How Weeds Take to Cities

3 March 2008 (All day)

Courtesy of Gilles Przetak

Urban adjustment.
By producing more heavy seeds (inset, top), this weed makes the best of a hard situation.

Speedy evolution may be the key to surviving city life, at least for weeds. In less than 12 years, a Mediterranean weed has adjusted its reproductive strategy to deal with the challenge of concrete, researchers say. The findings could help scientists understand how species evolve in fragmented habitats.

Like other members of its family, Crepis sancta produces two types of seeds. Heavy seeds fall into the grass below the plant, whereas lighter seeds with feathery tails drift in the wind to new habitats. Ecologists have long known that plants in patchy habitats, such as islands, for example, produce more heavy seeds than light seeds, presumably because wind-swept seeds tend to get lost in the ocean. But controlling for environmental changes has been difficult.

A good study system turned up in Pierre-Olivier Cheptou's backyard. The population ecologist at CNRS, the French national research agency in Montpellier, France, and colleagues realized that they could do experiments on the Crepis sancta that invaded grassy patches around trees planted along the sidewalks in the city. After all, it stands to reason that a seed that lands on concrete would be as doomed as a seed that falls in the ocean; thus, it would be more advantageous for city plants to produce heavy seeds. So the team designed experiments to test whether the city environment had spurred evolution.

The fate of the city seeds clearly depended on their type. By counting the various kinds of seeds on the plants and then counting the seeds that fell on a sticky surface just below the plant, the team found that heavy seeds always landed on ground beneath the plant whereas light seeds had only a 45% chance of sticking around. Then, the researchers took seeds from city weeds and countryside weeds and grew them under similar conditions in the lab, comparing heavy seeds to light seeds. Whereas the urban dwellers yielded roughly 15% heavy seeds, the country plants produced closer to 10%. Using genetic models, the researchers confirmed that the changing pattern of seed dispersal took place in the past 12 years, about the time that the sidewalks in the city were constructed. "I was surprised that evolution can go so fast," Cheptou says. The next step is to follow the city plants to see if the proportion of light seeds decreases as the plants continue to evolve.

Cheptou says the findings suggest that deforestation and other human changes to the environment could lead to reduced dispersal among plants in fragmented habitats, further isolating them from other members of their species and therefore limiting the exchange of genes. Although this isn't necessarily bad for the plants, it is important information for conservationists developing habitat-management plans.

Ecologist Joshua Tewksbury of the University of Washington, Seattle, says that the study is a "great example of strong selection." However, he says that the city situation is also rather unique because of the size of the grassy patch and the concrete surface. "You may not see this level of dramatic change in natural systems," he adds.

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