Scientists had long thought that the main reason some invasive species are so successful is that they typically have no natural predators in the environments where they aren’t native. This theory has been backed up by evidence from experiments comparing the rates of reproduction of invasive species exposed to predators and those who haven’t been exposed.
But biologist Rob Colautti and his colleagues have found that in the case of the invasive European plant purple loosestrife it was the plant's remarkable ability to evolve quickly to adapt to different climates that was "just as strong" as the lack of predators.
The results were published this week in Science.
Purple loosestrife is a wetland plant that was introduced to the East Coast of North America during the 19th century, likely hitching a ride in soil in the ballast water of European ships. Since then, it has spread as far south as Texas, as far north as northern Ontario and Newfoundland, and as far west as B.C.
According to the Ontario’s Invading Species Awareness program, purple loosestrife is a concern because it spreads quickly and grows in dense stands, reducing nutrients and space for native plants, and degrading habitat for wildlife.
Colautti, a post-doctoral researcher at the University of British Columbia, conducted an experiment during his Ph.D. at the University of Toronto in which he transplanted loosestrife plants from Texas to Timmins, Ont., and plants from northern Ontario to northern Virginia.
Compared to the transplanted southern plants, the local loosestrife in Timmins flowered 20 days earlier. That allowed them to take advantage of the short growing season and produce 40 times as many seeds.
Similarly, when the northern Ontario plants were grown in Virginia, they produced just a 10th of the seeds that local plants produced because they flowered very early, when they were very small.
The increase in reproductive rates linked to local adaptation was comparable to that seen in the absence of natural predators.
Colautti told CBCNews in an interview that purple loosestrife has spread far and wide mostly in the past 50 years, suggesting that it evolved changes in growth and flowering patterns to adapt to different climates within just a few decades — very, very quickly.
Multiple introductions boost evolutionary speed
He said there is some evidence that this rapid adaptation ability may be "fairly common" among invasive species, especially those that have spread over a large area.
In fact, the way such species were introduced to North America from other continents may have helped them gain their unusual evolutionary speed. Similar to the pattern seen in many invasive species, genetic analysis of purple loosestrife suggests it was introduced to North America multiple times from different parts of Europe and Asia, which would boost the amount of genetic variation in the North American plants.
"One prediction we might make is that species with higher genetic variation for those traits that are important for local adaptation should evolve faster and spread faster," Colautti said.
The findings of the study have a number of implications for controlling the spread of invasive species.
For one thing, Colautti said, it suggests that limiting the number of times an invasive species is introduced may help prevent it from gaining the genetic diversity needed to spread quickly.
It also suggests that strategies for controlling an invasive species should take into account different populations adapted to different climates, rather than just treating them as a single species.
For example, to control the spread of purple loosestrife, two European beetles that eat the plant’s leaves were introduced to North America by the U.S. and Canadian governments in 1992.
Strategies must target distinct populations
Colautti said to maximize their effectiveness, control programs that use beetles need to ensure the insects are feasting at the right time to damage seed production for a given population of loosestrife.
"I think there might be ways to improve them by taking into accounts those differences… among populations," Colautti said.
In theory, he added, the plants’ genetic diversity could even be used against them, by transplanting northern plants south and southern plants north. That would introduce "maladaptive" genes for flowering and growth rates that are "wrong” for the climate into local populations.
Another thing that the study’s findings suggest is that purple loosestrife will be very resilient to climate change, and that a warmer climate could help it expand its range even further.
"As bad as some of the climate predictions are," Colautti said, "the difference between Texas and Northern Ontario is much larger than the difference from current climate to future climate."
The research was funded by the Natural Sciences and Engineering Research Council of Canada.