Most people reflexively think cities don’t contain nature, but in fact, they are complex ecosystems that just happen to be shaped by different evolutionary forces than an untouched forest or pristine lake.

A set of 14 studies packaged together in the journal Biogeochemistry this month shows this in force, by examining the way cities are all over the United States–from Boston, Massachusetts to Tucson, Arizona- are shaped in the same consistent ways by the presence of modern urban societies.





It’s a fascinating look at the universal environmental footprint of urban infrastructure. And in an overview article, researchers from the University of Maryland and the University of New Hampshire point out that these ecosystems can quickly improve or quickly degrade depending on a how they’re managed.

Because of road salt and even dietary salt that is excreted in wastewater, the streams, lakes and land surfaces of a city environment become saltier over time. Some of the waste also leaks into groundwater from crumbling pipes. Tracking the chloride content of a city’s waters is a good way to gauge the spread of urbanization, the researchers believe.

Waterways in cities carry the chemical signature of dissolving concrete. When buildings are exposed to acid rain, the limestone weathers, forming surfaces that are similar to a kind of natural limestone called karst. Urban streams, therefore, contain more calcium and carbonate minerals from this urban karst, which alters their acidity and the kind of life they can sustain.





Road crossings and other urban features often create specific places where chemicals in the air and water–from car exhaust, road salt, sewage overflows, and other pollutants–are in highest concentration, especially after heavy rainstorms. Though in nature, a hotspot is usually a healthy thing–in cities, this might be less so.

The authors caution that efforts to restore city ecosystems to their formerly “natural” states may be fruitless against the inevitable tides of urban evolutionary patterns as seen across cities. They say, instead, city managers should work to better understand the complex factors and design sustainable restoration projects that create a healthy city habitat for humans, animals, and plants together.