Survival of the urban species

Anyone plagued by urban foxes might not be surprised to hear that biodiversity is increasing in cities. This was the consensus at the Urban Biodiversity Conference held in Glasgow at the end of October. Even though the overall impact of humans on biodiversity is disastrous, cities are providing new opportunities for wildlife through habitats such as parks, allotments, railway embankments and cemeteries. How and why some species thrive in close proximity to humans, while others don’t adapt fast enough, is the question that conservationists are trying to answer.

Although urban habitats cannot on their own offset the impact of human activity, they have an important role to play. With the help of initiatives such as roof gardens, species can be preserved despite living close to humans, sometimes with greater protection than in rural areas where farming and poaching are a threat.

Animals, including squirrels and birds, have adapted successfully to cities. Among plants, some weeds have adjusted well. One is crepis sancta, a member of the daisy family. The lightweight seeds of many weeds often disperse only to fall on barren urban ground, but c sancta produces heavier seeds more likely to fall back on the soil occupied by the parent. London is now home to the peregrine falcon, which has learnt how to hunt by night instead of by day, according to a 2008 report in the journal British Birds.

Of course, the first impact of urban development on wildlife is negative, but over time many species that were initially expelled from their habitats return to their original hunting grounds. Some take longer than others. Natural scavengers like pigeons, starlings and seagulls quickly adapted to the urban sprawl that has grown up in the past 100 years or so. Others such as foxes took longer, but now co-exist with humans at a much higher population density than in country areas.

The rapid adaptation of urban animals can be compared with humans themselves during the transition to agriculture, starting 10,000 years ago. This led initially to lower life expectancies because diet became less varied and infectious disease more common in denser populations. But over several thousand years new gene variants coped with changing diet and improved immunity to disease. In the animal kingdom, about 97 per cent of Britain’s urban foxes were wiped out in the 1990s by mange, a skin disease caused by mites. But the small fraction which were immune thrived, eventually reaching the heart of London—evidence that near-extinctions accelerate adaptation for the lucky survivors.

Urban noise and pollution are challenges for wildlife. Blackbirds have learned to sing at a higher pitch to communicate with potential mates above traffic noise. Other urban birds sing at night, causing increased production of stress hormones such as corticosterone. Yet they eventually adapt by producing less of it than rural counterparts exposed to stressful situations such as an encroaching human population.

It has taken songbirds much longer to adapt to city life than pigeons or starlings, which explains why the avian life of a typical city diversified only recently, with increasing numbers of tits seen in urban gardens.

There is even evidence that cities can provide havens for rare animals. A Dutch study involving eight universities in 2007 discovered 18 species of bird on the endangered list living happily within urban business parks around the country.

A number of big cities across the world, including Toronto and Singapore, are taking a lead by encouraging inhabitants to record biodiversity. Glasgow has recently launched a plan for volunteers and community groups to record information via questionnaires, leading to schemes promoting wildlife by exploiting roofs, allotments and parks.

Such projects should show that encouraging city dwellers and businesses to provide a helping hand to wildlife can yield clues to how natural selection can cope with rapid change, and to how providing opportunities for animals and plants to adapt at their own pace.