The flowering plant purple loosestrife - Lythrum salicaria - has been heading north since it was first introduced to the eastern seaboard from Europe 150 years ago. This exotic invader chokes out native species and has dramatically altered wetland habitats in North America.

But as this invasive plant has spread north it has run into challenges posed by a shorter growing season, according to a study conducted by researchers from the University of Toronto's Department of Ecology and Evolutionary Ecology. Scientists have found that adapting to the Great White North carries a severe reproductive penalty that may limit its spread. The results are highlighted this week in Nature.  

The authors used modeling and experimental studies of 20 purple loosestrife populations along a 1200 km latitudinal gradient from Maryland to Timmins, Ontario, representing a one-month difference in growing season. They found that northern populations have become locally adapted and flower earlier in response to a shorter growing season. However, early flowering plants suffer a cost in terms of smaller size and reduced seed production. The reason: a genetic constraint.

"Genes that cause early flowering also reduce plant size, so early flowering and small size evolve together," says University of Toronto ecologist Robert Colautti. "Smaller size results in lower seed production, which is likely to limit the spread of purple loosestrife in northern regions."


Citation: Robert Colautti, 'Evolutionary biology: On the invasion front', February 2010, Nature 463, 1002; doi:10.1038/4631002e