UW Today

February 1, 2011

Invader abundance not always explosive

News and Information

When invasive plants gain a foothold in new territory they become about as abundant as on their home turf.

The vast majority of the species pictured at this Whidbey Island site are invasives. They include nine of the 26 study species such as common dandelion and sweet vernal grass. Credit required: University of Washington

The vast majority of the species pictured at this Whidbey Island site are invasives. They include nine of the 26 study species such as common dandelion and sweet vernal grass. Credit required: University of Washington

The new findings published online Feb. 1, 2011, in the journal Ecology Letters, challenge the widely held assumption that invaders are more plentiful in introduced areas because, for instance, they dont face as many natural competitors or enemies in new places.

While that assumption holds true for a small number of species, the new study demonstrates that it is not a universal pattern. The authors measured the abundance of species at 39 grassland sites around the world, with UW co-authors contributing data from three sites in western Washington. They then compared the abundances of 26 species at sites within their home ranges and sites into which they were introduced.

It turns out that 20 of the 26 invasive species – about 75 percent – were not more abundant in territories they invaded than in sites within their home ranges. UWs Jon Bakker, assistant professor of forest resources and Janneke Hille Ris Lambers, UW assistant professor of biology, are co-authors on the paper.

“The results of this ‘home and ‘away comparison are helpful for scientists and managers trying to predict which species are likely to pose problems if introduced to new areas,” Bakker says. “Knowing the abundance at native sites helps predict the abundance of a species at introduced sites.”

Sites with greater numbers of introduced plant species were also more compositionally similar to each other than sites with few introduced species, even when on different continents, the paper says.

“This supports the idea that species introductions result in reduced biodiversity,” says Hille Ris Lambers.

Graduate students Karen Regan, biology, and Rachel Mitchell, forest resources, hold a frame of PVC pipe that goes on the ground as a step in measuring species abundance. Credit required: University of Washington

Graduate students Karen Regan, biology, and Rachel Mitchell, forest resources, hold a frame of PVC pipe that goes on the ground as a step in measuring species abundance. Credit required: University of Washington

The UW plots are on land owned by Pacific Rim Institute for Environmental Stewardship on Whidbey Island, on Thurston County land near Olympia and on San Juan Island National Historical Park land. Graduate students who are primary contributors to the work are Karen Reagan, Susan Waters, Rachel Mitchell and Ryan Haugo.

UW data were shared with the Nutrient Network, a National Science Foundation funded collaboration that involves standardized data collection and experimentation in grassland sites around the world.