How Are Invasive Plants so Good at What They Do?

Research reveals the tricks that invasive plants use for taking over an ecosystem.

Image: kudzu vine.
Kudzu vine overwhelms all in its path, including trees. traveler1116/Getty Images

Ever wonder what exactly makes an invasive plant so good at taking over an ecosystem? And, if a plant from another part of the world is so much better at it than its native counterpart, why not let it have the job?

Survival of the fittest, right?

The trouble, of course, is that these foreign invaders are too good at their job. Take kudzu, for instance. Since arriving in the United States back in 1876, these vigorous vines have taken so well to the local soil, they’re literally suffocating huge swathes of the American South. Today some 7.4 million acres in the South are covered in kudzu.

No ecosystem can thrive on one plant alone. But kudzu vines, also appropriately known as monsters, are not the sharing types.

The same goes for Japanese knotweed, another foreign marauder that suffers no competition — as its tough, bamboo-like thickets choke out the local plant life. That’s bad news for wetlands and other ecosystems where biodiversity is vital for wildlife to thrive.

But why are these invaders so much more relentlessly efficient than the local vegetation? You might think, for example, that Japan — where kudzu was originally spawned — would have been swallowed up by the vine long ago.

And if buckthorn, which originally hails from Europe, is such a ferocious grower, why isn’t the Old World covered in it?

Superman Didn't Get His Superpowers until He Left Home

The answer, according to a recent study published in the journal Science, is that plants gain their superpowers when they leave home. Think Superman — and ordinary Kryptonian on his homeworld. But when he shows up here on Earth, he’s suddenly the Man of Steel.

In the case of non-native plants, there’s something in the water — or, rather the very microbes in the soil — that makes them heartier than the locals. The study suggests they interact differently, not only with those microbes with the local insects. As a result, they don’t just grow bigger and stronger. They also unleash more carbon dioxide into the atmosphere.

And the last thing a planet that’s already struggling to put a cap on greenhouse gas emissions needs are plants that cycle more CO2 into the atmosphere.

For their study, Lauren Waller of the Bio-Protection Research Centre at Lincoln University in New Zealand and her colleagues built 160 experimental mini ecosystems.

Each tiny ecosystem featured a unique combination of invasive and non-invasive plants. Even the soil featured microbes with varying levels of foreign microorganisms. And researchers topped some ecosystems with a sprinkle of weevils, moths, aphids and other critters.

“We created communities varying in exotic plant dominance, plant traits, soil biota, and invertebrate herbivores and measured indicators of carbon cycling,” the researchers note in the study.

Bugs Love International Cuisine

Ultimately, insects proved the real difference-maker. Mini ecosystems that didn’t have bugs, regardless of whether the plants were native or non-native, maintained a consistent CO2 output.

Introduce a few weevils or aphids, on the other hand, and the picture changes dramatically. In the mini ecosystems with non-native soil and exotic plants, the local insects seemed to get extra busy helping the vegetation release 2.5 times as much CO2 as their local counterparts.

The foreign plants interacted vigorously with certain types of soil bacteria. At the same time, those plants showed a much stronger resistance to fungi — pathogens that most often cause plant diseases.

The bottom line? In lab tests, foreign plants grew stronger in non-native soil — and staved off killer fungi more effectively than their local counterparts.

But insects, particularly the destructive kind, also loved them. Maybe that’s because they were the new plants on the block. Who doesn’t like to hang around a new haunt? But more likely, the researchers suggest, the foreign plants had certain physical characteristics that appealed to insect ravagers — like thick, dense leaves.

Those munching insects would hasten the rate of a plant’s decay, also speeding up its carbon cycle. As a result, if the research holds up in the real world, invasive plants would be exhaling much more CO2 into the atmosphere. And that may explain why not all plants are equally good for a particular ecosystem.

“Are all trees good?” David Wardle, a professor of forest ecology at Nanyang Technical University in Singapore asks Axios. “Do we really want trillions of trees if they are nonnative species that are transforming the ecosystem? Probably not.”