Life in the Soft Edges
Rethinking the reeds of the mid-Atlantic tidal marsh
by Judith S. Weis, marine biologist, Rutgers University
Phragmites australis. Common reed. Maryland Biodiversity Project | Kerry Wixted
The common reed, Phragmites australis, is an invasive species in tidal marshes in the US mid-Atlantic, frequently removed and replaced with native Spartina alterniflora (cordgrass) during restoration projects. Most people in the US dislike Phragmites and wish to have it removed, while people in Europe and Asia appreciate it. The species is actually native to the US, with the native form comprising a minor part of the high marsh. A new genetic variety arrived many decades ago and proceeded to “take over” many brackish and freshwater marshes, and to reduce the diversity of plant life. Incidentally, in Europe and Asia, it is valued as an important wetland species. And in China, where Spartina alterniflora has arrived, they are concerned with the effects of this invasive species replacing their beloved Phragmites!
Spartina alterniflora. Smooth cordgrass. credit: seashore to forest floor
Habitat for Aquatic Animals
Around 1990, we reviewed the science that was known, and found there weren’t many studies, but what we found indicated that fish in the tidal creeks of Phragmites marshes in New England and Chesapeake Bay were about as abundant and diverse as those in cordgrass (Spartina) marshes. We undertook studies to look at the animals in marshes to see their relationship with Phragmites and Spartina. In behavioral studies, we found that grass shrimp, fiddler crabs, and killifish chose both plants equally and that both plants provided comparable protection for grass shrimp against their predator, killifish.
These previous studies were conducted in lab aquaria, so we did field studies at a tidal creek with cordgrass on one side and reeds on the other. Sampling on both yielded comparable numbers of animals, including invertebrates in the mud on both sides of the creek. Other investigators have found that fish assemblages tend to be similar in Phragmites and Spartina marshes or less dense in the Phragmites. The killifish (Fundulus heteroclitus), especially small ones, are reduced in Phragmites marshes. However, ribbed mussels and fiddler crabs are comparable in both types of marshes.
Another important role of marsh plants is that their detritus provides food for many animals. When decaying plant material from Spartina, Phragmites, and restored Spartina was ground up and fed to fiddler crabs and grass shrimp, they provided equivalent nutrition, which supported survival and growth of fiddler crabs; but did not support shrimp survival.
Terrestrial Animals and Plants
Bird use of Phragmites varies depending on the species, location, stand architecture, and other available plant species. Some birds clearly do not like, and some prefer Phragmites. Studies comparing the density of individuals or the numbers of species in reeds versus alternate plants show variable results. Extensive, dense beds of tall reeds seem to support fewer species of breeding birds than do smaller beds, those with reeds mixed with other plants, sparse stands, and stands with Phragmites patches mixed with pools or clearings. In some cases, Phragmites appears to benefit roosting birds, songbirds eating seeds during migration or winter, animals taking refuge from flooding in high reed stands, and small mammals like cottontail rabbits hiding in reed stands. In other cases, Phragmites appears detrimental; shading turtle nesting sites and displacing short plant species in the high salt marsh.
Ecosystem Services: Pollution
Both Spartina and Phragmites take up comparable amounts of metal pollutants from the sediments into their roots, but Spartina sends more up into the stems and leaves from which they are excreted back into the ecosystem, while Phragmites sequester more metals in the roots, keeping them away from the rest of the ecosystem. Reeds are also better at sequestering other pollutants, including nitrogen and carbon dioxide. Phragmites marshes can remove lots of nitrogen from surface waters and soil due to its greater biomass relative to other plants. The nitrogen it takes up is not available for causing algal blooms and eutrophication. The CO2 it takes up is “blue carbon,” which does not contribute to climate change.
Ecosystem Services: Resilience
Phragmites traps more sediments and enables a marsh to elevate more rapidly than Spartina does. Phragmites builds and stabilizes marsh soils and stores carbon in litter and soils more effectively than Spartina and can protect tidal marshes from erosion associated with sea level rise, as well as helping to mitigate global climate change. Some marsh restoration projects lower the marsh surface after removing the reeds and before re-planting cordgrass. At this time of climate change and accelerating sea level rise a plant that enables a marsh to increase its elevation more rapidly should be valued, not destroyed.
Some people dislike reeds because they grow tall and dense and block their view of the water. Think about storm surge and sea level rise: what type of plant would you prefer in front of your coastal property: one that is denser and taller, or one that is shorter and sparser?
Piermont Marsh is located along the Hudson River. The New York State Department of Environmental Conservation (DEC) had set aside over $1 million to compensate for the environmental damage resulting from building the new Tappan Zee Bridge. They proposed to use the funds to eradicate Phragmites in order to restore the marsh. A group of residents of the town opposed the removal because they felt that the reeds had protected their neighborhood from damaging effects of Hurricane Sandy. After the outcry, DEC revised the plan: to eradicate much less Phragmites.
Phragmites supports many animals and performs valuable ecosystem services. The assumption that it degrades marshes must be re-thought at this time when we must be concerned with resilience. We should shift from attempted eradication everywhere, to modification of stands to create habitat for particular species while retaining non-habitat ecosystem services. Because Phragmites can provide substantial ecosystem services, as well as being an invader, it requires an approach that is tailored to individual sites and management goals. Killing it everywhere is impractical, expensive, damages sensitive species, reduces resilience, and wastes resources.
Want to learn more? References are available upon request.
Judith S. Weis is a marine biologist, Professor Emerita at Rutgers University. Learn more and read more at: wisem.rutgers.edu/weis-judith