Great Lakes coastal wetlands improve after invasive species removal, study finds
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LANSING -- Removing an invasive wetland plant can make space for native species to return, but researchers say restoration doesn't necessarily mean a damaged ecosystem can return to exactly what it was like before the invasion.
However, recent research on Great Lakes coastal wetlands suggests native plant communities can respond quickly after invasive phragmites australis is removed.
Phragmites australis, also known as common reed, is a large wetland grass that can grow 5 to 15 feet with rigid hollow stems that can spread aggressively through deep underground stems.
According to the Department of Environment, Great Lakes and Energy, invasive phragmites can threaten wetlands with dense tall stands that crowd native plants and reduce biodiversity while disrupting wildlife habitats. They also block shoreline access, increase fire hazards and harm the ecological health of wetlands and the Great Lakes.
In a study on Great Lakes coastal wetland plant biodiversity, researchers found that a treated wetland looked similar to a healthy, non-invaded wetland just two years after management.
"I compared the plant communities among these different sites and I found that, based on one year of data that took place two years after management, the treated site looked really similar to the non-invaded site," said researcher Meghan Ward, a PhD candidate at Trent University in Ontario who studies changes in wetland biodiversity.
The result suggested that restoration could produce rapid changes in plant communities.
"My conclusion was, even two years later, we're seeing a convergence of these two sites together," Ward said. "They're becoming more similar."
One possible reason for that rapid response is a seed bank - a collection of seeds that exist in the mud or in the wetland soil. The seeds don't necessarily grow until the environment is more conducive, Ward said.
When phragmites is removed, those conditions can change.
"Some think that if there's a healthy seed bank and the phragmites is removed -- which allows more light to penetrate temperatures to get warmer in the soil -- there's more room for growth of new plants, then those seeds will start to germinate and we'll see native plants come back," Ward said.
The surrounding landscape can also play a role in determining how quickly a treated wetland is rehabilitated. A treated site near a healthy wetland could have greater opportunities for plants to arrive naturally.
"Seeds blowing over from the healthy wetland -- or maybe a deer or a raccoon walking through that healthy wetland and bringing with them burr seeds and dropping them in the treated site so that there's space for them to grow," Ward said.
One plant that seemed to benefit from the removal was large-fruited bur-reed, which looks similar to a cattail without the corn dog structure.
"It can spread in a couple different ways," Ward said. "It also has a persistent sea bank. So likely those seeds were able to stay in the muck when the phragmites were growing. Then once the phragmites was removed, this plant came about again."
These circumstances appear to have helped the species expand quickly.
Still, researchers caution that rapid recovery should not be confused with a complete return to the ecosystem's previous condition.
"We can't realistically say now that if you take out phragmites, you're 100% guaranteed to return to pre-invasion status," Ward said. "Instead, you might see a new community coming up, which is just as positive, but different."
"The wetlands are providing similar function to different plants and animals, but the individual species of plants are different is what we're finding," Ward said. "So both are still very healthy wetlands, but they just look a little bit different."
The same principle applies when considering animal communities. Research suggests that no single management method that consistently works best at every site and animal group.
The effects of phragmites extend beyond vegetation.
Dense stands can alter the physical structure of wetlands, affecting animals that live both above and below the water. Fish and aquatic invertebrates may encounter more stems and potentially shallower water, while the plants can also reduce wave action.
"Fish communities, aquatic invertebrate communities and aquatic insect communities might change because not only are we seeing way more individual stem-like barriers that fish have to navigate, but there's also potentially less water depth for them to navigate," Ward said.
Above the water, dense invasive plant stands can create a different challenge.
"Invasive phragmites grow and stand so dense that it could be difficult for large water birds to navigate through," Ward said. "One sort of trend we've seen in the data is that large water birds are excluded from sites where phragmites have invaded."
Phragmites can still provide habitat for some species. Red-winged blackbirds, for example, use native phragmites patches and can also use the invasive plant because of its physical structure. The researchers say that does not necessarily mean invasive phragmites is the best habitat.
"While some can still function there, they would likely be functioning better if there was a whole mix of native plants, native phragmites, and not only the invasive phragmites," Ward said.
The study found that insect populations increased after treatment.
"Invertebrates are a key component of aquatic food webs, and the fish, the birds, the amphibians, et cetera, all rely on healthy insect populations so that they can grow," Ward said. "We found that insect populations increased after the phragmites were removed, which is really positive, because it means not only are those insect populations healthy, but that then maybe leads into a more stable food web system."
The findings also show why restoration cannot necessarily be judged after only a few years.
"I think wetlands should be monitored for a decade after large scale restoration, if not longer, because the more we can learn about short-term trends, intermediate trends and long-term trends, the better that we can do restoration on a global scale moving forward," Ward said.
Anna Ironside writes for Great Lakes Echo.