More than 77 million people live in the Southeast, a part of the United States that began to grow rapidly in the latter half of the 20th century, thanks in no small part of the rise of air conditioning and automobiles (and some of the trends we discussed here).

Development there looks notably different from urbanized parts of the Northeast that were built in a very different era (think Boston, Philadelphia). Southern cities and their suburbs tend to be lower-density and car-dependent, with more large-lot, single-family homes. Communities often leap-frog across the countryside, creating a kind of patchwork development that consumes a lot of land. And amid all of these patterns, the region's rate of population growth over the last six decades has been about 40 percent greater than the rest of the country.

This recent history raises some unsettling questions for conservationists and urban planners about the future: If some of the fastest-growing parts of the country are also among the most sprawling, what will the Southeast look like a few decades from now? And what will that future mean for natural habitats, climate change and the people who live there?

In a provocative illustration in the online journal PLOS ONE, researchers with North Carolina State University and the U.S. Geological Survey have modeled some answers to at least the first question. From their work, this is what urban land cover in the region looked like in 2009:

"The Southern Megalopolis: Using the Past to Predict the Future of Urban Sprawl in the Southeast U.S" by A. Terando et al. in PLOS ONE

They then simulated how the picture might appear in 2060, if we assume a "business as usual" scenario in which future growth in the Southeast continues to look a lot like the recent past, with the same policies, living preferences and growth rates:

A. Terando

In those scenarios, land used for urban (and suburban) purposes would increase by anywhere from 101 percent to 192 percent over the next 50 years. And a completely connected megalopolis would stretch from Raleigh all the way to Atlanta:

A. Terando

Without "significant changes to the status quo," Adam Terando and co-authors write, we can expect the same type of growth to continue into the future, creating a picture like this one. Why might that be cause for concern?

Urban sprawl increases the connectivity among urban habitats while simultaneously fragmenting non-urban habitats such as forests and grasslands. These changes have a variety of effects on species and ecosystems, including impacts to water pollution, disturbance dynamics, local climate, and predator-prey relationships [2][5]. Urban sprawl will also, almost certainly, influence the ability of species to respond to climate change, in as much as it creates barriers to the movement of species that cannot survive in cities and corridors for those who can.

That's the ecological argument against the status quo. There are others, as well: Low-density living can come with higher energy and transportation costs for local residents, not to mention higher societal costs in building utilities and roads. It can consume rural land otherwise devoted to farming (or to people who prize rural settings). Sprawling development may also have economic consequences, for instance for social mobility.

The researchers don't sound too optimistic about our chances of getting off this course:

Given that urbanization has many consequences for how both humans and other species live, optimizing such growth could become a key national and regional priority, where optimization includes providing for biodiversity as well as economic development and cultural desires. However, history suggests humans, in contrast to ants and slime molds (e.g. [33]), rarely optimize growth, particularly when multiple objectives such as profit, equity, and ecological integrity come into conflict.