The spread of invasive species in local ecosystems could spell trouble for native plants and insect colonies, as a new study out of the University of Toronto suggests that the interaction between multiple foreign species help each other accelerate their dominance.

The findings say that the combination of two or more invasive species interacting with one another could hasten their spread in habitats foreign to them, leading to further degradation and environmental change.

"When an invasive species moves into an ecosystem, for some reason it does very well there. Maybe they're highly predatory and can wipe out prey or they can be highly competitive and influence native species," said Kirsten Prior of the department of biology at the University of Florida, and one of the researchers of this study.

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The findings represent a phenomenon called "invasional meltdown," whereby multiple invasive species facilitate the success of each other and cause invasive species to dominate local habitats.

Megan Frederickson, one of the study's researchers, who works in U of T's department of ecology and evolutionary biology, said invasive species also tend to drive native species extinct, reducing biodiversity.

"Some of those species that they drive extinct are important for us in a variety of different ways, either providing important ecosystem services, or they're native species that we're actually quite attached to and have some kind of cultural significance," she said.

The researchers compared how the European fire ant, an invasive ant species known for taking over yards and delivering a nasty sting, dispersed seeds of both native and invasive plants as compared to various local ant colonies.

They created artificial ecological communities called mesocosms inside 42 small plastic children's swimming pools at U of T's field station, the Koffler Scientific Reserve, to test this interaction.

The researchers filled each pool with soil and planted four species of spring wildflower plants – three native species (sharp-lobed hepatica, Canadian wild ginger and bloodroot) and one invasive species (greater celandine). They then collected colonies of European fire ants or native woodland ants and added one of the two to each of the pools.

The ants picked up and moved seeds of these plant species and the researchers watched what happened, observing that the pools with the invasive ant were overrun by the invasive plant, but pools with the native ant had lots of native plants.

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"This actually might be pretty common in ecosystems but there aren't a whole lot of examples of it," said Ms. Frederickson.

Ms. Prior said the nature of this study differed from most that have tested this same trend.

"This is the only example of a manipulative experiment to examine how invasive ant species influence myrmecochorous (native ant-dispersed) communities," she said. "A lot of these studies are what we call observational. That means you go out into nature, you go to an area that is dominated by the invasive ant and an area that is not, and you look for differences in the plan communities."

Jenny McCune, a postdoctoral fellow at the University of Guelph's department of integrative biology, said the findings are important in examining the interactions of plants with other factors in the ecosystem, such as seed dispersers in this case.

Ms. McCune, whose current research is about conservation of rare woodland plants in southern Ontario, said the results aren't surprising, but scientists will need to follow long-term real invasions in order to test whether the findings hold up.

"This is a study that only lasted two years and they're dealing here with an invasive plant that's biennial and their three native plants are perennial," she said. "So the questions that I have are, if you followed these native plants longer, would you maybe see a benefit of being dispersed by the exotic ant? Is it just something that takes more time?"

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The authors of the study say the next step is already underway, which is to determine if the invasive plant is also helping the invasive ant to spread. The plant seeds contain an attached fleshy appendage known as an elaiosome, which ants pick up and feed to their larvae before dispersing the seed.

"The question is whether the invasive ant does better feeding on the elaiosomes of the invasive plant than those of the native plant," said Ms. Frederickson. "Basically, are the elaiosomes on these seeds an important food resource for the invasive ant that might help them to grow faster and reproduce more?"