CITIES: SOURCES OF SOME SURPRISING ADAPTATIONS

CITIES: SOURCES OF SOME SURPRISING ADAPTATIONS

• BY FANNY ROHRBACHER

By building cities, human beings have altered the environment and created new habitats. But some plants, birds and insects have shown an impressive ability to adapt. A closer look at certain species.

AN OPPORTUNITY FOR SOME PLANT SPECIES?

According to some studies, dandelions, poison ivy and ragweed could benefit from the increase in CO 2 in the air. CO 2 is absorbed by plants during photosynthesis, so its greater concentration in cities may well lead to a marked increase in the number of dandelion flowers and seeds between now and the end of the century. At the same time, poison ivy will reportedly become more toxic as it develops a more allergenic form of urushiol (the substance that causes a painful skin rash). It is also anticipated that ragweed will produce more pollen, which will make it more allergenic.

ACROBATIC SWALLOWS

The case of the cliff swallow is a typical example of urban adaptation. Some of this swallow’s favourite nesting sites are highway bridges and overpasses. By choosing to live near roads, these birds run the risk of colliding with motor vehicles. Yet in just a few decades, the cliff swallow’s morphology has evolved to adapt to this hazard. With shorter, rounder wings, it is now capable of making faster turns and vertical take-offs to avoid traffic. This evolutionary change has allowed it to survive better than other swallows with longer wings that are less skillful flyers.

TAMER URBAN BIRDS

In all the world’s cities, birds are becoming increasingly tolerant of human proximity. This behavioural adaptation is sometimes even printed in their DNA, and so is transmissible to following generations. Argentine burrowing owls, European common blackbirds, Australian black swans—when a pedestrian approaches, these birds that have become urban dwellers now take longer to fly off than their country cousins, and don’t retreat as far. It seems that urban crowds are influencing the evolution of these birds’ anxiety and distrust genes, to the point where they have virtually no fear of possible threats to their safety.

WEAVING THEIR WEBS IN THE CITY

In urban areas, some spiders are no longer afraid to come out of their dark hidey-holes. The triangulate cobweb spider, a cousin of the black widow, seems to have adapted to the glare of public lighting. While young country spiders avoid well-lit places and weave their webs in the dark, young urban spiders weave them in the light. As a result of this change in behaviour, they can now capture the many kinds of prey that are attracted by light, such as moths, which are a meal of choice for triangulate cobweb spiders. This adaptation to artificial light now appears to be broadly anchored in the DNA of these specific populations. The new behaviour does not seem to be shared by other species of spiders, however.

HIGHER TEMPERATURES ARE A HUGE CHALLENGE FOR SOME INSECTS

Warmer than the country, the urban environment has an influence on a key characteristic of insects: their size, making some of them bigger and others smaller. When temperatures are higher, insects have to expend more energy to survive. As a result, some of them, like beetles and weevils, grow less if they are unable to offset their growth-related energy losses by eating more. Conversely, the rise in temperatures associated with longer growing seasons could allow other species, like the Russian wheat aphid, to harvest more resources and get bigger. These two mechanisms can also act simultaneously, compensating for one another or cancelling each other out. How will smaller or bigger insects affect their predators? Only time will tell.

CITY DWELLERS, FIELD DWELLERS

Fast, persistent urbanization is also having an influence on o ur own species. Natural selection can occur following epidemics: people who have lived in urban areas for a long time are better adapted and have greater natural resistance to certain infections, like leprosy and tuberculosis. In the future, extreme pollution in megacities could also play a role in changing our genome, activating or deactivating some of our genes or causing mutations. For example, a quarter of the people living in San Antonio de los Cobres, Argentina, now carry mutations that protect them against the arsenic in their drinking water. In short, around the world, people’s genomes are adapting to urban life and will continue to do so in future.