Just how far will urban sprawl spread?

We have been hearing for a while now about the continued explosive growth of cities around the world. According to the World Health Organization, 40 percent of the global population lived in an urban area in 1990, we crossed the 50 percent threshold by 2010, and we’ll be at 60 percent by 2030 and 70 percent by 2050. That means more cities, bigger cities, and, importantly but often forgotten, more sprawl.

“Urban sprawl increases the connectivity among urban habitats while simultaneously fragmenting non-urban habitats such as forests and grasslands,” write researchers in a new PLoS One paper. “These changes have a variety of effects on species and ecosystems, including impacts to water pollution, disturbance dynamics, local climate, and predator-prey relationships.” The researchers, from the U.S. Geological Survey and North Carolina State University, modeled how urban sprawl—defined as low-density development outside the “urban core”—is going to change in the American Southeast over the next few decades to see how ecosystems in that area may change as well.

They found that sprawl is expanding in a big, big way. Overall, the simulations pointed to an expansion of “urbanization” in the Southeast by between 101 and 192 percent over the next 50 years. The median number was 139 percent, representing an overall expansion of 90,700 square kilometers (~35,000 square miles); that’s bigger than the entire area of one southeastern state, South Carolina.

The specific types of land that end up as urban sprawl, of course, matter greatly as to how the expansion affects habitats. The paper found that the largest conversion will be from agricultural land to urban uses, with up to 21 percent of farmland yielding to the cities’ tentacles. Grasslands (up to 17 percent) and forests (up to 12 percent) are next, followed by wetlands (up to 7 percent).

“The increasingly fragmented natural landscape would reduce habitat availability, suppress natural disturbance processes (such as wildfires), hinder management actions that come into conflict with urban areas, and likely eliminate existing corridors,” the authors write. “Furthermore, all these impacts could occur simultaneously, posing a particularly devastating threat to already vulnerable species and systems.” They note the endangered red-cockaded woodpecker as an example; it lives in longleaf pine forests, which will likely be among the forest casualties in this new urban wasteland.

And there are secondary effects to simply eliminating forests and grasslands where animals live. The urban heat island effect (and all its attendant causes, effects, and cause-effect mishmashes) will expand its reach, for example raising temperatures in the Piedmont region by between 2 and 6 degrees Celsius. And while sprawl destroys some wildlife corridors, it will open up other, weirder ones: the simulation suggests that some currently disconnected urban environments will expand enough to join forces. Creation of “urban corridors” could create novel habitats that allow totally different groups of species to flourish; one giant megalopolis will exist, unbroken, all the way from Raleigh, North Carolina, to Atlanta, Georgia, 400 miles distant.

Given the knowledge we have that urban sprawl is likely to expand rapidly, in the Southeast and in many other areas, one could argue that we are well prepared to adjust and ease back on the throttle. The researchers, in concluding, take a cynical view of that possibility: “History suggests humans, in contrast to ants and slime molds, rarely optimize growth, particularly when multiple objectives such as profit, equity, and ecological integrity come into conflict.” And since we aren’t quite as good at this as slime molds are, there is the distinct possibility that we should plan for the worst rather than assume we’ll fix the problem ahead of time. – Dave Levitan | August 5 2014

Source: Terando AJ, Costanza J, Belyea C, et al (2014). The southern megalopolis: Using the past to predict the future of urban sprawl in the Southeast US, PLoS One, 9 (7) e102261. DOI: 10.1371/journal.pone.0102261

Image: shutterstock.com, trekandshoot