Two thousand coyotes are estimated to live in the Chicago metropolitan region. Three thousand wild boars roam the parks of Berlin. The underbelly of the Congress Avenue Bridge in Austin, Texas, is home to approximately 1.5 million bats—almost twice the urban population.
Nature and culture are often contrasted with one another, yet the cities of the world are home to so many different animal species—pigeons, raccoons, skunks, squirrels, mice, rats, rabbits, foxes, opossums, armadillos, hawks, seagulls, ducks, geese, hedgehogs, deer, bobcats, snakes, monkeys, baboons, reptiles, birds and insects of all kinds—that cities are more appropriately understood as continuous with the wild.
The field of urban ecology is young, relatively speaking. As Tristan Donovan writes in his book Feral Cities, “Cities are possibly the most exciting, most surprising, and least understood ecosystems on the planet. They are places where much of what we think we know about the natural world doesn’t apply.”
Cities exert such a strong force on animals that they have visibly affected their behavior. The great tit, for example, sings at higher frequencies than it does in rural areas in order to compete with low frequency noise pollution (i.e., cars). Urbanized peregrine falcons are becoming nocturnal, using streetlights to spot prey. Some of Moscow’s estimated thirty-five thousand feral dogs have learned to ride the subway and obey traffic lights. Seals and porpoises cruise the waterways of London. Old deer look both ways before crossing the street.
The Urban Wildlife Working Group categorizes urban wildlife into four main types, based on “the degree to which urban wildlife benefits from or is harmed by anthropogenic habitat change”:
- Human obligates: Consisting largely of domestic animals and urban livestock, obligates “compete with, disturb, and most importantly, predate upon native species… Domestic cats in particular are known for their impressive predatory skills and their impacts on native and migratory bird species.” They are a deliberately included form of wildlife.
- Human associates and exploiters: Often generalists when it comes to eating, associates and exploiters take advantage of such human-based food sources as gardens, garbage, domestic animals, and pet food. They are able to achieve higher numbers in the city due to the prevalence of available food and/or the release from predation. Opinions on this group are wide-ranging: “songbirds that use backyard feeders are often regarded positively, whereas predators that kill pets are likely to have negative associations. Property damage and disease transmission can also generate negative attitudes toward certain exploiters, including raccoons.” Pigeons, mice and squirrels fall under this category as well.
- Human adapters: While adapters may survive in or around urban areas with little trouble, even making use of human resources, they do not receive any added benefit from the arrangement. “These species are often located on the periphery of development… Deer are sometimes regarded as human adapters, as they can achieve high population sizes from wild areas to suburban habitats.” Red foxes, skunks, coyotes and bobcats could be considered adapters.
- Human avoiders: Avoiders occasionally find themselves in human contexts when attempting to migrate or disperse. With specific habitat requirements for reproduction or foraging (unlike the earlier groups) these species can experience high mortality rates in human environments. They also have a history of conflict with humans. “Mountain lions, for example, are human avoiders, but occasionally come into conflict with human communities by eating livestock or pets.” Grey wolves, grey foxes and pileated woodpeckers are human avoiders.
Sharing the city with hosts of animals, we live in a complex, multi-species nest. As cities continue to grow in size and encroach upon the surrounding regions—nearly two-thirds of the population are expected to live in cities by 2050—the field of urban ecology grows alongside.