Invasive species are often seen as a destructive – almost malign -- force in the natural world. For example, greenish flying beetles from Asia -- the emerald ash borer -- are killing tens of millions of stately ash trees across Maryland and the United States.
Another example: In Chesapeake Bay wetlands, many landowners perceive invasive reeds from Eurasia – the tall, fringe-topped Phragmites australis – as an enemy because it crowds out native wetlands grasses and birds.
Some homeowners and wildlife managers spend countless hours and thousands of dollars spraying herbicides and setting fires to try to kill off phragmites, although these efforts can have unwanted side-effects
As it turns out, some exotic plants are as much friend as foe.
A new study by researchers at the Smithsonian Environmental Research Center and allied institutions found that invasive species of plants can help fight climate change. Exotic plants can increase by about 40 percent the amount of carbon dioxide pollution absorbed by wetlands, underwater grass beds, and other coastal areas, according to the research, published in the Journal Global Change Biology.
Christina Simkanin is a biologist at the Smithsonian center in Edgewood, Maryland. She took me out into a field of phragmites, beside a Chesapeake bay tributary, to explain this phenomenon.
“Phragmites has been seen as kind of a Jekyll and Hyde species,” Simkanin said. “It can have negative impacts on marshes, but it can also have positive impacts by storing carbon in its tall and quickly growing plant roots and stems and leaves.”
Not only do the invasive plants grow much faster than native grasses – thereby consuming more carbon dioxide—their massive, interwoven root systems can help build up marshlands being flooded and eroded by rising sea levels.
The downsides are that native plants can not compete. The Chesapeake’s birds and mammals have not adapted to eat the seeds of phragmites, or nest among its dense bamboo-like walls. So Simkanin does not advocate planting more of it.
“No. We certainly are not advocating planting a non-native species into an area where it currently isn’t introduced,” Simkanin said. “But in areas that have already been impacted by a non-native species? This study provides greater information for determining whether eradication is worth the cost and effort to be done.”
Her research did not focus only on phragmites. She and her colleagues studied 104 scientific papers from around the world on the amount of biomass produced, and carbon dioxide captured, by wetlands, seagrass beds and other coastal areas that have been invaded by exotic species. The researchers compared these areas to places dominated by purely native plants.
One interesting finding is that invasions by similar types of plants more than double the amount of carbon dioxide captured – for example, when exotic reeds invade wetlands grasses. But this pollution control benefit did not happen when dissimilar types of plants invaded: for example, when bushes move in on wetlands. And there was no upside when exotic insects or animals invaded, Simkanin found.
“An animal invading a marsh grass or a mangrove or a seagrass system generally has a negative impact,” Simkanin said. “So they eat the plants that form the habitat, they can dig within the soil so they cause disruption. And overall they decrease the ability of marshes, seagrasses and mangroves to store carbon.”
So introducing, for example, Asian oysters into the bay would not help. But preserving – instead of poisoning – phragmites and other exotic plants… just might.
The study “Differential effects of biological invasions on coastal blue carbon: A global review and meta‐analysis" can be found in Global Change Biology.