The Force Behind Shrinking Salmon, Killer Mice, and Silent Songbirds
We’ve long understood that over time, humans can cause “micro-evolutionary” changes in other species: small genetic tweaks that help a plant or animal survive and thrive in human communities.
But as urbanization accelerates—more than half the world’s human population now lives in cities—so does evolution, according to researcher Marina Alberti. Other organisms are changing to keep up with the pollution, light, noise, human crowding, and changes in land and water that urbanization brings.
This is happening not over centuries or millennia but in decades or even years.
In her research, published in the journal Trends in Ecology & Evolution, Alberti described well-documented shifts in animal traits in response to urbanization: Pacific salmon in rivers are getting smaller, for instance, and some songbirds are changing their mating songs to find each other amid a city’s noise.
That could mean that animals like endangered Southern Resident killer whales that depend on salmon for food will have to hunt harder and longer to get the same amount of calories, hurting their chances for survival. Or that those songbirds could become less genetically robust, because newcomers will not be able to find them to mate.
“What we know at the moment is that the changes have reduced ecosystem function, not increased them,” Alberti said.
In one change with a direct impact on human health, a urban mouse species that is a primary carrier of Lyme disease ticks is becoming hardier. Upswings in these mouse populations are connected to more widespread outbreaks of Lyme.
“Urbanization is not only reducing the number of species and biodiversity in general, but actually we are making, selecting, which species can coexist in urban areas,” said Alberti, the director of the Urban Ecology Lab in the University of Washington’s planning department.
She worries that this could easily affect the natural systems we depend on, including food production, clean air and water, and even the genetic diversity that creates resilience to changes in the environment like global warming.
For example, earthworms in urban areas have become better able to survive in soils contaminated with heavy metals. That sounds like a pretty good outcome, but we have no idea how it will affect animals that eat worms, say, or the living ecosystem within the soil, or even the survival of the worms themselves.
“Those organisms have specific importance for nutrient cycling, decomposition, and other ecosystem functions,” Alberti said. “So those changes have enormous implications over where this planet will go.”
As troubling as this all sounds, Alberti said, her point in doing the research was not to chart an inevitable apocalypse.
“I’m asking the question, can urbanization affect the course of evolution to a point that we are shifting to a completely different planet?” Alberti said. “I’m an optimist. I believe that if we become aware of that, we can steer the changes in the direction of a sustainable future.”