Trees are crucial to insects, birds and even people in a city. They lower the temperature, purify the air, reduce the risk of flooding and have a positive impact on people’s mental and physical health. For animals, they offer a habitat in an otherwise barren urban environment.
In Sweden – and abroad – it is common to plant non-native tree species in cities. In the parks of the city of Malmö, for example, one in four trees is a non-native species. The reason is that some of the exotic or foreign species are considered particularly hardy, or easy to maintain, in a tough urban environment. Now, however, researchers in Lund have established that the non-native tree species are significantly less beneficial to insect life in cities. An investigation of Malmö’s parks showed that the number of insects and spiders in some cases was two to three times higher on native trees compared to non-native species.
Although the researchers expected to find a difference, they were surprised at its extent.
A strong and important factor
“We can show that this effect is strong and an important factor to be taken into account. At the same time, it is relatively easy to address – increasing the proportion of native trees in the city would be sufficient. We investigated parks, where the usual argument for planting non-native species – that they must withstand a harsh urban environment – doesn’t really hold. In parks, it is possible to choose more freely among species”, says Johan Kjellberg Jensen, a doctoral student at the Centre for Environmental and Climate Science (CEC) and the Department of Biology at Lund University.
Researchers already knew that non-native trees were bad for insects and other bugs, because they lack the evolutionary history of coexistence required for the species to adapt to one another. However, what researchers did not know was how strong or important this effect is in an urban environment where many other factors negatively affect insect life, such as artificial light, higher temperatures and air pollution. The aim of the study was to understand the importance of tree choice in an urban context.
“Our results show that importing foreign tree species to the city can have a greater negative impact on insect life than other urban factors such as light and air pollution”, says Johan Kjellberg Jensen.
The study is published in the scientific journal Ecological Applications.