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22 August 2014

City living is making spiders bigger, study finds

According to new research, city-dwelling spiders are larger and more fertile than their rural-dwelling relatives.

By Fiona Rutherford

The growth of cities destroys the homes of many animals. With habitats drenched in cement, their only option is to relocate – and in the worst cases, urban expansion can cause species extinction. However, it’s not all doom and gloom for some species, as city living appears to bring out the best in them. Have you noticed that spiders are looking meatier nowadays?

According to new research, city-dwelling spiders are flourishing. Similar to rats and pigeons, spiders are “urban exploiters”. This means that they thrive in urban surroundings. Despite our squeamish attitudes towards them, spiders are important creatures: they greatly impact food webs, plant colonies and herbivore abundance, and they help to control the insect populace.

Most research on the expansion of cities focuses on birds. For instance, noise pollution has forced birds to change their calls, while other studies show that some birds have even stopped migrating. Researchers from the University of Sydney, however, were interested in the changes in spider anatomy that might have come from living in cities, and specifically Golden Orb-Weaving (Nephilla Plumipes) spiders, which are common in both urbanised and rural Australian landscapes. They build strong semi-permanent webs with golden-looking silk, and, once matured, they reside in this web their entire life. Thus, their “coach potato” lifestyle makes them ideal research subjects.

The researchers collected 222 spiders from rural and urbanised areas. All spiders were female and had reached maturity, and their body size, fat reserves, and ovary weight were recorded. The findings showed that rural-dwelling spiders had smaller bodies, but that city-dwelling spiders were larger, and more fertile. Places with roads, buildings and a lack of greenery typically denoted larger spiders. The researchers proposed that temperature differences could explain these results, as cities are much warmer than rural areas thanks to the urban heat island effect.

Their results could also reflect differences in prey availability. The study’s lead author, ecologist Elizabeth Lowe, explained in more detail:

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There were strong associations in particular between spider size and the presence of hard surfaces (such as roads and buildings) and lack of vegetation. These hard surfaces contribute to the urban “heat island” effect, which makes it warmer in cities than surrounding areas. It is also likely that there is more food for the spiders in the city as a result of night lighting and increased resources. This combination of warm temperatures and more food would allow spiders to put more energy towards growth and reproduction. Fewer predators and parasites in urban areas could also allow urban spiders to grow larger.”

This study is only applicable to one species of spider in one Australian city, so it’s not fair yet to say that all urban spiders are getting bigger – so don’t panic about huge spiders with larger ovaries just yet. However, the researchers pledge that this is promising news:

The fact that some spiders benefit from urbanisation is a good thing. In order to maintain biodiversity in cities, we need to be able to support diverse populations of spiders and other invertebrates” Lowe said. “By gaining a better understanding of the impacts of urbanisation on wildlife in cities, we can work towards creating healthy, functioning ecosystems in urban areas.” 

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