Certain bugs are the winners of urbanization, which affects the insect diversity of cities

Cities are connected to lives, both human and animal. The smallest of them—insects, spiders and ants—are easy to ignore, but their presence or absence in cities has a far-reaching impact. Scientists in Austria published a study i Frontiers in Ecology and Evolution, who found a correlation between the presence of arthropods – invertebrate animals with exoskeletons; among them are bees, insects, and spiders – and a level of urbanization.

“We show that the richness and diversity of arthropods on trees and shrubs decreases along the rural-urban gradient,” said first author Dr. Marion Chatelain, postdoctoral researcher at the University of Innsbruck, Austria. “Specifically, we show that urbanization affects wingless groups, especially trees. In fact, spiders and wasps are less likely to be found in the city, where aphids, whales and flies are common. otherwise.”

From bushes to treetops

“In this study, we compared how different indices of urbanization shape arthropod communities,” explained Chatelain. To do so, they collected arthropod samples at 180 sites within an area covering 56.5 square km in and around the Austrian city of Innsbruck.

At each site, samples were collected in three microhabitats: the canopy, tree bark, and shrub layer. By measuring the percentage of paved and built-up areas, vegetation or trees, Chatelain and her colleagues assessed the level of urbanization within 100 metres, 500 meters and 1,000 meters around each site.

They then tested its effect on the total number of arthropods (abundance), how many different taxonomic groups were present (richness), and which arthropods were present. The team also considered diversity, a metric that took into account both abundance and richness. By measuring the level of urbanization at different scales the researchers were able to better explain the effects of urbanization on arthropod communities.

Urban ‘bugs’ wingless web-building arthropods, carnivores

Their results showed a correlation between the level of urbanization and the total number of arthropods in the shrub layer. The more urbanized the site, the more bark beetles and crab spiders lived on the bushes – a pattern that may be due to more nutritious leaves in the shrub row in cities. In the canopy, certain species such as flies increased in more urban areas, but certain groups of spiders were found less frequently. This could indicate that winged arthropods have an advantage in cities, probably due to their increased ability to move between isolated green spaces.

Chatelain and her team also observed type-specific effects on bugs. For example, they found web-building spiders at a consistently lower density than those that actively hunt, such as crab spiders. This suggests that there is a correlation between the decline or increase of spider groups with their hunting methods. The lower frequency of four out of ten spider families examined in the study suggests that they have a direct impact on plant-eating bugs, which were found more often in urban settings.

Some groups of arthropods thrive in cities while others do not, the scientists said. This, however, does not give any direct conclusion on the total number of bugs: “Because some groups become and others[s] screened from urban areas, there are at least as many arthropods in the city as in the surrounding rural area,” said Chatelain. “In fact, in bushes, arthropods, especially bark lice and crab spiders, are actually more abundant in the city . ”

The researchers also suggested that there may be bottom-up effects on insect-eating birds. “Our results suggest that urbanization affects arthropod prey availability, which is expected to have consequences for predator nutritional status, foraging behavior, reproductive success, survival and distribution within the urban landscape,” said Chatelain. “This study is part of a larger project that aims to understand the effects of urbanization on food availability, diet and nutritional status of great tits and blue tits.”