Nature, wrote Henry David Thoreau a century and a half ago, 'flourishes most alone, far from the towns where [people] reside.'

But was he right? The long-standing assumption that cities and nature are fundamentally incompatible is giving way to the idea that nature encompasses all things, including cities and the people in them. Ecologists traditionally have studied wilderness areas, old-growth forests, and other 'untarnished' landscapes. In recent years, however, more than a few have joined with geographers and other scientists to examine the urban jungle instead. They're now looking at cities as ecosystems in their own right, with their own air chemistry, energy flow, species mix--even their own weather.

To do so, ecologists have had to change old models of, say, energy flow to incorporate the effects of streets, buildings, canals, and other human modifications of the landscape, which can have a profound effect on the environment. L. Helmuth reports in Science News (March 27, 1999), for example, that atmospheric scientists are now studying the fact that cities generate their own weather systems, a finding that is 'fueling a growing area of meteorology--weather prediction for urban microenvironments.'

As a case study, Helmuth writes, climate specialists are now closely tracking weather data for Atlanta. Metropolitan Atlanta has grown from a population of about 1.7 million to about 3 million in the past 25 years, swallowing up what had been 380,000 acres of forest. One result is an urban 'heat island,' a permanent low-pressure system that produces its own thunderstorms and weather fronts.

The presence or absence of forested areas influences more than the weather. Garry Hamilton, writing in New Scientist (March 20, 1999), observes that different combinations of trees can affect the energy efficiency of buildings, making structures easier to cool and heat. Some trees, such as oaks, help produce ozone, which can temper heat emissions from power sources, whereas others help filter particulate pollutants. Groves of trees, depending on how they are situated with respect to buildings and open spaces, can also affect wind speed, temperature, humidity, and other microclimatic variables. Conversely, the absence of trees can negatively affect the quality of a city's water supply by contributing to increased sedimentation as the soil once held in place by tree roots is washed into storm sewers that muddy reservoirs and streams.

Baltimore and Phoenix are now the subject of federally funded, long-term ecological studies. Hamilton calls the Baltimore Ecosystem Study, involving more than two dozen scientists and a $1 million annual budget, 'one of the U.S. government's most radical large-scale expeditions into the field of ecology.' Planned to extend over the next several decades, the Baltimore study will examine the traditional stuff of ecological research--hydrological systems, climate, nutrient cycles, energy transfers, speciation, predator-prey relationships--but in an urban setting. Says project manager Stewart Pickett, an ecologist with the Institute of Ecosystem Studies in Millbrook, New York, 'I'm excited to find out how [nature] goes on in cities because before now, we just haven't looked. This is ecology's last frontier.'