Humans share cities with many other creatures, some of them small and easy to overlook, but even the smallest insects or spiders have a big effect on the world around them.
Austrian scientists have studied the correlation between the presence of these arthropods and urbanisation, and the results don’t look good for those without wings.
“We show that richness and diversity of arthropods on trees and bushes decreases along the rural-urban gradient,” says Dr Marion Chatelain, a postdoctoral researcher at the University of Innsbruck. “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.”
The researchers collected arthropod samples from 180 sites in a 55.6 square kilometre area around the Austrian city of Innsbruck, which has a population of about 130,000.
Chatelain and her colleagues collected in three micro-habitats: tree canopies, tree bark, and bush or understory. They measured the percentage of paved-over and built-up areas, vegetation and trees to estimate the level of urbanisation within 100, 500 and 1,000 metres of the collection sites.
How many different taxanomic groups were present and overall diversity, or richness, was assessed in relation to the level of urbanisation.
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More urbanised sites showed a higher incidence of bark lice and crab spiders in bushes – likely due to more nutritious leaves in the bush layer in cities.
In the canopy, flies were more common, while spider numbers dropped. The researchers say this may be due to winged arthropods’ ability to move more easily between pockets of greenery in cities.
The team also noted that web-building spiders were more negatively impacted by urbanisation than those that actively hunt, such as crab spiders. Four of the ten spider species examined in the study showed a decline with urbanisation, which is linked to a higher density of plant-eating bugs.
“Because some groups thrive while other are filtered from urban areas, there are at least as many arthropods in the city as in the rural surrounding,” Chatelain explains. “In fact, in bushes, arthropods, especially bark lice and crab spiders, are actually more abundant in the city.”
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The team also notes that such changes in insect and spider density will have a bottom-up effect on other species in and around urban areas.
“Our results suggest that urbanization 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,” Chatelain says. “This study is part of a larger project aiming at understanding the effects of urbanization on food availability, diet and nutritional status of great tits and blue tits.”
The research is published in Frontiers in Ecology and Evolution.