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How does urbanization shape arthropod communities?

Urban areas are bursting with both human and animal life. Although some of the smallest life forms, such as insects, spiders, and ants are easily overseen, their presence or absence in cities can have wide-reaching effects on a variety of ecosystems. Now, a team of researchers from the University of Innsbruck in Austria has examined the correlation between the presence of arthropods – invertebrate animals with exoskeletons including bees, insects, and spiders – and levels of urbanization.

“In this study, we compared how different indexes of urbanization shape arthropod communities,” said lead author Marion Chatelain, a postdoctoral fellow in Urban Ecology at Innsbruck. “We show that richness and diversity of arthropods on trees and bushes decreases along the rural-urban gradient. More specifically, we show that urbanization disfavors 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 scientists collected arthropod samples at 180 sites within an area covering 56.5 square kilometers in and around the city of Innsbruck, from three types of micro-habitats – the canopy, the tree bark, and the bush layer – at each site. Then, they estimated the level of urbanization within 100, 500, and 1000 meters around each site by measuring the percentage of paved-over and built-up area compared to vegetation or trees, and tested its impact on the total number of arthropods (abundance), the number of taxonomic groups (richness), and the types of arthropods present.

The analysis revealed a significant correlation between the level of urbanization and total arthropod numbers in the bush layer, with higher urbanization levels linked to more bark lice and crab spiders in the shrubs (most likely due to more nutritious leaves in urban bushes). In the canopy, the number of winged arthropods such as flies increased in more urbanized areas, while certain species of spiders were found less frequently, suggesting a possible advantage of winged arthropods in cities, due to their increased ability to move between isolated green spaces. 

The researchers also observed reduced numbers of web-building spiders in comparison to species that actively hunt, suggesting that the increase or decline of spider groups correlates with their hunting modes.   

While certain arthropod groups seem to have adapted better to urban life than others, no effect on total bug numbers were found. “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 explained. “In fact, in bushes, arthropods, especially bark lice and crab spiders, are actually more abundant in the city.”

Further research is needed to better understand how arthropod distribution and abundance in urban areas may impact insect-eating bird communities. “Our results suggest that urbanization affects the availability of arthropod prey, which is expected to have consequences on predator nutritional status, foraging behavior, reproduction success, survival, and distribution within the urban landscape. 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,” Chatelain concluded.

The research is published in the journal Frontiers in Ecology and Evolution.

By Andrei Ionescu, Staff Writer

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