Heatwaves and heavy rainfall are becoming more frequent due to climate change. This has a particularly severe impact on cities. Engineers and local authorities are therefore working to better protect built-up areas from natural hazards. They are increasingly turning to nature-based elements such as permeable surfaces and paving, green roofs or retention ponds. This ‘blue-green infrastructure’ offers advantages over ‘grey’ protective measures such as concrete rainwater basins: permeable surfaces, for example, store rainwater directly in the ground, thereby relieving the strain on sewer systems.

Blue-green infrastructure also provides habitats for plants, animals, fungi and microbes. When properly designed and interconnected, it serves as a crucial element in maintaining and promoting biodiversity in urban areas. “Yet for engineering professionals, the biological aspects of blue-green infrastructure are often still an afterthought,” says Kilian Perrelet. In his doctoral thesis, supervised by Lauren Cook and Florian Altermatt (both at the Eawag Water Research Institute) and Marco Moretti (from the Swiss Federal Research Institute WSL), he investigated how biodiversity can be promoted through blue-green infrastructure. Perrelet now works as a postdoctoral researcher at the WSL.

Part of the thesis consisted of a literature review published in the journal *npj Urban Sustainability*. For this, Perrelet compiled studies demonstrating that biodiversity is a key factor in the effectiveness of blue-green infrastructure. “There is evidence, for example, that an increase in plant diversity improves rainwater infiltration, water quality and heat reduction,” says Perrelet. “These improvements are not vast, but they are significant.”

The benefits of diversity

One important effect is that different species can utilise resources in a complementary way. Infrastructure with varied foliage or root systems can therefore absorb more heat or retain and purify water more effectively. “Generally speaking, the greater the morphological differences between species, the greater the benefits,” says Kilian Perrelet. Evidence from the field has also shown that ecological functionality improves when humans do not interfere too intensively with the vegetation. “This could be because it allows species to establish themselves that use resources differently in order to avoid competition,” says Perrelet. But other factors could play a role as well, namely the greater number of species in unmanaged sites relative to heavily manicured ones.

Biodiverse infrastructure elements are not only more efficient but also more resilient. They typically comprise several species that fulfil the same function but respond differently to disturbances. This makes systems less vulnerable to droughts, floods or pest infestations. Studies on this topic in the field of blue-green infrastructure are still relatively rare. “But the existing ones confirm this pattern,” says Perrelet.

Ultimately, biodiversity can also reduce maintenance costs. In parks or gardens with a wide diversity of flowers, for example, there are more natural enemies of pests, reducing the need for pesticides. On green roofs, meanwhile, studies have shown that stonecrop species alter the microclimate, thereby protecting other plants from drying out. “This reduces the effort required for watering or reseeding,” says Perrelet.