By Menno Schilthuizen, The New York Times, July 23, 2016
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| Blackbird in New York |
Amsterdam — A FRIEND recently invited me over to see the blackbird that had taken up residence in a potted plant on her balcony.
Serenely incubating eggs in the inner city, this bird had little in common with its shy, reclusive ancestors that nested in Europe’s forests. Early in the 19th century, probably in Germany, blackbirds began settling in cities. By the mid-20th century, they were hopping around on stoops all over Europe.
Many “wild” bird species — like the peregrine falcons, red-tailed hawks and laughing gulls of New York — have set up camp in cities. But the thing about Europe’s urban blackbirds (a relative of the American robin, not to be confused with North American blackbirds, which belong to a different family) is that they are very different from their forest-dwelling relatives. They have stockier bills, sing at a higher pitch (high enough to be heard over the din of traffic), are less likely to migrate (in cities there’s food and warmth year-round), and have less nervous personalities.
For many of these differences, genes are responsible. The birds’ DNA, after 200 years or less of adaptation, has diverged from that of their rural ancestors.
For a long time, biologists thought evolution was a very, very slow process, too tardy to be observed in a human lifetime. But recently, we have come to understand that evolution can happen very quickly, as long as natural selection — the relative benefit that a particular characteristic bestows on its bearer — is strong.
And where else to find such strong natural selection than in the heart of a big city? The urban environment is about as extreme as it gets. Temperatures in the city center can be more than 10 degrees higher than in the surrounding countryside. Traffic causes continuous background noise, a mist of fine dust particles and barriers to movement for any animal that cannot fly or burrow. Much of the city is clad in impervious surfaces of stone, glass, steel and tarmac. There is pollution of soil, water and air, mainly human-derived food sources, and an especially motley crew of local and invasive flora and fauna.
With urban environments expanding all over the world, wildlife and biologists alike are starting to treat the city as a true ecosystem. Many species’ original habitats are being squeezed into annihilation. For them, it’s adapt or die. And field biologists like me are following suit. As we have to travel ever farther to find untouched wilderness, we are beginning to realize that the expanding urban sprawl is perhaps not something to be depressed about, but rather something very exciting, as entirely novel forms of life are evolving right under our noses.
A Fordham University biologist, Jason Munshi-South, studies the populations of white-footed mice marooned in New York City parks. These native mice once lived all over the place. But as the city expanded, they became confined to the small pockets of forest left behind in parks. Thus isolated, the mice in each park began evolving a park-specific genetic blueprint. In some parks, Dr. Munshi-South found mice carrying genes for heavy metal tolerance, probably because soils there are contaminated with lead or chromium. In other parks, the animals have genes for increased immune response — maybe diseases spread more easily in some high-density populations.
French biologists have been studying a daisylike weed called Crepis sancta, which normally produces two kinds of seeds: heavy ones that fall to the floor, and light seeds that drift in the wind for long distances. But in Montpellier, in southern France, C. sancta makes reduced numbers of the airborne seeds. Small wonder: The plants grow in pockets of soil on sidewalks, and any seeds that are carried on the wind are likely to land on concrete. The heavy seeds that land at the parent plant’s feet, on the other hand, are pretty certain to find a patch of fertile soil. So plants genetically predisposed to produce more heavy seeds have been favored by urban evolution.
THERE are more examples: Spiders in Vienna are evolving to build their webs near moth-attracting streetlights. In some cities, moths, in turn, are developing a resistance to the lure of light bulbs. Certain Puerto Rican city lizards are evolving feet that better grip urban surfaces like concrete. Some grass is adapting to the relentless regime of the lawn mower by acquiring a shorter stature.
The most exciting projects are perhaps no longer in faraway forests and canyons, but just there on our doorstep. We evolutionary biologists are trading our expedition gear for subway tickets and studying street grass and house mosquitoes instead of jungle orchids and mountain birds.
And we have millions of city dwellers to help us. Citizen science projects on urban ecology and evolution are springing up everywhere. This year, my students and I will introduce a smartphone app to measure how snail shells in hot inner cities in Europe and North America are evolving lighter colors to shield against overheating. Adeline Murthy of the University of New Mexico used the Christmas Bird Count, an annual census conducted by volunteers, to show that North American cities harbor an avifauna that is pretty much homogenized across the continent. At least 18 bird species are shared by all of them — something not the case in non-urban areas.
In fact, that Christmas data highlights one feature of urban nature that sets it apart from all other ecosystems: globalization. City-adapted wildlife is likely to hitch rides on human transportation and colonize other cities — at least within the same climate zone.
What’s more, as cities continue to grow, they will exchange more goods, people and information over greater distances. So each change in the environment (a particular pollutant, a certain novelty in road construction, a new kind of food source) will spread quickly across the world, and urban wildlife everywhere will be faced with the same novel challenge. Those that evolve adaptations will also easily spread to other cities, leading to a truly globalized urban flora and fauna — continually evolving at breakneck speed to keep up with an increasingly human-dominated world.
Back on my friend’s balcony, I peered through the branches at that nesting blackbird. She returned my stare with one glistening eye, as if to say: “Consider me your Darwin’s finch. And this city is my Galápagos.”
Menno Schilthuizen, an evolutionary biologist at the Naturalis Biodiversity Center in the Netherlands, is the author of “Nature’s Nether Regions” and the forthcoming “Darwin Comes to Town.”
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