Student Erik Verhagen concludes in Bachelor's thesis

Bees happy with Utrecht’s green bus stop roofs

Bushokjes met groene daken Foto: Erik Verhagen
A bus stop with a green roof. Photos: Erik Verhagen

The idea to investigate the bus stops came from his supervisor but Erik had wanted to study bees all along. He was not very familiar with the bus stops with green roofs before he started researching, however. “I saw them around the city and I thought it was a nice concept, but I didn’t pay much attention to them.”

Since 2019, Utrecht has gained a total of 316 bus stops covered in succulents. The plants are meant to serve many different purposes, such as absorbing dirty air and retaining water, not to mention they have a cooling effect. But do bees like them? And do these green roofs help attract more bees to the area? That's what the student wanted to find out.

Erik’s fieldwork consisted of visiting a number of bus stops to count how many bees were hanging out there. “A stepladder wasn’t high enough, so I went to places where I was\ able to see whether bees were flying back and forth from the roof,” he explains. Initially, he and his supervisor were hesitant about whether the research would even succeed, as bees' behaviour can be affected by the weather. Fortunately, Erik had little trouble with changing weather conditions when he conducted his study, between the end of May and the beginning of June 2023.

Bushokjes met groene daken Foto: Erik Verhagen

Green environment
In addition to the fieldwork, Erik also used data from the Geographic Information System (GIS). Using a dataset, he was able to determine where green areas were located around the bus stops. The definition of a "green area" used is an open environment with green elements such as trees and shrubs, but it is located within an urban area. Therefore, this notion includes not only parks and public gardens but also football fields and vegetable gardens. Erik wanted to find out to what extent the presence of green areas in the vicinity impacts how bees use the bus stops. He also investigated whether the distance between the green areas and the bus stops matters. “Bees should also be able to fly there.”

The research showed that most roofs are visited by bees, although not a single bee could be found on some of the roofs examined. He also discovered that bus stops are visited by bees more often when there is a large green area nearby. The student was already expecting that the roofs would be a great place for bees. “The layer of sedum (succulents) on the roof provides a lot of nectar and pollen,” he says. Still, the results surprised him. “I didn’t expect that there would be a really visible association in my research.” His pride regarding the outcome of the research is clearly visible, too. “This way we can provide a tangible example that this is an effective way to make cities more green."

Erik decided to send his findings to the municipality, which is also conducting its own research into the effectiveness of these green roofs. “I don’t know if they’ve read it or not, but I would appreciate it if they did,” says the student, explaining that the municipality could use the results to quantify how green certain neighbourhoods are.

But how are bees able to find these roofs so well? “The shape of a neighbourhood or the variety of flowers planted nearby can have an influence on this. It would be nice to demonstrate a causal relationship but that is not possible with my research.”

In addition to the municipality and the university, Erik hopes that Utrecht's inhabitants will also learn something from his work. “I hope that people will make their roofs greener, for example. Many people want to support nature and ensure that there are more butterflies and bees flying around, and this type of solution is very beneficial without taking up extra space." Thanks to his research, Erik feels a bit more connected to the city now. “I haven’t been to all the bus stops in the city but if you visit several shelters and stand there for ten minutes, you get to know the city rather quickly.”