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Urban life is making some spiders bigger, more numerous

Scott Sutherland
Meteorologist/Science Writer

Tuesday, August 26, 2014, 12:16 PM - Dislike spiders? Avoid leaving the city because you expect that you'd find more of them, and perhaps more importantly bigger ones, in the country? Well, according to some new research, cities are apparently a paradise for spiders that may have you fleeing at your earliest convenience!

Compared to rural areas, cities definitely have less wildlife and fewer wildlife species, as the noise, pollution, lack of habitat and overall increased danger-level drive most species out. Some do quite well, though, like pigeons, crows and (especially in Toronto) raccoons, and insect species like flies have it made in the urban environment, with the abundance of garbage and waste to feed off of and breed in.

With a reduced number of species around to prey on them, ample food (the flies) and warmer temperatures due to the urban heat island effect - not to mention plenty of nooks and crannies to live in amongst the buildings - cities are a paradise for spiders.

The largest spider found, from Sydney, and the smallest,
from Brisbane Waters National Park

University of Sydney PhD student Elizabeth Lowe and her colleagues conducted a study of one particular spider species - the golden orb-weaving spider, which is known for the golden colour of the silk webs they spin, and for having one of the most extreme size differences between males and females of any spider species, with males being only about one-tenth the size of females. Collecting specimens along the east coast of Australia, from both rural and city environments, they compared the size and reproductive ability of the various spiders.

Using the length of the spiders' tibia (the longest part of their legs) as a measurement of size, and how much fatty reserves they had in their body and how heavy the females' ovaries as a gauge for their reproductive capability, they found that as they transitioned from rural to urban environments, the city spiders were larger and had the ability to produce more offspring than their country cousins.

"There were strong associations in particular between spider size and the presence of hard surfaces (such as roads and buildings) and lack of vegetation," Lowe wrote in The Conversation. "These hard surfaces contribute to the urban 'heat island' effect, which makes it warmer in cities than surrounding areas."

Now, as nightmarish as this might seem for some people, one thing that Lowe and her colleagues emphasize is that this is a good thing, and she says not to reach for the bug spray just yet.

"Although you may not like the idea of living in close proximity to spiders," she wrote, "we need them in cities because they are vital components of terrestrial ecosystems. The incredible diversity of spiders in Australia means that they play many important roles and contribute to biodiversity — the total number of different species found in an environment."

Besides, without the spiders there to control their population, many plant-eating insects would multiply and become pets, and the spiders, in turn, become food for city-dwelling birds.

The study is available from the online journal PLOS ONE (click here).

(Images courtesy: Elizabeth Lowe, University of Sydney)

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