Practice Test 12: Urban Wildlife

A Until fairly recently, the word ‘wildlife’ conjured images of remote forests, mountain ranges and untouched wilderness far from human settlement. Today, however, a very different picture is emerging. Red foxes trot along London pavements after dark, raccoons raid rubbish bins in Toronto’s suburbs, and coyotes have been spotted crossing car parks in downtown Chicago. In Mumbai, leopards patrol the fringes of slum housing, while in Berlin, wild boar dig up flowerbeds in public parks. Scientists estimate that hundreds of species once confined to rural habitats now make at least part of their living inside city limits. This is not simply a matter of animals passing through; increasingly, urban environments are becoming permanent, self-sustaining habitats in their own right, complete with breeding populations that rarely, if ever, venture beyond the city boundary.

B Why should a fox or a raccoon choose to live among concrete and traffic rather than in a forest or field? The answer, in large part, comes down to resources. Cities generate a staggering quantity of discarded food, from overflowing bins outside restaurants to fallen fruit in gardens, providing a reliable and calorie-rich diet that requires far less effort to obtain than hunting or foraging in the wild. Urban areas are also considerably warmer than the surrounding countryside, an effect known as the ‘urban heat island’, which allows animals to expend less energy on staying warm during winter months. Predators, too, are generally scarcer within city limits; a fox that might once have been killed by a wolf or a larger predator in open countryside faces comparatively little such danger on a suburban street. Taken together, these factors make the city, for many species, a surprisingly hospitable place.

C Moving into the city, however, requires more than simply tolerating noise and traffic; it demands genuine behavioural flexibility. Many urban animals have shifted their activity patterns to become more strictly nocturnal, even in species that are naturally active during the day in rural settings, presumably to avoid the peak hours of human activity. Others have grown noticeably bolder: urban-dwelling birds and mammals alike tend to flee from an approaching human at a much shorter distance than their rural counterparts, a trait researchers refer to as reduced ‘flight initiation distance’. City-dwelling animals have also become adept at exploiting human infrastructure for purposes it was never designed for, turning storm drains into den sites, roof cavities into nesting sites, and railway embankments into a means of moving unseen across a city. Such behavioural adjustments, made within a single generation, illustrate just how adaptable certain species can be.

D Perhaps the most extensively studied urban adapter is the red fox. A long-term comparison of fox skulls held in museum collections, some collected in London and others from rural parts of England, revealed a striking pattern. Urban foxes, the researchers found, tend to possess shorter, wider snouts than their rural relatives, a shape better suited to feeding from bins than to the swift, precision bites required to catch small prey. The same study also detected differences in brain case size relative to body size between the two populations. While the authors were cautious about drawing firm conclusions on intelligence, they suggested the changes were consistent with a population gradually adapting across generations, rather than individual foxes simply learning new tricks within their own lifetimes.

E Life in the city is not without its dangers, of course. Roads present a constant hazard, and collisions with vehicles are now among the leading causes of death for many urban mammals, including foxes and hedgehogs. Rubbish, while a convenient food source, can also introduce animals to plastics, poisons and discarded medication, with sometimes fatal consequences. Disease transmission is another concern: dense urban animal populations, living in close proximity to one another and to humans, can allow pathogens to spread more readily than they might in a sparser rural population. Conflict with people presents a further complication, ranging from minor nuisance complaints about noise or damaged gardens to more serious concerns raised where larger predators, such as coyotes or leopards, come into contact with pets or, occasionally, people themselves.

F For biologists, the city has become something unexpected: a living laboratory in which evolution can be observed unfolding in real time. Because urban and rural populations of the same species are often separated by just a few kilometres, yet experience such different pressures, researchers can compare the two directly, without waiting centuries for change to accumulate. What is emerging from this research is a picture of cities not as sterile, wildlife-free zones, but as dynamic new ecosystems, shaping the animals that inhabit them just as surely as any forest or grassland once did. Understanding these changes, many argue, will prove essential if humans and wildlife are to share increasingly crowded urban spaces successfully in the decades ahead.

Charles Darwin imagined evolution as a process so gradual that its effects could only be perceived across many thousands of years. Yet a growing body of research suggests that, within cities, natural selection can act with startling speed, sometimes producing measurable change within a few decades or even a handful of generations. Because urban environments differ so sharply from the rural or wild habitats many species evolved in originally, city-dwelling populations face intense new pressures, from traffic and artificial light to unfamiliar building materials and altered food supplies. For evolutionary biologists, this presents an unusual opportunity: rather than reconstructing change from fossils laid down over millennia, they can observe adaptation unfolding almost in real time.

One of the clearest examples comes from a decades-long study of cliff swallows nesting beneath highway overpasses in Nebraska. Researchers led by biologist Charles Brown began collecting swallows killed by passing vehicles along a stretch of road in the 1980s, and continued doing so for more than thirty years. Over that period, even as traffic volumes rose considerably, the number of swallows killed on the road fell sharply. When Brown’s team measured the wingspan of the birds killed by vehicles and compared it with the wingspan of the wider living population, they discovered that the roadkill birds consistently had longer wings than average. Longer wings, it appears, make it harder for a swallow to execute the tight, rapid turns needed to dodge an oncoming vehicle, meaning that birds with shorter, more manoeuvrable wings were more likely to survive and breed.

Traffic does not only pose a direct physical danger; it can also interfere with communication between animals. Many birds rely on song to attract mates and defend territory, but low-frequency traffic noise can mask these calls in busy urban areas. Studies of great tits in European cities have found that birds nesting in noisier locations tend to sing at a higher pitch and with greater volume than birds of the same species in quieter woodland, a shift that allows their song to be heard above the low rumble of traffic. Similar patterns have since been documented in a range of other species. Whether such changes reflect genuine evolutionary adaptation passed down through generations, or simply the ability of individual birds to adjust their singing according to their immediate surroundings, remains a subject of active investigation.

Adaptation to city life is not confined to birds. Research on Anolis lizards living in Caribbean and Latin American cities has revealed physical changes linked to the urban environment. Compared with lizards living in forests, where they typically climb rough tree bark, urban lizards living on smooth surfaces such as concrete walls, glass and metal railings tend to have relatively longer limbs and toe pads equipped with a greater number of adhesive scales. Biologist Kristin Winchell, who has studied these populations extensively, has suggested that such features improve grip on artificial surfaces that offer far less texture than natural bark. Laboratory tests support this interpretation, showing that lizards with these urban-associated traits are able to cling to smooth surfaces considerably more effectively than lizards lacking them, a clear advantage in an environment built largely from glass, metal and painted concrete.

A central challenge for researchers studying urban wildlife is determining whether an observed difference reflects genuine evolutionary change, meaning an inherited alteration passed from parent to offspring, or merely developmental plasticity, in which an individual’s traits are shaped directly by its environment without any underlying genetic change. To distinguish between the two, scientists frequently use what is known as a common garden experiment, raising offspring from both urban and rural populations under identical conditions and comparing the results. If differences between the two groups persist even when both are raised in the same environment, this is generally taken as evidence that the trait has a genetic basis rather than being a simple response to local conditions.

Some of the most compelling evidence for genuine evolutionary change in cities comes from an unlikely source: white clover, a common lawn weed. Many clover plants produce a chemical defence, releasing small amounts of cyanide when their leaves are damaged by grazing animals, a trait known as cyanogenesis. A large international research project, coordinating researchers in more than 160 cities worldwide, found that clover growing in city centres was consistently less likely to produce this cyanide-based defence than clover growing on the outskirts of the same cities. The likely explanation is that herbivorous animals, such as rabbits and slugs, are less abundant in city centres, reducing the evolutionary pressure to maintain a costly chemical defence. Remarkably, this pattern of reduced cyanogenesis in urban centres was found to repeat itself independently across cities on different continents, a phenomenon researchers describe as convergent evolution.

Taken together, these findings are prompting a broader rethink of how quickly evolution can proceed under the right conditions. Rather than a force that operates imperceptibly over geological time, natural selection in cities appears capable of reshaping populations within a human lifetime. For conservationists, this raises important and largely unresolved questions about how urban evolution might affect a species’ long-term resilience, and whether traits favoured in the city could prove disadvantageous should a population ever need to return to a more natural habitat.

As evidence for wildlife’s remarkable capacity to adapt to city life continues to accumulate, planners, architects and conservationists are increasingly asking a different question. Rather than treating the city and the natural world as separate, competing domains, how might urban areas themselves be deliberately designed to support biodiversity, rather than merely tolerating whatever wildlife happens to move in uninvited? This shift in thinking, still in its early stages in most parts of the world, has given rise to a wide range of experiments, from small-scale changes to individual buildings through to ambitious city-wide planning strategies, each attempting to reconcile the needs of a growing human population with those of the species sharing its space.

Rethinking Urban Green Space
Traditional parks, with their mown lawns and ornamental flowerbeds, offer comparatively little of value to most wildlife. In response, a growing number of cities have begun investing in what is often termed ‘green infrastructure’: rooftop gardens, vertical planting on building facades, and public spaces planted with native species chosen specifically for their value to local insects and birds, rather than for visual appeal alone. Singapore, frequently cited as a leader in this field, has pursued a long-term strategy to transform itself, in the words of one planning document, from a ‘garden city’ into a ‘city in a garden’, weaving greenery through and around its buildings rather than confining it to designated parks. Elsewhere, smaller-scale interventions such as rain gardens, which capture and filter rainwater while providing habitat for insects, and ‘pocket parks’ created on vacant urban lots, aim to bring similar benefits to cities with fewer resources for large-scale redevelopment.

Corridors and Crossings
Even where green spaces exist within a city, they are often isolated from one another, separated by roads, car parks and buildings that many species are reluctant, or simply unable, to cross. This fragmentation can leave small populations of animals cut off from one another, unable to find new mates or replenish their numbers should local conditions deteriorate. Wildlife corridors, strips of vegetation deliberately planted to connect one green space to another, aim to address this problem by giving animals a relatively safe route between habitats. In places where a busy road cannot easily be avoided, some cities have gone a step further, constructing dedicated wildlife crossings, vegetated bridges or tunnels built specifically to allow animals to pass beneath or over traffic. Although expensive, such structures have been shown to significantly reduce both roadkill and the genetic isolation of divided animal populations.

The Rewilding Debate
A more contested approach involves deliberately reintroducing species, sometimes including larger or more demanding animals, to urban and semi-urban waterways and green spaces. Beavers, for example, have been reintroduced to a number of rivers running through or near cities in recent years, valued for their ability to build dams that slow the flow of water, reduce downstream flooding and create wetland habitat for numerous other species. Such projects are not without controversy, however. Landowners and residents have sometimes objected to beaver dams flooding agricultural land or gardens, while the reintroduction of larger predators closer to urban areas raises understandable concerns about safety, both real and perceived. Advocates argue that the ecological benefits, from flood control to increased biodiversity, generally outweigh the drawbacks, provided reintroductions are carefully managed and local communities are consulted from the outset, though not everyone affected by such projects has been persuaded.

Balancing Competing Interests
Even without deliberate reintroductions, the presence of adaptable wildlife in cities inevitably generates friction. Residents may welcome an occasional glimpse of a fox in the garden, yet object strongly when the same animal damages property, raids bins or is perceived as a threat to pets or small children. Local authorities, caught between residents demanding action and conservationists urging restraint, must navigate these competing pressures carefully. In several cities, non-lethal management strategies, including secure bin design, targeted public information campaigns and, in more serious cases, humane relocation, have been favoured over the culling programmes once considered standard practice. Public education plays a considerable role in these efforts; research suggests that residents who understand why an animal behaves as it does are considerably more tolerant of its presence than those who regard it simply as a nuisance to be eliminated.

Looking Ahead
For many working in this field, the ultimate goal is to move wildlife considerations from an afterthought, addressed only once a building or development is complete, to a factor built into planning decisions from the very beginning. This might mean requiring new developments to include green roofs, incorporating wildlife crossings into road design from the outset, or setting aside a minimum proportion of any development for habitat, rather than retrofitting such features later at greater expense. Whether such measures become standard practice or remain isolated examples of good intentions will depend largely on political will, public support, and continued evidence that cities designed with wildlife in mind are, in the end, more pleasant places for their human residents too.