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Invasive species get by with a little help from their alien friends

Foreign fire ants and alien wildflowers are a recipe for 'invasional meltdown,' Canadian researchers say

Scientists call it "invasional meltdown," and it's the theory that once an alien species takes root in a new land, it opens the door to intrusions from other invading organisms.

In a paper published Tuesday in the Proceedings of the Royal Society B, Canadian researchers suggest that just such a meltdown may be occurring  in some North American forests.

Specifically, a tiny, stinging Eurasian invader is hastening the spread of a greater celandine, a yellow-flowered herb that entered North America from Europe, according to researchers.

"Ecologists think invasive species might help each other to spread," said senior author Megan Frederickson, an evolutionary biologist at the University of Toronto.

"Our results suggest that invasional meltdown could be happening right under our noses here in Ontario," she said in a statement.

In a series of lab and outdoor experiments, Frederickson and her colleagues focused their attention on the behavior of Myrmica rubra, or the European fire ant, which entered North American in the early 20th century. 

Like other native ants, the reddish fire ant just can't resist certain plant seeds and hauls them back to their colony whenever they get a chance.

The ants don't actually eat the seed, however; they just strip off the fleshy fat- and protein-filled cap and feed it to their young. When they're done, they haul off the neglected seed to a waste pile where it can happily grow into a plant.

That process, whereby foraging ants help to transport and sow the seeds for earthbound plants is called myrmecochory, and it goes on all the time with native plants and ants. 

However, study authors wondered whether the alien fire ants were giving celandine an unfair advantage over native flowering plants like sharp-lobed hepatica, Canadian wild ginger and bloodroot.

To test their hypothesis, they buried 42 children's swimming pools in a red pine plantation, filled them with soil and planted representatives of each plant species. Next, they added a colony of European fire ants or a colony of native woodland ants known as Aphaenogaster rudis.

Once they finished building these tiny walled environments, or mesocosms, they began the tedious task of watching and counting as the ants hauled seeds to and fro.

What they saw alarmed them.

"The pools with the invasive ant were overrun by the invasive plant, but pools with the native ant had lots of native plants," said lead author Kirsten Prior, an ecologist at the university's Koffler Scientific Reserve. 

Although the native and invader ants had similar tastes for plant seeds, the foreign ants were far hungrier and far more industrious. They moved twice the number of seeds than the native ants.

That, combined with the alien plant's ability to flower sooner and spread faster than the native plants, showed that the invaders had a clear edge.

"Unfortunately, as a result of humans rapidly moving species around the globe through trade and traffic, most ecosystems are now home to numerous invasive species," Prior said.

"Our finding that multiple invasive species can accelerate invasion and cause ecosystems to become dominated by invasive species is a troubling one."

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