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NATURE, it seems, is suddenly fashionable. Earlier this year, millions of British television viewers tuned in to Springwatch, a daily natural history programme that deployed hidden cameras in nests, on cliff tops and outside burrows to give an intimate view of the everyday lives of native animals. Even seasoned TV producers found it hard to explain what made this such a hit, but whatever the reason, a taste for wildlife-watching has come just in time for the non-human species with which we share the planet. As the global human population grows, people are appropriating ever more of Earth’s natural resources and encroaching on more of the remaining wild spaces.

But the world’s expanding cities and towns also provide niches in which wild species can try their luck. And as wildlife ventures into the city, urbanites are showing an increasing tolerance, and even affection, towards their new neighbours. The resulting efforts to make towns and cities more wildlife-friendly seem like good news for humans and non-humans alike. But they also pose a challenge. As we lure animals into urban environments with promises of food and shelter, we could be exposing them to hazards they are ill-equipped to handle.

A few non-human species have always had a genius for urban living: gulls, pigeons, rats and cockroaches spring to mind. In the past few decades, however, these long-established city-dwellers have been joined by a growing gang of newcomers. In British cities, foxes are now commonplace. In southern California, bighorn sheep journey down from the mountains to feed on lush lawns. Further up the Pacific coast, Canada geese nest on balconies and roofs in Vancouver. And in Australia, Melbourne is witnessing the return of rainbow lorikeets, flashy parrots not seen in the city since the 1920s. Even peregrine falcons – majestic birds of prey redolent of lonely sea cliffs and wide-open country – have gone urban. In the heart of New York city, 16 pairs are now raising their young.

But not all creatures have fared so well as cities expand. Consider, for example, the plight of the populations of dung-feeding scarab beetles living on land that became trapped by the growth of the city of Rome. Their patch was surrounded by the sprawling Italian capital in the 1950s, but continued to be grazed by sheep until 1989. Since then it has been the exclusive preserve of humans and their dogs. This transition was a disaster for 10 of the 19 resident species of scarab beetle. They relied upon copious quantities of high-cellulose sheep droppings to nurture their young, and when dog faeces became the sole food available, they could not survive. The nine other scarab species have coped only slightly better, eking out a living on dog droppings only because they contain small amounts of cellulose from the cereals added to dog foods.

It is unlikely that many Romans noticed the loss of the useful scavengers from their midst, let alone cared. But increasingly city folk are coming to appreciate the bonuses of living alongside wildlife. In 1980, when engineers reconstructed the Congress Avenue bridge in downtown Austin, Texas, for example, they had no idea that the crevices beneath the roadway would make an ideal bat roost. Now the bridge provides shelter for 1.5 million Mexican free-tailed bats, North America’s largest urban bat colony. At first the locals were outraged at this influx of migrants from Mexico – the bats arrive in mid-March to breed and return south in November. But conservationists soon won them over, not least by pointing out that the bats consume up to 15 tonnes of insects a night. Now more than 100,000 people visit the bridge every year to witness the bats emerging at dusk, bringing Austin around $8 million in tourist revenue.

“Even peregrine falcons, majestic birds of prey redolent of lonely sea cliffs, have gone urban”

Undoubtedly, wildlife can make our urban environment a healthier, happier and more lucrative place to live. But there is sometimes a downside. After a colony of great-tailed grackles built their nests alongside county hall in Houston, Texas, this year, pedestrians found themselves being dive-bombed by angry birds keen to defend their young. Such conflicts can only become more common as the countryside comes to the city.

“In 2003, 30,000 bats set up camp in Melbourne’s botanic gardens”

Despite this, interest in urban natural history is growing, a trend reflected in the number of people taking part in regular surveys to track the changing face of urban wildlife. Last year, 5000 participants in a US nationwide study organised by the Cornell Lab of Ornithology in Ithaca, New York, sent in their sightings of city pigeons, doves, gulls and crows. In the UK too, thousands of ordinary people are monitoring the wild birds that visit their gardens. The observations, analysed by the British Trust for Ornithology (BTO), are already beginning to point to changes in the way 80 different species of birds are using gardens.

“Town planners are tweaking their designs to encourage wildlife to stay or return”

One outcome of such large-scale surveys is the discovery that certain species that once thrived in our inner cities are now in decline. Just a few decades ago, for example, house sparrows occupied central London in their tens of thousands. Now they have all but disappeared from the heart of the city. In Kensington Gardens, numbers have fallen from 2603 birds in 1925 to eight in 2000 and apparently none today. Paris still has its sparrows, so what has gone wrong in London? There has been no shortage of theories: predation by a plague of cats, a shortage of nest sites and lack of food have all come under suspicion. Some allege that a chemical blitz on weeds and pests may have wiped out the seeds and insects they needed to feed their chicks. Or gardens landscaped with decking and gravel instead of trees and shrubs may have destroyed many of the birds’ potential nesting sites.

Changing cityscapes

Now surveys carried out for the BTO’s London Bird Project have found that the remaining birds are most likely to be found in places where there are plenty of deciduous bushes. This suggests that a loss of nesting sites has been the key. In recent years, many of the deciduous bushes that used to be a feature of London parks have been rooted out in a bid to cut crime and make people feel safer by removing hiding places for would-be attackers. The Royal Society for the Protection of Birds is planning to counter this with a four-year project to introduce nest boxes and bird feeders into suburban London gardens to see whether they can attract the sparrows back. If the move proves successful, the birds may eventually be persuaded to recolonise central London’s gardens and parks.

The story of London’s sparrows is a dramatic illustration of how sensitive animals can be to changes in cityscapes. But this has its positive side too. Architects and planners are starting to tweak their designs in an effort to encourage wildlife to stay or return. One potential beneficiary is the swift. These spectacularly aerobatic birds migrate every spring from their winter home in sub-Saharan Africa to nest under the eaves of European roofs. As the poet Ted Hughes wrote of their annual return: “They’ve made it again/Which means the globe’s still working…” Across Europe, however, swifts are not as numerous as they were, in part because their traditional nesting places are disappearing, as old buildings are re-roofed, or replaced by new ones. The Netherlands and Switzerland now have laws requiring developers to incorporate nesting places for swifts. In the UK, the conservation group Concern for Swifts is campaigning for access holes and specially designed rectangular nest boxes to be fitted under eaves in new and renovated roofs on public and industrial buildings.

Planners are incorporating similar ideas into large-scale housing developments. Take the derelict sites and old docks east of London earmarked for the massive Thames Gateway project. This “brownfield” site supports a band of black redstarts, attractive birds which moved in after the second world war. The wasteland is perfect for them because they feed on weedy plants that grow on land with few nutrients. What they cannot tolerate is the sterile bark-mulched landscaping routinely used on new developments. So eco-planners at Thames Gateway advocate building roofs with sparsely vegetated areas where the black redstarts could forage. Such “green roofs” have already proved their worth, providing refuge for a variety of plants, insects and birds in Germany, Switzerland, Sweden and the US.

Harmonious cohabitation between people and wildlife seems on the surface to be a guaranteed win-win situation. But it is not always so, as the tale of the Cooper’s hawk reveals. At first glance it looks like another urban success story. Cooper’s hawks occupy the city of Tucson, Arizona, at much higher densities than the surrounding countryside, and they nest earlier and lay more eggs. The trouble begins after the eggs hatch. More than 50 per cent of urban nestlings die, compared to less than 5 per cent of chicks reared in the country. The young city hawks are dying from trichomoniasis, a disease carried by pigeons and doves, which make up the bulk of the urban hawks’ diet. Yet Tucson’s hawk population is not declining. The losses among urban birds are masked by a continuing influx from the surrounding area, lured into the city by the prospect of pigeons galore and a wealth of nest sites.

Springing the trap

This is a textbook example of an ecological trap, a concept first described in the 1970s. Ecological traps arise because animals can make mistakes: they can be seduced by a man-made habitat that looks good but in fact has a hidden downside. Animals trying to breed there will fail over and over again to raise young, yet new pairs will keep moving into the neighbourhood to keep the population up. The bighorn sheep near Palm Springs, California, are another example. Tempted down from the arid mountains to feed on well-watered urban lawns, they are brought into contact with a parasitic nematode called trichostrongyle, which reduces their general stamina, making them more vulnerable to pathogens and less likely to raise healthy young. Clear-cut, documented cases of ecological traps are still few and far between, but ecologists fear there are a lot more out there.

Predation is another factor that can spring an ecological trap. The notion that urban living might guarantee birds extra protection from predators has been dealt a serious blow by a study in which researchers in Finland, Italy and Spain compared the fate of birds breeding in towns, villages and forests. Their findings, published this year, indicate that nest predation is, if anything, higher in town centres, where magpies, crows, cats, rats and human disturbance take their toll (Ecography, vol 28, p 59). And in Seattle, predation by cats and rats seems to be the major reason that most birds nesting in the smallest urban spaces raise no chicks at all.

“Animals can be seduced by a man-made habitat that looks good but has a hidden downside”

Such findings highlight the fact that if we truly want wild species to feel at home in our cities, we need to understand them better. Without more long-term ecological research our efforts to encourage urban wildlife will be futile: green streets and gardens for us; ecological traps for the animals. But armed with improved knowledge, it should be possible to introduce policies that can satisfy the public’s new-found taste for watching wildlife without moving outside the city limits.