As climate change erodes US coastlines, an invasive plant could become an ally

Many invasive species are found along U.S. coasts, including fishes, crabs, mollusks and marsh grasses. Since the general opinion is that invasives are harmful, land managers and communities spend a lot of time and resources attempting to remove them. Often this happens before much is known about their actual effects, either good or bad.

The common reed Phragmites australis is a tall perennial grass with long leaves that invades fresh and brackish wetlands. There it crowds out native species, reducing plant diversity. Managers frequently kill it with herbicides and replace it in brackish marshes with native Spartina alterniflora, or cordgrass, during restoration projects.

But despite its bad reputation, Phragmites provides many benefits that are generally unknown and unappreciated. After studying salt marsh ecology and the impacts of stressors, including invasive plants, for many years, I have concluded that removing this invasive species wherever it is found – especially along vulnerable coastlines – is a very expensive and often foolish procedure.

Providing food and shelter

Phragmites actually is native in the United States, but the native form comprises only a minor component of the high marsh – the zone that typically is above water. A new genetic variety arrived many decades ago and invaded brackish marshes.

In Europe and Asia, where Phragmites is also native, it is valued as an important wetland species. In China, where Spartina alterniflora has arrived, marsh scientists and managers are concerned with the effects of that invader replacing their beloved Phragmites. Human attitudes toward invasive species can be a bit subjective.

An Ohio field overgrow with invasive Phragmites. USGS
In the 1990s, my research group reviewed the limited state of knowledge on how Phragmites invasions were affecting mid-Atlantic coastal areas. We found a few studies indicating that fish and invertebrates in tidal creeks of Phragmites marshes in New England and the Mid-Atlantic were roughly as abundant and diverse as those in Spartina marshes. In other words, we did not find a major negative impact from Phragmites invasions.

We also did some behavioral laboratory studies examining relationships of estuarine animals to Phragmites (reeds) and Spartina (cordgrass). These investigations showed that grass shrimp, fiddler crabs and killifish chose both plants equally, and that both plants gave grass shrimp comparable protection against their predator, killifish.

To check these results in the field, we did studies at a tidal creek with cordgrass on one side and reeds on the other. Here we again found comparable numbers of animals in the mud on both sides of the creek.

Findings can vary in different locations. Some researchers have found that fish assemblages are similar in both marshes, while others have shown them to be less dense in Phragmites. Some studies found that killifish were clearly reduced in Phragmites marshes.

Marsh plants also provide food for many animals after they die and decay, producing detritus that enters estuarine food webs. When we ground up decaying leaves from Phragmites and Spartina and fed it to fiddler crabs and grass shrimp, the two plants provided equivalent nutrition. Both plants supported survival and growth of fiddler crabs, but neither one supported shrimp survival beyond three weeks.

Phragmites can spread through aerial seed dispersal or via rhizomes (underground stems). Great Lakes Phragmites Collaborative, CC BY-ND

Habitat for terrestrial animals and plants

Marshes are habitat for many kinds of birds, both year-round and during migration. Phragmites supports many birds and other land animals, though not as many as Spartina.