Seed Keepers

["Lawrence F. London, Jr." <lflj@intrex.net>]

<http://news.bbc.co.uk/2/hi/science/nature/7277057.stm>
Last Updated: Wednesday, 5 March 2008, 20:14 GMT
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Plant seeds 'adapt to city life'
By Mark Kinver
Science and nature reporter, BBC News

Weed non-dispersing (top) and dispersing (bottom) seed (Image: Eric Imbert)
Heavier seeds had a better chance of germinating in urban areas
A species of plant found in cities has evolved rapidly in order to adapt to the challenges of surviving in the concrete 
jungle, a study suggests.

Crepis sancta growing in urban areas produces heavy seeds that fall to the ground rather than lighter seeds that are 
dispersed by the wind.

Wind-blown seeds are less likely to germinate because most end up on concrete 
surfaces, scientists say.

The findings appear in Proceedings of the National Academy of Sciences.

Researchers from the Centre d'Ecologie Fonctionelle et Evolutive (CEFE), based in Montpellier, estimate that the change 
in the way the plant disperses its seeds has taken place in as little as five to 12 generations (five to 12 years).

Co-author Pierre-Oliver Cheptou said the team was surprised by the speed of 
the change.

"Logic would assume that this sort of evolutionary trait would develop more slowly, which is probably the case in less 
fragmented situations," he told BBC News.

"However, at the same time, it is consistent with the estimation we had for the 
trait in a fragmented urban situation."

Risky business

The team collected seed samples from the weed, which grows on wasteland or next to roadside trees, from various 
locations around the French city.

        
Crepis sancta (Image: Gilles Przetak)
In fragmented situations, the evolution towards lower dispersal leads to more isolated populations; increasing the risk 
of extinction
Dr Pierre-Olivier Cheptou,
Report co-author

They then grew them in a greenhouse to see what fraction of the resulting plants' seeds were of the light, 
wind-dispersible variety.

Compared with specimens taken from the countryside, the urban samples 
produced far fewer of these seeds.

The researchers said "dispersing" seeds had a 55% lower chance of germinating because the majority ended up on concrete 
surfaces.

The heavier seeds were at an evolutionary advantage because they would fall down into the patch of soil that had 
supported the previous generation of the plant.

They added that their findings supported "cost of dispersal" theories.

"When dispersal is passive (wind or water transport) and habitat choice is random, the probability of settling in a 
suitable site is positively dependent upon the frequency of suitable sites in the landscape," they wrote.

"Many empirical studies have reported a reduction in dispersal structures in organisms that live on islands, such as 
plants or insects."

Dr Cheptou said that their study showed the same also applied to plants in urban areas, where suitable soil was widely 
fragmented by buildings, pavements and roads.

He explained that such a strategy, while increasing the odds of survival for 
the next generation, could have drawbacks.

"We can hypothesise that in fragmented situations, the evolution towards lower dispersal leads to more isolated 
populations, increasing the risk of extinction."
                
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