Not just creepy-crawlies: What spiders can reveal about city life

Spiders can help us understand the complexity of how animals are affected by and respond to environmental changes.

We have a problem. Humans are changing and disturbing the environment. From cutting down a backyard tree to building roads and cities, our actions influence the ecosystems around us. The animals inhabiting our neighbourhoods are constantly exposed to the by-products of our presence. Traffic, artificial lights, and pollution all commonly have negative consequences for animals. Yet, some animals show an incredible capacity to overcome the challenges associated with city life. Just think about the classic examples of true urban survivors, like rats, raccoons, and those pesky pigeons many of us love to hate.

With a changing climate thrown into the mix, however, biologists are increasingly concerned with how plants and animals will be affected by things like changes in temperature and rainfall, combined with the stress of metropolitan life. Cities are already experiencing higher temperatures than surrounding areas. Heat captured by impervious surfaces, like concrete and asphalt, and heat produced by machinery like cars elevates city temperatures in a phenomenon coined the “urban heat island effect.” 

Dr. J. Chadwick Johnson, from Arizona State University, says that studying the urban heat island phenomenon is important because it provides a preview of how ecosystems will shift given current climate change projections, especially because “the environment is critical in determining how an animal acts.” Biologists have begun examining the effects on a number of animals, but surprisingly little is known about the implications for the smaller members of urban wildlife, our neighbourhood creepy crawlies.

If these are not your favourite animals, you might find comfort at the thought of their absence during the Canadian winter. Or you could let Johnson reassure you that animals like spiders are “not just aggressive robots.” After all, he has worked with several thousand of them.

As it turns out, spiders can reveal a surprising amount of information about how animals respond to changing temperatures. Johnson and colleagues set out to investigate how urban heat influences Western Black Widow spider behaviour and development in Phoenix, Arizona. Spider habitats in urban Phoenix are on average 6°C warmer than the surrounding Sonoran Desert. Since spiders are ectothermsthey rely on external heat sources to warm their bodies and be able to go about their daily spider businessurban heat islands could either be a great thing for the spiders, or the increasingly warmer conditions could exceed their thermal limits with serious or even fatal consequences. 

Johnson’s research unfortunately suggests the latter. It appears that Western Black Widow spiders would struggle to survive in the hotter urban areas. Specifically, spiderlings developed slower, were smaller, and had lower survivorship. Similarly, adult males experienced extremely high mortality and adult females spent less time building webs. Despite these concerning findings, Phoenix’ wild Widow spiders appear to be thriving and are abundant city dwellers across western North America. Why has yet to be discovered, but Johnson and colleagues are keen to keep searching for answers. 

One theory put forward by Johnson’s team suggests that urban spiders may be able to exploit complex refuges and behaviourally compensate to find and use the most favourable city microhabitats. A concept supported by research and Johnson’s own observations is that spiders can change their behaviour if they are moved from urban to desert temperatures, and vice versa. This suggests a surprising level of resourcefulness and flexibility for an animal most people tend not to think highly of.

Although many people dislike or even fear spiders, they are as valuable as any other animal in shedding light on the complexity of life and the implications of the multitude of threats animals face because of humans. Even someone who dislikes spiders, like Lauren Hennigar, a Mt. A student in psychology, can still appreciate their value. At the local level, “we need them to keep the mosquitoes at bay,” Hennigar says, and “a world without spiders is kind of scary, think about all the insects that would still be around.” So, if for nothing else, perhaps we can find comfort in knowing that as humans continue to change and disturb the world around us, spiders may be able to overcome the challenges we create and keep the insects in check. Perhaps they are not that bad after all.

To learn more, check out: Urban heat island conditions experienced by the Western black widow spider (Latrodectus hesperus): Extreme heat slows development but results in behavioral accommodations, by Johnson and colleagues (2019).

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