Can Cities Change Earth's Evolution?
When Charles Darwin first taught us how to think about evolution, he also was teaching us to think about time. By allowing natural selection to work over millions of years, what might seem like a divine miracle (the creation of a new kind of animal) became something much more grounded (though equally wondrous).
But what if evolutionary change could operate much faster? What if evolution could dance to the same beat as something like ecology — the interaction of life with changing environments? This idea of "eco-evolutionary dynamics" is a relatively new one that, on its own, is stunning enough. But include the possibility of rapid, evolutionary change a la the Anthropocene — the brave new human-dominated world we're creating — and things can get really interesting.
Eco-evolutionary dynamics in a rapidly urbanizing world is the subject of a new paper by Marina Alberti, a professor at the University of Washington's College of the Built Environment. I first met Alberti a couple of years ago, as my own research into "the physics of cities" began. What is fascinating about her Urban Ecology Research Lab is the breadth of its efforts — and I've been happy to learn and collaborate with them. As an astrophysicist, what intrigues me about the new paper is that its perspective takes rapid urbanization as not just a social phenomena, but also as a planetary one.
Over the past decade or so, Alberti argues, scientists have recognized that changes in ecosystems can drive changes in evolution, such as the emergence of new species' traits. And as these traits rapidly evolve, novel ecosystem functions will appear as well. As Alberti puts it: "There is significant evidence that changes in ecological conditions drive evolutionary change in species traits that, in turn, alters ecological interactions."
In the modern era of a human-dominated planet, we are likely to have a special role to play in eco-evolutionary dynamics. Alberti writes:
"In human-dominated environments, selection pressures acting on traits can affect population dynamics by changing organisms' rates of survival or reproductive success, leaving a genetic signature that might affect community dynamics and ecosystem functions... These effects drive changes in energy and material fluxes that, in turn, influence ecosystem functions, such as primary productivity, nutrient cycling, hydrological function, and biodiversity, which provide essential services for human wellbeing."
The fact that human beings can shape ecosystems is, of course, nothing new. The really interesting thing in Alberti's study is the fact these ecosystem changes are shifting the evolution of species right now. And, just as important, the potential feedback of those evolutionary changes on ecosystem function will happen on a similar timescale.
City building is the most dramatic example of human beings altering ecosystems, and Alberti provides a long list of ways in which life is already evolving in response to the pressures of urbanization.
Songbirds are becoming tamer and bolder — and also are changing their tunes to ensure their acoustic signals don't get lost in the noise of cities. European blackbirds are becoming sedentary and have changed their migratory behavior in response to the growth of urban regions. Then there are the earthworms that are evolving increased tolerance to metals, plants that are modifying their seed dispersal "strategies" and an urban variety of mouse that's becoming a "critical host" for Lyme disease carrying ticks.
In all these cases, Alberti contends that the "urbanization" of ecosystems is, in a sense, leading to an urbanization of evolution. On the genetic level, life is responding to us. If she is right, however, the implications may stretch far beyond just evolving new species of "loud and proud" songbirds. Given the pace, scope and consequence of accelerating urbanization, Alberti thinks this new form of eco-evo dynamics might have global consequences, too. She writes:
"From a planetary perspective, the emergence and rapid expansion of cities across the globe could represent another turning point in the life of our planet. For most of its history, Earth has experienced long periods of relative stability, dominated primarily by negative feedbacks. However, the recent increase in positive feedback (i.e., climate change) ... could trigger transformations on the scale of the Great Oxidation Event that introduced oxygen into the atmosphere more than 2 billion years ago."
In other words, the evolutionary changes driven by our impact on the planet could drive the Earth through a dramatic and, perhaps, nonreversible tipping-point.
It's important to understand, however, that Alberti is not arguing for a "catastrophic" perspective. Instead, she says the recognition of eco-evo dynamics highlights both the challenges and the unique opportunity that humans have in shaping the Earth. Ecosystems in urban environments are a sort of hybrid, she said: "It is their hybrid nature that makes them unstable, but also capable of innovating."
Innovation is the key word here. Evolution has always been about innovation in the endless exploration of new niches. What I like about Alberti's perspective is it offers a powerful shift in how we view ourselves in the natural world. Rather than seeing human beings as a nexus of greed and evil, we become part of evolution's ongoing experimentation.
"We can drive urbanizing ecosystems to collapse — or we can consciously steer them toward a resilient and sustainable future," Alberti says. "The question is whether we become aware of the role we are playing."
Adam Frank is a co-founder of the 13.7 blog, an astrophysics professor at the University of Rochester, a book author and a self-described "evangelist of science." You can keep up with more of what Adam is thinking on Facebook and Twitter: @adamfrank4.
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