The ability of invasive plants to rapidly adapt to local climates -- and potentially to climate change -- may be a key factor in how quickly they spread.

According to new research published in Science by UBC evolutionary ecologist Rob Colautti, it is rapid evolution -- as much as resistance to local pests -- that has helped purple loosestrife to invade, and thrive in, northern Ontario.

"The ability of invasive species to rapidly adapt to local climate has not generally been considered to be an important factor affecting spread," says Colautti, an NSERC Banting Postdoctoral Fellow with the UBC department of Botany who started the research in 2007 as a PhD student at the University of Toronto.

"Instead, factors such as escape from natural enemies including herbivores, predators, pathogens or parasites were thought to explain how species become invasive," says Colautti.

"We found that the evolution of local adaptation to climate in purple loosestrife increased reproduction as much as or more than escaping natural enemies. Understanding that species can evolve rapidly to local climates is important for predicting how invasive species spread and how native and non-native species alike will respond to climate change."

To determine whether populations have evolved local adaptation, Colautti and University of Toronto professor Spencer Barrett collected seeds from three different climatic regions and grew them at three sites spanning the distribution of the species in eastern North America.

They found that 'home' plants collected from latitudes most similar to each common garden location always had higher fitness than the 'away' plants. For example, plants collected from northern latitudes had the highest fitness when grown at the northern site in Timmins, Ontario but the lowest fitness when grown at a southern site in northern Virginia relative to plants collected from southern latitudes.

The researchers then directly measured Darwinian natural selection on flowering time at each of the common garden sites. They found that early flowering was adaptive at the most northern site because early-flowering plants produced the most offspring while plants with delayed flowering began reproduction near the end of the growing season, when pollinators were scarce and flowers were prone to frost damage.

But later flowering was favoured by natural selection at more southern sites because delayed reproduction allowed plants to grow larger and produce more seeds when the growing season is longer. The differences have evolved over the past 50 years as the species moved northwards, following its initial introduction to the east coast of the USA.