My Times column on the evolution of urban wildlife:
Easter Monday bank holiday feels like a good moment to put aside politics and consider something far more portentous: evolution. Recently I was walking alongside a canal in central London, surrounded by concrete, glass, steel and tarmac, when I heard the call of a grey wagtail. Looking to my right I saw this bold, fast, yellow-bottomed bird, which I associate with wild rocky rivers in the north, flitting into a canal tunnel. Later that week I stared up at two peregrine falcons circling high above parliament — and got funny looks from passers-by.
Like other cities, London is increasingly home to exotic wildlife and is as biodiverse as some wildernesses. Mumbai has leopards, Boston turkeys, Chicago coyotes and Newcastle kittiwakes. Suburbs are already richer in wildlife than most arable fields in the so-called green belt, making environmental objections to housing development perverse. Gardens, ledges, drains, walls, trees and roofs are full of niches for everything from foxes to flowers and moths.
Two Czech scientists counted the species of plants in the city of Plzen compared with a similar area of surrounding countryside. In the city the number of species had risen from 478 in the late 19th century to 773 today. In the countryside it had fallen from 1,112 to 745.
Since most animals have shorter lifespans than us and no welfare state, they are genetically adapting faster to the concrete world than we are. A fascinating book by a Dutch biologist, Menno Schilthuizen, called Darwin Comes to Town, documents just how wide and deep this urban wildlife evolutionary pulse is. We have unleashed an unprecedented burst of natural selection.
Once a species thrives in a man-made habitat, it may find itself giving up living elsewhere. This must have happened to swallows and sparrows a long time ago: they became so successful nesting in buildings that the genes of their tree or cliff-nesting cousins died out. Today it is probably happening with peregrine falcons and herring gulls: urban ones are having more young than rural ones, so will soon swamp the whole species with their genes.
Urban landscapes present new evolutionary pressures. Street lights confuse and massacre moths and cause songbirds insomnia. Metal concentrations can be toxic. Noise drowns out birdsong. Instead of remaining insuperable, however, these novelties bring out the ingenuity in evolution. Urban insects may be changing their genetic make-up so they no longer fly towards lights: suicide as a selective force. One Swiss study found that ermine moths from the countryside are almost twice as likely to fly towards a light as their cousins from the city of Basel.
Other examples of urban evolution abound. Killifish in polluted American harbours have developed genetic resistance to the effect of polychlorinated biphenyls, an industrial pollutant. Acorn ants in Cleveland, Ohio, can withstand high temperatures better than ants from the country — which is necessary because city temperatures tend to be higher. Mexican sparrows that incorporate cigarette butts in their nests have fewer bloodsucking mites feeding on their chicks because nicotine is a pesticide.
Birds sing higher-pitched songs in cities — the ones that stayed low having attracted fewer mates over the sound of traffic. In the countryside, the opposite is true: female great tits mated to high-pitched males are more likely to stray. So the species is splitting into soprano town-tits and bass country-tits. In the Netherlands, chiffchaff warblers and grasshoppers both sing higher-pitched songs if they live near busy roads. Pigeons in big cities have darker plumage because melanin pigment binds zinc, excreting it from the body and improving the birds’ health.
Human beings, too, have been forced to evolve by urbanisation. For centuries cities such as London were population “sinks”, killing more people with disease than their birth rates could match and sustaining their population only by immigration from the countryside. That put a premium on genetic mutations that resisted urban diseases. People with long histories of urban living tend to have genes that resist tuberculosis and leprosy, for example. It would not be a surprise to find that an ability to tolerate continual noise may also be partly genetic as well as learnt.
Walking to the Tube in London each morning at this time of year I hear goldcrests and goldfinches, parakeets and dunnocks, wrens and long-tailed tits, none of which lived in the middle of cities in my youth. Experiments show that urban tits, finches and sparrows are less “neophobic” than rural ones: they have evolved to be less fearful of the appearance of new objects on bird tables, for example. Compared with the egg-stealing, catapult-wielding youths of previous centuries, young people today simply do not pester animals as much.
Blackbirds first showed up in London in the 1920s, later than in continental cities. Studies in France and the Netherlands found that urban blackbirds were rapidly diverging from rural ones. They tend to have shorter beaks and wings, longer intestines and legs, as well as higher-pitched songs. They may soon count as a separate species, just as town pigeons are very different from their rock-dove cousins. Dr Schilthuizen argues that “as the urban environment expands its reach, it will become more and more an ecosystem in its own right, writing its own evolutionary rules and running at its own evolutionary pace”. Wildernesses experience very slow rates of species formation because they are already mature ecosystems. Cities, like archipelagos of islands, experience a much faster rate of change.
The immediate reaction of many people to this tale of urban biodiversity might be to lament the human interference in nature and discount urban wildlife as artificial. We sometimes despise rather than admire creatures that become urban: town pigeons are “feathered rats”, urban foxes “mangy vermin”.
An increase in urban wildlife cannot compensate for the extinction crisis in wilder spaces. But thanks to increased awareness and new techniques, we have shown we can halt extinction if we try.
In recent centuries we have lost 61 of 4,428 species of mammals and 129 of 8,971 birds. Thanks to the genetic change that is happening in the urban Galapagos, we can create new species too, albeit unwittingly. A small cheerful thought for a festival of chicks and bunnies.
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