Vegetated roofs, or green roofs, add pockets of natural environments to urban areas and may mitigate the habitat loss caused by urbanisation and increasing urban compactness. Vegetated roofs can be used to provide habitats to not only plants, but also insects, spiders and other invertebrates.
Kukka Kyrö, who defends her doctoral thesis at the Faculty of Biological and Environmental Sciences, University of Helsinki, has studied the true bug, ant and spider communities found on meadow-like vegetated roofs located in the Helsinki Metropolitan Area. Together with Swiss colleagues, she has also investigated beetles living on vegetated roofs in Basel, Switzerland.
“The insect and spider species that thrive on roofs are either associated with dry habitats or fare well in a wide variety of environments. Diverse roof vegetation that resembles open, ecologically valuable local ground-level habitats makes it possible for a diverse community of arthropods to thrive as well. Of course, the species have to be able to spread to the roof, for example, by flying or other means,” Kyrö says.
Vegetated roofs are fairly challenging habitats. Typically, the substrate is thin. In the case of succulent-moss roofs it could be just a few centimetres thick, and usually not much more than 30 centimetres in meadow roofs with forbs and grasses. Such roofs with thin substrates dry up easily, but, at the same time, they recover quickly when it rains. In the wintertime, the thorough freezing of the substrate can also hamper arthropods’ overwintering.
Succulent plants are unattractive to many herbivorous arthropods
As a rule, the vegetation on the roofs studied was drought-tolerant and composed of combinations of grasses and forbs as well as succulents and mosses.
“Most arthropods grow in abundance as the number of grasses and forbs increases. There is little food available for species which feed on forbs and grasses on roofs whose vegetation consists mostly of succulents and mosses, which is why the richness of herbivorous species in particular is poorer on succulent roofs compared to meadow roofs.”
Many arthropod groups arrived quickly at the roofs that were studied soon after their creation, but younger roofs had fewer species than the open green areas located on the ground next to the roofs. However, the community of true bugs, spiders and ants on Finnish roofs remained fairly scarce even as the vegetation grew older.
“In contrast, we found some rare beetle species on Swiss roofs. In Switzerland, regulations pertaining to vegetated roof design direct people to build vegetated roofs that support biodiversity. In our research area in Basel, a significant share of the vegetated roofs are designed to provide habitats to local species which have adapted to dry habitats,” Kyrö notes.
At the same time, the roofs can serve as platforms through which alien species spread, for example, when plant material is imported to Finland. In Kyrö’s studies, a true bug species new to Finland was discovered on the roofs, which probably arrived with prefabricated succulent roof mats imported from Sweden.
Kukka Kyrö’s doctoral thesis: Vegetated roofs as habitats for arthropods in urban areas