Scientists studying how urbanisation impacts insects found that some bugs are better adapted to urban environments than others
Cities are bursting with life, both human and animal. The smallest of them, insects, spiders, and ants are easily overseen, but their presence – or absence – in cities has wide-reaching effects.
Scientists in Austria have published a study, which found a correlation between the presence of arthropods – invertebrate animals with an exoskeleton; among them are bees, insects, and spiders – and level of urbanisation.
First author Dr Marion Chatelain, a postdoctoral researcher at the University of Innsbruck, said: “We show that richness and diversity of arthropods on trees and bushes decreases along the rural-urban gradient.
“More specifically, we show that urbanisation disfavours wingless groups, particularly so on trees. Indeed, web spiders and springtails are less likely to be found in the city, where, on the contrary, aphids, woodlice and flies are common.”
Level of urbanisation
Chatelain explained: “In this study, we compared how different indexes of urbanisation shape arthropod communities.”
To do so, they collected arthropod samples at 180 sites within an area covering 56.5km2 in and around the Austrian city of Innsbruck.
At each site, samples were collected in three micro-habitats: the canopy; the tree bark; and the bush layer. By measuring the percentage of paved-over and built-up area, vegetation or trees, Chatelain and her colleagues estimated the level of urbanisation within 100m, 500m and 1,000m around each site.
Then they tested the impact it had on the total number of arthropods (abundance), how many different taxonomic groups were present (richness), and what arthropods were present. The team also considered diversity, a metric taking both abundance and richness into account.
Measuring the level of urbanisation at different scales allowed to better explain the effects of urbanisation on arthropod communities.
Their findings showed a correlation between the level of urbanisation and total arthropod numbers in the bush layer. The more urbanised the site was, the more bark lice and crab spiders dwelled on the shrubs – a pattern that may be due to more nutritious leaves in the bush layer in cities.
In the canopy, certain species, like flies, increased in more urbanised areas, whereas certain groups of spiders were found less often. This may indicate an advantage of winged arthropods in cities, likely because of 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 consistently lower density than those that actively hunt, such as crab spiders.
This suggests that the decline or increase of spider groups correlates with their hunting modes.
The lower occurrence of four out of ten spider families examined in the study, suggests a direct impact on plant-eating bugs, which were found more often in urban settings.
Certain arthropod groups do well in cities while others don’t, the scientists said. This, however, offers no direct conclusion on total bug numbers.
Chatelain stated: “Because some groups thrive while others are filtered from urban areas, there are at least as many arthropods in the city as in the rural surrounding.
“In fact, in bushes, arthropods, especially bark lice and crab spiders, are actually more abundant in the city.”
From bugs to birds
The researchers also pointed out possible bottom-up effects on insect-eating birds.
Chatelain said: “Our results suggest that urbanisation affects the availability of arthropod prey, which is expected to have consequences on predator nutritional status, foraging behaviour, reproduction success, survival and distribution within the urban landscape.
“This study is part of a larger project aiming at understanding the effects of urbanisation on food availability, diet and nutritional status of great tits and blue tits.”
The research is published in Frontiers in Ecology and Evolution.
Image: A ‘cucumber green spider’. © Dr Marion Chatelain.