Purple loosestrife: Climate may keep beautiful invasive plant in check

Purple loosestrife (Lythrum salicaria) destroys wildlife habitats by displacing native vegetation that provides food, shelter, and breeding areas for wildlife. In urban areas, it invades ditches where it can block or disrupt water flow. It has few pests and diseases, resists various control methods, and plants can produce as many as 3 million seeds a year.

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 Robert Colautti, who recently obtained his Ph.D. from the University of Toronto's Department of Ecology and Evolutionary Ecology. The results are published online this week in the Proceedings of the Royal Society of London, series B and featured 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 Colautti. "Smaller size results in lower seed production, which is likely to limit the spread of purple loosestrife in northern regions."

Co-authors of the study are Colautti's supervisor Professor Spencer Barrett of the Department of Ecology and Evolutionary Biology and Christopher Eckert of Queen's University. The research was funded by the Natural Sciences and Engineering Research Council of Canada, an Ontario Graduate Scholarship and a Premier's Discovery Award from the Ontario Government.