Some Spiders Grow Bigger in Urban Areas

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Artificial lighting, lawns, urban “heat islands,” and other features of city and suburban life are leading to bigger spiders with an increased ability to reproduce, a new study finds.

The study, published in the latest issue of PLoS ONE, could help to explain why some homeowners are finding particularly big spiders in their gardens. Researchers say the effect is noticeable among common orb weaving spiders. (Orb weavers are spiders that build spiral wheel-shaped webs.)

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Like an unintended Frankenstein experiment, we are facilitating spider growth with light posts.

“Artificial night lighting has many implications for spider fitness as it leads to local increases in insect abundance, and increased prey capture for spiders in lit habitats,” wrote lead author Elizabeth Lowe from the University of Sydney, Australia, and colleagues.

The researchers focused on a particular common orb weaver, Nephila plumipes, because it lives in both urban and rural settings. They measured and then compared the body size, fat reserves and ovary weight of the spiders in each habitat. Ovary weight is an indicator of reproductive ability.

Lowe and her team found that spiders had smaller bodies in areas with more vegetation cover, and larger, fatter bodies in areas associated with urban development. One would think that clean, country living would be optimal for the spiders, but not so. To paraphrase the old Alicia Bridges song, they “love the nightlife” and thrive under the bright lights. The spiders also seem to like living on and around hard surfaces.

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The researchers theorize that the spiders also do well with increased heat. Cityscapes create “heat islands” where the temperature is significantly hotter than other regions. According to the EPA, the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4 degrees Fahrenheit warmer than its surroundings.

Heat islands occur where buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry. This is obviously a problem for most, but apparently not for these spiders.

The spiders create a base for themselves in and around structures and often near a prey-filled lawn. When given the choice between being near leaf litter or a lawn, the researchers found the orb weavers selected the latter.

Another important difference between these city and country spiders is that country spiders seem to suffer from more parasites. The heat, hard surfaces and more discourage the growth of the tiny parasites that prey on spiders. City spiders therefore don’t have to deal with this problem as much as their country cousins do.

As for ovary weight, it was greater in the city/suburban spiders. This measurement suggests that they will reproduce more often, according to the researchers.

News stories often highlight animals that suffer as a result of human-changed landscapes, but we would also do well to watch out for “urban exploiters” such as these spiders.

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Lowe and her team concluded, “By identifying the elements of cities that influence the success of urban exploiters we gain a better understanding of what drives changes in the biodiversity of urban systems.”

Photo: the spider Nephila plumipes. Credit: Elizabeth Lowe