University of Minnesota
Elizabeth Borer and Eric Seabloom are dedicated to answering fundamental questions in ecology.
Photo: Tim Rummelhoff
A new side of invasive species
Most introduced plants don't run rampant, a network of ecologists finds
By Deane Morrison
If somebody says "invasive plant," the first thing that pops to mind is probably a nuisance like Eurasian watermilfoil or purple loosestrife.
But it may be more on the mark to think about bluegrass or clover, according to a new study by a worldwide web of ecologists headquartered at the University of Minnesota.
The ecologists studied 26 species of grasses and forbs--flowering relatives of grasses that include clover, sunflowers, and milkweed--in North America, Europe, China, Australia, and New Zealand. Each species had moved from its native range to at least one new range, usually by jumping continents.
But when the data came in, the popular image of the over-achieving super-invader plant melted away. Instead, the researchers found that most, including bluegrass and two species of clover, were similarly abundant in their native and new ranges. Only 4-6 were more abundant in their new ranges than in their home ranges.
The work should bolster efforts to contain the spread of destructive exotic plants by providing solid data about the likelihood of such events, says Elizabeth Borer. She and Eric Seabloom, both associate professors of ecology at the University, are among the authors of the study, which is published in the journal Ecology Letters.
"USDA and other agencies want to decrease the risk to ecosystems from invasive species," says Borer. "For example, yellow star thistle decreases the potential for cattle grazing. But the surprising result here is that in general, a species' abundance in its native range predicts its abundance in a novel range. It doesn't appear to be the case that new species necessarily take over, although some do."
"It's as if you only read medical journals. You'd conclude that all fungi and bacteria are bad for people, but that's just what doctors study."--Eric Seabloom
Too often, the ones that do take over the new territory just get all the press, says Seabloom.
"It's as if you only read medical journals. You'd conclude that all fungi and bacteria are bad for people, but that's just what doctors study," he says.
The researchers conclude that scientists and biosecurity professionals should regard species that show similar abundances in their native and new ranges as the norm and concentrate on the reasons why some are exceptions.
Ecologists join forces
The study is near-unique in that it was carried out by ecologists around the world who all used the same experimental procedures so that they could confidently pool their data and so achieve much stronger conclusions. Called the Nutrient Network, the new collaboration is the brainchild of Borer, Seabloom, and a handful of others.
"About five of us in the National Center for Ecological Analysis and Synthesis in California were looking at fundamental questions about what structures [ecological] communities," Borer recalls. "We had lots of data, but we wondered how comparable it all was. Then we thought, 'gee--we can each develop an experiment, and we can each do it [using the exact same procedures].'" The idea soon spread to ecologists running experiments at grassland sites around the globe.
The Nutrient Network ecologists realized that while invasive species don't necessarily run rampant in a new range, invasions do change native plant communities in other ways.
"As sites around the world get more and more invaded, they become more similar to each other," says Seabloom.
That's not too surprising, given that most invasions happen as a result of human travels, the researchers say. Humans change ecosystems in predictable ways, such as by adding nutrients like nitrogen, phosphorus, and potassium; removing predators; adding grazing animals; or, indirectly, raising the numbers of mice and other native herbivores.
Borer and Seabloom have a National Science Foundation grant to coordinate the Nutrient Network as it takes on more fundamental questions in ecology, including how human activity affects biological communities. It is sure to influence, for example, rates at which carbon is stored in biomass or soil and released to the atmosphere through decomposition.
"We can't answer any of that with current data, but it's an important component of climate models," notes Borer.
Published in 2011