Mortality of bumblebee queens increases with traffic volume
Written by Juliana Dániel-Ferreira
Universidade de Évora, Portugal
July 18th, 2023
Can flower-rich road verges be beneficial for pollinators?
Europe has undergone an extensive loss of one of its most species-rich habitats: semi-natural grasslands. Because of this, species that are tightly linked to these habitats such as plants and pollinators have also declined. For pollinators, the loss of their habitat implies not only a reduction in foraging areas, but often also creates a scarcity of nesting sites. Furthermore, what remains of semi-natural habitats is often scattered across the landscape as isolated patches, which organisms may struggle to move between.
Due to their pervasiveness, road verges have been proposed as a tool to promote insect conservation. If managed to promote flowers, road verges can potentially benefit pollinator communities. However, it is unclear whether flower-rich road verges are a threat to pollinator populations by acting as ecological traps. In other words, roadsides may attract animals whilst also having severe detrimental effects, potentially killing entire populations through traffic mortality and/or acting as barriers to movement. We explored the combined effects of traffic volume and flowering plant diversity in road verges on the mortality and the foraging and nest-seeking behaviour of bumblebee queens as an indirect step in determining whether road verges are ecological traps for bumblebees.
Why study queens?
Bumblebee colonies are founded by a single queen which means that they have small effective population sizes (i.e. the number of individuals that contribute in producing the next generation is small). These populations are more likely to suffer from demographic stochasticity and reduced genetic diversity and are therefore more susceptible to local extinctions. Therefore, the mortality of queens before they have established a colony can have negative repercussions on population persistence.
Proving the existence of ecological traps requires detailed demographic data to be able to compare between the potential trap and the regular habitat. While not impossible, this is difficult to achieve, especially with insects. We predicted that if flower-rich road verges were acting as ecological traps for bumblebees, we would find that the probability of observing dead queens would be higher along roads with flower-rich road verges than along roads with fewer flowering plants.
To investigate this, we studied 20 sites with contrasting flowering plant diversity (low vs. high) and a gradient in traffic volume. At these sites, we counted the number of dead and alive bumblebee queens in a 2 km transect (1 km in each side of the road), and also recorded their behaviour.
More queens die when the traffic increases
We found that the probability of observing a dead queen quadrupled as traffic volume increased from 100 to 6000 vehicles per day. However, we did not find any evidence that flower-rich road verges acted as ecological traps (i.e. there were not more dead queens along these roads). In fact, we found that there was a tendency for a lower mortality risk in flower-rich sites. Importantly, we also did not find a moderating effect of the road verge on traffic mortality, indicating that mortality risk of bumblebee queens along roads increases with traffic volume regardless of the flowering plant diversity in the road verge.
But what about their behaviour?
The movement patterns of queens and therefore their responses to roads can differ depending on the activity they are performing. For instance, queens that have not yet established a nest can move randomly in search of a nesting site throughout the landscape. Nesting sites differ among species of bumblebee. Some species take over abandoned underground mice burrows while others build a nest aboveground in grass tussocks. Queens that are looking for nesting sites often fly in a typical zigzag trajectory near the ground and land repeatedly to investigate potential sites. When foraging, their movements can resemble those of their workers as they move from flower to flower. The type of movement can therefore influence the effect of traffic on mortality.
We found that the number of queens looking for nests decreased with increasing traffic volume at sites with low flowering plant diversity, but not at sites with high flowering plant diversity. This, in combination with the previous result, could indicate that the nest-seeking behaviour is a key factor in traffic-related bumblebee mortality which could potentially expose queens for an increased risk of collision and death. However, as we could not directly observe what the queens were doing when killed, this connection remains speculative.
Implications for conservation
We estimated that between 0.2% and 32% of all bumblebee queens in a 1 x 1 km landscape can be killed by traffic along roads with more than 2000 vehicles per day. Therefore, we argue that road mortality can be a potential source of bumblebee decline. Nevertheless, our study provides preliminary evidence that flower-rich road verges are not an ecological trap for bumblebees. Whether road verges are ecological traps for most pollinating insects or not is an issue that needs to be investigated more directly by exploring how roads affect population growth. In the meantime, our study suggests that roads with low traffic intensity should be prioritised when aiming to promote flower-rich road verges, and that measures to reduce bumblebee queen mortality (e.g. reducing the amount of traffic or the speed limits to reduce the risk of collisions) should be considered to prevent population declines.
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.
Riccardo Bommarco, Professor in agricultural entomology 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:
Sophia Cooke
Cite this summary:
Dániel-Ferreira, J. (2023). Mortality of bumblebee queens increases with traffic volume. Edited by Cooke, S. TransportEcology.info, Accessed at: https://transportecology.info/research/bumblebee-queen-road-mortality