Greening the City

Surviving in the urban jungle

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Ville et biodiversité

One of the goals of urban ecology is to understand how species colonize cities and adapt to urban ecosystems. “Cities are a fabulous laboratory for the study of biodiversity and evolution, because species are put under considerable pressure, as they come up against novel situations as to luminosity, food availability, predation, etc,” explains Philippe Clergeau, Professor at the French Muséum national d’histoire naturelle and a specialist in urban ecology. “However, this field of research is relatively recent. In France, the first research dates from around ten years ago.”
Clergeau runs the ECORURB program, which was launched in 2003 for a period of ten years, and brings together ecologists, meteorologists, geographers and sociologists who are carrying out a comparative study of sites in the city centers of Angers and Rennes and the surrounding countryside. The first findings in plant ecology show that the combustion of hydrocarbons increases the nitrogen content of soil in cities. It turns out that the availability of nitrogen for plant growth is 160% greater in the city center of Rennes than in the surrounding countryside. “This gives an advantage to nitrogen-loving plants such as nettles,” Clergeau says. In addition, the air in cities contains three times fewer seeds than the air in the surrounding countryside, because walls and buildings affect air currents. Meteorological research has also shown that there is a correlation between early blossoming of cherry trees and the warmer climate found in city centers. In downtown Rennes, blossoms appear a week early.
As for animal ecology, three groups have been studied: birds, small mammals and ground beetles. “The mobility of animals is the main factor that explains their distribution in cities. Bird communities, for instance, are richer in cities than in rural environments, since their dispersion capacity enables them to take advantage of the wide range of habitats,” says Clergeau, “whereas ground beetles, which are unable to fly long distances, are less diverse in cities.” This is also the case for small mammals, which also have to deal with cats, the urban jungle’s top predators.
There are other factors that explain why some species do better than others. “Generalists, which can make use of a variety of habitats, do very well,” Clergeau explains. “Species that can adapt their behavior to the specific nature of the urban environment are also at an advantage.” This is true of kestrels: in a rural environment, these small birds of prey feed on tiny mammals, which they watch for while hovering. But in the city, they eat practically nothing other than sparrows, which they hunt by lying in wait.

CNRS    sagascience