Ecosystems

(1) More than half of the world's people now live in cities, and cities are growing faster than any other habitat on Earth. (2) For a long time, ecologists treated cities as biological deserts. (3) Recent studies show that many species not only survive in cities but change in response to them. (4) In several European cities, blackbirds sing at a higher pitch than blackbirds in nearby forests. Low-frequency traffic noise masks lower notes, and birds that sing higher are more likely to attract mates. (5) A small fish called the Atlantic killifish has evolved resistance to industrial pollutants in some heavily contaminated estuaries. Resistant populations carry mutations in genes coding for a receptor protein that normally triggers a toxic response to these chemicals. (6) Moths that live near bright street lights are, in some populations, less strongly attracted to light than moths from dark rural areas. (7) Critics warn that some reported differences may be due to phenotypic plasticity rather than evolution: individuals may simply respond to their surroundings during their lifetime, without any change in allele frequency. (8) To tell the difference, researchers raise animals from city and rural populations under identical conditions. If the differences remain, a genetic basis is likely. (9) Urban green spaces, such as parks, gardens and green roofs, can act as stepping stones between habitats, but small isolated populations may lose genetic diversity.