Ours is an increasingly urban nation – over 80 percent of the U.S. population now dwells in cities and towns, a figure that’s only rising. Nowhere is that trend more pronounced than in the West: Phoenix, Salt Lake City and Denver are among the country’s fastest-growing cities. Our metropolitan migration has environmentalists and planners dreaming of efficient high-density housing and a public transportation revolution. But before we wholeheartedly embrace urban living, it’s worth asking what the growth of American cities means for our wildlife.
Two recent studies approach that question in very different ways. The first, published this month in the journal PLOS ONE, suggests that cities could be hazardous to the health of animals dwelling in and around urban areas. To arrive at that conclusion, researchers at Arizona State University trapped house finches – small, crimson-breasted birds native to southwestern deserts – across a range of habitats in the Phoenix area, from the heart of downtown to a park twenty miles outside the city, and examined them for parasites and a disease called avian poxvirus. What they found was troubling: Finches that lived in areas with higher human populations and greater habitat disturbance were more likely to be infected.
What explains the urban illness? One possibility, says Kevin McGraw, the study’s senior author, is that bird feeders encourage high population densities and unhygienic conditions that lead to infection. “When you’re around many other members of your species, you’re more likely to get infected,” explains McGraw. “Dirty birdfeeders themselves might also be a route of transmission.” The pollutants, pesticides, and junk food that urban birds encounter may also make the creatures more susceptible to illness by putting stress on their bodies. “Anytime you’re stressed, you could be immunologically compromised,” McGraw says.
Madhusudan Katti, an ecologist at California State University, Fresno who has studied birds throughout the Southwest, proposes a fourth hypothesis. “Because food is readily available in cities, birds that might die in the wild survive,” says Katti. “You can cause an unhealthy population to persist.”
Katti’s own research suggests that the relationship between birds and cities is immensely complicated. To wit: Abert’s towhee is a large sparrow, native to the Sonoran Desert, which ordinarily sticks to riparian corridors. In Phoenix, however, where manmade features like sewage ponds and groundwater recharge basins provide watery habitat, Katti found that the towhee sometimes strays miles from stream channels. “Desert birds take advantage of any water, anywhere,” he says. “Cities create urban oases, which bring in a lot of opportunistic species.”
But those opportunists don’t always flourish. Cities and other human-altered habitats can become ecological traps that lure wildlife with the promise of easy food and shelter, and then suppress reproduction and survival. After all, cities, with their cats and collision-inducing windows, are perilous places for birds, and our well-intended aid may not help. “We attract them with bird feeders, but I wonder about the nutritional value of the food we’re giving them,” says McGraw. “They’re eating the equivalent of bird candy.”
The task for planners and conservationists, then, is to ensure that cities function less like traps and more like oases. Another study, published in Proceedings of the Royal Society B last week by researchers affiliated with U.C. Santa Barbara's National Center for Ecological Analysis and Synthesis (NCEAS), offers a partial solution to that challenge. The paper, a sweeping analysis of urban plant and bird biodiversity in 147 global cities (including some in the American West like Los Angeles, Tucson and Seattle), found that cities retain a surprisingly rich suite of native species. Sure, urban areas can’t match the diversity of undeveloped landscapes, but they’re not homogenous wastelands of pigeons and starlings, either: the median city contained 112.5 bird species, only 3.5 of which were exotic. (If you don’t think 109 native birds is impressive, try naming 50 in your hometown, and your neighbor’s cockatiel doesn’t count.) “Most people think of cities as concrete jungles that aren’t important for conservation, but they actually support 20 percent of bird species worldwide,” says Myla Aronson, a Rutgers University biologist and lead author.The NCEAS study also demonstrates that if an urban area’s vegetative cover increases, so does its species density. “That’s a message to urban planners about the value of green space,” says Frank Davis, director of NCEAS. “What you plant, the way you design streamside corridors – there are ways to make urban areas more friendly to biodiversity.”
To be sure, increased green space isn’t a panacea for wildlife health: Although habitat loss has been associated with the spread of animal disease elsewhere in the world, an extra park or two likely won’t prevent birds from crowding around dirty feeders stocked with non-nutritious seed. The presence or absence of high-quality green space habitat, says McGraw, still lies within the “big black box of stress” — meaning it’s a variable that could compromise birds’ immune systems, but, compared to other factors like feeder hygiene, is so complex that it’s hard to directly connect to animal health. “We still don’t understand a great deal about disease dynamics in urban areas,” adds Katti (who’s also one of the authors of the NCEAS study).
When it comes to retaining native species, however, creating green space is still the best tool urban planners have. Los Angeles’ Debs Park, site of a new Audubon nature center, hosts over 140 different avian species, Phoenix is using a U.S. Fish and Wildlife Service grant to create nesting habitat, and surveys on the green space-rich Berkeley campus suggest that avian diversity hasn’t declined in the last 75 years. “We go to cities and appreciate the language and culture, but there’s another side, and that’s their nature,” says NCEAS’ Davis. “It may be diminished biodiversity, but it’s still native biodiversity, and we can’t give up on trying to restore it.”
Ben Goldfarb is an editorial intern at High Country News. He tweets @bengoldfarb13.