Big cities lead to bigger spiders and lazier birds

Chuck Bednar for – @BednarChuck

As humans continue to build and live in bigger and better cities, they are driving evolutionary changes in creatures that could have “significant implications” for their own well-being, experts from the University of Washington report in a new study.

Writing in the journal Trends in Ecology & Evolution, Marina Alberti from the UW College of Built Environments’ Urban Ecology Research Lab explained that those changes have been small thus far, but noticeable. They include spiders that are growing larger, salmon that are shrinking, and urban birds that are growing tamer and bolder than their rural counterparts.

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If, as the evidence suggests, that human-driven evolutionary change is having an impact on the way that ecosystems function, it “may have significant implications for ecological and human well-being,” Alberti explained. It had long been assumed that these changes would take too long to have an immediate impact on ecological processes, but new evidence suggests otherwise.

“We now have evidence that there is rapid evolution. These changes may affect the state of the environment now,” the urban design and planning professor said. “This is what’s called eco-evolutionary feedback. Cities are not simply affecting biodiversity by reducing the number and variety of species that live in urban habitats.”

Spiders and worms and birds, oh my!

In fact, she said that organisms are undergoing accelerated evolutionary changes directly caused by humans living in cities, Alberti said. Those changes have had an impact on biodiversity, food production, nutrient cycling, seed dispersal, and other ecosystem functions, and that they will also eventually have an impact on the health and well-being of humans as well.

In her study, Alberti conducted a systematic review of various examples of human-caused trait changes in birds, mammals, fish and plants, and the effects of these so-called human signatures. In addition to the changes in spiders and salmon, she found earthworms that have an increased tolerance to metals, a type of city-based mouse that has become a host for ticks that carry Lyme disease, and plants that are now dispersing their seeds far less effectively than before.

She also found that songbirds have been changing their tunes to make sure that their acoustic signals do not get lost in the cacophony of urban noises, and that European blackbirds have been growing more sedentary and have altered their migratory patterns due to urbanization.

But why is this happening?

Humans dwelling in cities have caused these changes to take place in several different ways, according to Alberti. Urbanization changes and distrupts natural vegetation patterns, introduces toxic pollutants and novel disturbances such as noise and light, and causes the temperature to increase. Our presence also changes the availability of food, water, and other vital resources.

The researcher says that her work has prompted serious questions that could be used to guide future studies, including whether or not global urbanization will ultimately alter the course of the Earth’s evolution, and if urbanization is bringing our planet closer to an environmental tipping point such as the event that introduced oxygen to the atmosphere over two billion years ago.

However, Alberti emphasizes that she is not looking at the phenomenon through a “catastrophic” perspective. Rather, she wants to emphasize the challenges that we face, as well as the one-of-a-kind opportunity that we currently have in shaping the evolution of the planet Earth.

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It is the “hybrid nature” of ecosystems in urban environments “that makes them unstable, but also capable of innovating,” she concluded. “We can drive urbanizing ecosystems to collapse – or we can consciously steer them toward a resilient and sustainable future. The question is whether we become aware of the role we are playing.”


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