There Goes the Neighborhood

Flying cross-country on a clear day forces you to notice, right from your plane window, how profoundly humans have reshaped the American landscape. From the East Coast megalopolises to the Midwest’s giant circles and squares of irrigated fields to southern California’s sprawling suburbia, pristine American wilderness is now a rare exception to the rule.

Way back in 1997, ecologist Peter Vitousek estimated that humankind had modified roughly 50 percent of the planet’s land, and that number has certainly risen since then. This level of alteration is bound to have consequences, and a study published yesterday in the journal Nature suggests they are extreme. According to the authors’ models, human land use changes have diminished the total number of organisms living on earth by more than 10 percent over the past 500 years, and the number of species by 13.6 percent.

For the study, Tim Newbold of the United Nations Environment Programme World Conservation Monitoring Centre and his colleagues reviewed hundreds of land use studies, breaking the planet into very small units, each about the size of a football field. The granularity of their data enabled them to make findings that previous studies have not. Urbanization, for example, tends to cause the greatest loss in total individual organisms living on a plot of land. In terms of the change in the variety of species living on a plot, however, turning wild land into farms or pastures may be even more devastating than building condominiums. That finding may not impress hardened urbanites, who are accustomed to encountering a menagerie in the subway system, but it may surprise people who think of farmland as God’s country. Clearing a plot of land, planting a single crop, then spraying chemicals to kill any other life forms is a rather effective destroyer of biodiversity.

The United States has experienced some of the world’s most profound changes in species richness. Over the past few centuries, biodiversity has decreased over much of the United States by nearly a third. The most obvious losses have been extinctions, like the disappearances of the California grizzly, Merriam’s elk, passenger pigeon, or the eastern cougar. But a significant portion of the decline is due to the local disappearance of species from specific areas. This type of biodiversity loss, while less dramatic and permanent than total extinction, is sadly overlooked in conservation literature. Healthy ecosystems require a diversity of flora and fauna, and local declines portend global losses.

Land use, in general, is arguably not given the attention it deserves among environmentalists. University of Minnesota ecologist Jonathan Foley wrote in 2009, “I worry about this collective fixation on global warming as the mother of all environmental problems…I fear we are neglecting another, equally inconvenient truth: that we now face a global crisis in land use and agriculture that could undermine the health, security, and sustainability of our civilization” [emphasis in original]. Foley points out that worldwide agricultural land is approximately 60 times larger than all cities and suburbs combined.

Why the land-use crisis gets so little publicity is not entirely clear, but I suspect scientists haven’t done a great job of expressing the consequences. Climatologists can estimate in fairly precise terms how much a given quantity of carbon emissions will affect the climate. We also know what a ton of chlorofluorocarbons can do to the ozone layer. But it’s much harder to say what will happen to species when we convert a few acres of secondary forest into a wheat farm or a parking lot. You can’t pin the loss of a species on any single land-use change, so biodiversity is dying a death by a thousand cuts.

Hopefully Newbold’s study will go some way toward improving how we develop—or rather, don’t develop—land. The research, however, suggests things are only going to get worse. In the next century, biodiversity will decline by an average of 3.4 percent across the earth. It may not be the mother of all environmental problems, but it shouldn’t be the neglected middle child either.