering species, the cover of bare soil and the side of the road as explanatory variables in the final models.
All analyses were performed using R 3.2.2 (
In total, 111 individuals of 39 species of bees and wasps (Aculeata) were collected in the 20 yellow pan traps. The trap catch comprised 20 bee species (Apoidea), while the remainder consisted of various wasp species, such as Crabronidae (8 species), Pompilidae (6 species), Sphecidae (3 species), Tiphidae (1 species) and Vespidae (1 species) (see Suppl. material
The canonical correspondence analyses (CCAs) showed a significant difference in species composition between the eastern and the western side of E4 (Table
Results from tests of the explanatory variables in the CCA-models with all species included and with the largest species (>16 mm in maximum body length) excluded.
Explanatory variable | All species | Large species excluded | ||
---|---|---|---|---|
F | p-value | F | p-value | |
East/West | 2.04 | 0.021 | 2.60 | 0.003 |
North coordinate | 2.12 | 0.016 | 2.75 | 0.005 |
We found no significant difference in the number of flowering plants (p = 0.10), the cover of flowering plants (p = 0.16), or the cover of bare soil (p = 0.81) between the two sides of the E4. The abundance and number of species of bees and wasps did not differ significantly between the two sides either (Table
Ordination plots for communities of bees and wasps along the E4 in Sollentuna, where a shows the result for the analysis of the whole community and b the result for the analysis where the large species (>16 mm) were excluded. Grey small numbers refer to different species (Suppl. material
The abundance and number of species of bees and wasps (Aculaeta) in relation to the side of the road, number of flowering plants and the cover of bare soil based on generalized linear mixed models (Poisson error distribution).
Explanatory variable | Abundance | Number of species | ||
---|---|---|---|---|
Estimate (SE) | p-value | Estimate (SE) | p-value | |
Side of the road | -0.17 (0.22) | 0.44 | -0.33 (0.25) | 0.18 |
Number of flowering plant species | 0.30 (0.13) | 0.02 | 0.25 (0.14) | 0.08 |
Cover of bare soil (%) | -0.22 (0.14) | 0.12 | -0.21 (0.16) | 0.19 |
By analyzing differences in species composition along two sides of a large road we found indications that roads may act as barriers on the movement of bees and wasps. Even if the yellow pan traps were in most cases only separated with ~50 m across the road, the two analyses of the community composition both pointed in the same direction – the community composition differed between the western and eastern side of the road. Also when considering the fact that the trap catches where rather small (~10 individuals/site), the results show that species composition at sites situated on the same side of the highway were more similar to each other compared to the sites at the opposite side (Figure
Road verges have been highlighted as important grassland habitats, often rich in species of vascular plants and pollinators (
Even though using a rather limited data set, our results indicate that large roads may act as barriers to bees and wasps, especially for small species with poor dispersal ability. This means that large roads potentially can affect the ecosystem services these species provide (
We thank Claes Vernerback for carrying out fieldwork and Lars Norén for identifying all bees and wasps. The study was financed by the Swedish Transport Administration (contact persons: Kerstin Gustavsson and Anders Sjölund). The municipality of Sollentuna (contact persons: Rikard Dahlén, Paola Ponzio and Karin Hermansson) also contributed financially to the study. Moreover, we thank Heinrich Reck and one anonymous reviewer for valuable comments on the manuscript.
Species list and body lengths of the 39 Aculeata species
Data type: species data