Why do Some Species Thrive in Cities?

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0:01Since the mid-1800’s, Adelaide’s mammal diversity has fallen by more than a quarter,

0:06New York City has lost nearly half of its native plants, and at least 25 species of

0:10butterflies have disappeared from Rome. Our concrete jungles can be hard on wildlife,

0:18but there are plants and animals that survive – even thrive – in them.

0:23Some lucky species happen to be naturally suited to cities. For instance, English ivy

0:28and rock pigeons climb and roost on vertical structures like trees and cliffs anyway, so

0:34brick walls and sky-high ledges are fine substitutes. And omnivorous raccoons thrive

0:40on an endless buffet of everything from corn chips to cockroaches, helping them live ten

0:45times more densely in cities than in woodland habitats . Natural flexibility can also help

0:50animals cope with the stresses of metropolitan living: coyotes that colonize cities often

0:56become more nocturnal to minimize their encounters with humans.

1:00Most species can’t adopt an urban lifestyle so easily. But over several generations, genetic

1:06change can help some populations evolve into city slickers.

1:10New York City’s white-footed mice are one example: DNA sequencing suggests that these

1:15urban dwellers' genes differ from their country cousins' in more than 30 significant ways.

1:21We don't yet know exactly what effects these changes have, but we know they’ve shown

1:25up in genes involved in fighting disease and processing toxins, traits that likely help

1:30mice survive in crowded landscapes .

1:33We usually think of evolution as a slow process, so evolving fast enough to keep up with urbanization

1:40 may seem like a tall order. But when a big challenge springs up suddenly , an uncommon

1:45trait that helps individuals cope can become the genetic norm surprisingly fast - especially

1:51in speedy breeders - because those without it may not get a chance to reproduce.

1:56For example after factories dumped thousands of tons of toxic PCBs into the Hudson River

2:01in the mid-twentieth century, it took just just six decades for 99 percent of the local

2:06tomcod to evolve a protective mutation that blocks the toxin from entering their cells.

2:11And in Montpelier, France, it’s taken fewer than 12 years for urban weeds to start producing

2:17a larger proportion of heavy seeds, boosting their odds of settling in a patch of nearby

2:22soil instead of crash-landing on concrete. As urban and rural populations diverge genetically

2:28and geographically, there’s even a chance that some could split in two.

2:33This doesn’t mean that cities are a boon for biodiversity , but they aren’t biological

2:38dead zones either - they’re more like accidental laboratories where the limits of life’s