Bats thrive in Zurich

More bats flutter through Zurich at night than through Paris or Antwerp.

This is demonstrated by a new study led by the Swiss Federal Institute for Forest, Snow and Landscape Research WSL on the diversity of nocturnal animals such as insects and bats in cities. It shows that the connectivity of green spaces and the reduction of light pollution are necessary to promote this diversity.

Compared internationally, the city of Zurich supports a high diversity of nocturnal insects and bats — thanks to its proximity to forests and other green spaces.

As part of a European research programme conducted across seven cities, the research team had already investigated the occurrence of wild bees. The goal of this project, called BioVEINS, is to explore the importance of urban green spaces for biodiversity. Compared to diurnal organisms, relatively little is known about the relationship between nocturnal animals and green spaces. In three of the participating cities — Zurich, Paris and Antwerp — the researchers therefore additionally examined the diversity of nocturnal insects and bats. In the remaining cities, this was not possible for logistical reasons.

Proximity to forests makes the difference

The researchers used special devices to record the echolocation calls of bats at night in parks and other green spaces within cities. These calls allow species identification with great accuracy. They also caught nocturnal insects using light traps in order to compare the number and diversity of flying insects with that of bats. In Zurich, they discovered not only the greatest number but also the highest species diversity of bats. Study leader Martin Obrist, an insect and bat specialist at WSL, suspects this is due to the fact that near-natural areas such as forests can be found almost everywhere close to the city of Zurich.

Not all bat species cope equally well with urban life. The survey showed that species which hunt in forests and shrubland, as well as those that prefer a high, open flight space, are less common in cities. Bats that hunt in semi-open areas such as forest edges and are highly adaptable in their flight behavior are more frequently found in cities. 

The connectivity of green spaces is of central importance

«Nearby bodies of water proved to be an important factor for the survival of bats in Zurich", says Obrist. These served not only as a water source but also as hunting grounds. Insect diversity increased with the size of urban green spaces. Previous studies have also shown that insects benefit when a certain degree of natural growth is permitted in green areas.

By contrast, insect diversity was negatively affected when green zones were separated from other green spaces by major roads or built-up areas."It is of great importance that green spaces do not represent isolated islands, but form a network within the city", emphasizes Obrist. This increases not only the freedom of movement of insects, but also that of bats.

The light-tolerant common pipistrelle is displacing other species

By far the most common species found in cities by the researchers was the common pipistrelle (Pipistrellus pipistrellus), which tolerates artificial nighttime light well. This study thus confirmed earlier observations: increasing light pollution primarily disadvantages light-sensitive bat species, which are consequently losing their habitats. Artificial nighttime light attracts many insects, which become easy prey for those bats that are less affected by light. As a result, insect populations shrink, and bats — light-sensitive or not — lose their food supply.

What could help promote nocturnal biodiversity in cities? “Artificial lighting must be reduced under all circumstances,” says Obrist. Where this is impossible, LED lights can help reduce nighttime illumination: “LED lights can be precisely controlled and dimmed. The lights then only emit their full intensity when needed — for example, when a car drives past”, says Obrist. “During low-traffic periods, this protects both the insect world and the bats”. The colour temperature should also not exceed 3,000 Kelvin. At this level, the light has a lower blue and higher red component, which attracts fewer insects and therefore has less of an impact on bats as well.