How Do Humans and Plants Interact in Tidal Wetlands?
Part of Hall of Ocean Life.
Stepping off of our noisy bus into the frigid morning air, I am immediately astounded by the primal beauty of the marsh. The golden and magenta sunrise highlights the background, silhouetting the graceful cordgrass as it dances in the biting wind, and reflecting shards of sparkling light on the glassy water. The area appears untouched by human existence, a place completely separate from the commotion and insanity of the highway no more than 20 feet away; time seems to have stopped here. The marsh is so still and hushed, almost lifeless, that I can hear the wail of a lone osprey soaring high above me in the purple sky, hungrily searching for prey hidden in the vast mud flats. Awed by the striking landscape, it is difficult to focus on my intended purpose for visiting this place. I have come to study the marsh, focusing on plants as a vital component of this ecosystem.
The first few minutes of our expedition pass in a flurry of activity that disrupts the initial tranquility of the marsh, as different groups find a GPS reading of our location, begin to test the water chemistry of the creek, and unload our materials from the bus. During these moments, I pause to take my first photograph, which clearly chronicles the human interaction already underway in the marsh. Then, trudging into the knee-high cordgrass with my field journal in hand, I search for a quiet area where I can sit alone and write about my first perceptions of the area. I am still hypnotized by the marsh's serenity and beauty, and I find it hard to believe that this quiet place can shelter such a diverse range of organisms. The first thing I notice about the marsh is its ocean of vegetation that seems to stretch for miles. Looking out over this rambling wet prairie, temporarily invaded by amateur explorers, I begin to wonder how humans affect the natural order of wetland vegetation. Is human intervention in this ecosystem positive or negative?
The most noticeable feature of the area is the salt-marsh cordgrass (Spartina alterniflora), a thin green stalk that can grow to be four feet tall. Found mainly in the mud flats of the marsh, cordgrass is the dominant marsh plant, and it is also the only plant that can survive in this high-salinity environment due to an amazing adaptation -- specialized cells in the leaves. These glands are capable of secreting excess salt through the plant's leaves whenever the salinity reaches an intolerable point, and this allows the cordgrass to dominate the mud flats. As I walk through the marsh, I remove several of these leaves from one plant for later analysis, and I also quickly sketch a specimen in my field journal. The intricacy of the leaves is too difficult to draw, so I take note of this and continue across the marsh.
Salt-marsh cordgrass is an important aspect of tidal wetlands, and without it the marsh would cease to exist. Not only does it provide shelter for the vulnerable fiddler crab, but the hardy root system also helps to stabilize the unsteady mud flats. Decomposed cordgrass plays a major role in the detritus-based food web that supports the marsh ecosystem. Now, photographing the landscape, the only characteristic of the marsh I can see clearly through my lens is the rolling hills of cordgrass, which are the most prominent part of most of my pictures.
Another significant marsh plant is the Phragmites australis, which is an invasive, or non-native, species that is slowly taking over American wetlands. In the high marsh, I discover a large cluster of Phragmites, and I kneel on the frost-encrusted ground to sketch and photograph this striking purple-hued plant. The willowy, feathered reed has adapted to the anaerobic environment of the low marsh with a unique hollow stem, which acts like a snorkel in the waterlogged soil and transports oxygen to the roots. An additional adaptation of Phragmites is the location of its seeds.
Since they are gathered at the apex of the plant, they can be scattered throughout the marsh by the strong winds that prevail there; this allows for increased growth of Phragmites populations. Although this common reed is actually native to North America, a non-native strain of Phragmites australis is believed to have been transported here from Europe early in the 20th century, and scientists predict that this sturdy plant will soon take over the low marsh. I can clearly see that the invasion of Phragmites into the low marsh is an impending threat, as masses of the weed loom threateningly over the cordgrass at the boundaries of the high marsh.
Studying the marsh vegetation, I find myself wondering what other changes will take place here in the future. Will students going on this field trip in a few years see the same marsh that I am now observing? How will the marsh transform over the next decade, and how will humans have played a role in those changes?
Scientists are already trying to alter the natural order of our wetlands. While at the marsh, I join the botany activity, in which a large group of students will study several marsh plants with a biologist from the marine biology division of the University of Delaware. Dr. John Gallagher leads us around the different zones of the marsh, which are designated by different species of plants, and as we move through it, he asks us to collect seeds from certain plants. After dissecting a specimen of giant cordgrass, Spartina cynosuroides, he explains that scientists are currently studying this plant for its salinity-reducing properties, and, if successful, it will be used to decrease the high salt content of the brackish marsh waters. He continues to explain that Juncus gerardii, a wetland rush, is also being tested to see if it can be used to combat the rampant growth of the invasive Phragmites australis, which many scientists feel is choking the marsh and preventing the growth of a more diverse range of vegetation. I am fascinated by his lecture because I had not considered that scientists might be trying to reverse the gradual degradation of the marsh; instead, I have been focusing on how humans are affecting it negatively.
Bio-invasion is a major problem worldwide, and the situation in the marsh is a prime example. The invasive species Phragmites australis is slowly replacing native Spartina alterniflora in tidal wetlands throughout the Atlantic coast of the United States, and scientists fear that it will greatly impact the marsh ecosystem. Cordgrass is a vital part of the marsh because it is the foundation of the marsh's detritus-based food web, as well as a main source of shelter for thousands of organisms. Along the banks of the creek, I find three ribbed mussels attached by byssal threads to the hardy stem of a cordgrass specimen. I also observe several fiddler crabs scuttling into their burrows near the base of the cordgrass; these crabs, the most abundant arthropods in the marsh, are fully dependent on cordgrass for protection. However, Phragmites growth can drastically affect these environmental relationships, strangling a wetland's diversity by dominating the area, altering water flow patterns, and causing the accumulation of sediments. The most apparent causes of this rampant invasion of Phragmites into salt marshes are tidal restrictions that reduce the flow of saltwater to the area, landfills that raise the marsh ground levels and reduce wetness, and different forms of pollution, such as road salts.
Many scientists believe that the easiest and most effective solution for this problem is to remove large amounts of Phragmites and replace it with cordgrass. Unfortunately, these "constructed" wetlands are less varied and productive than natural marshes, and the harsh removal process can also shock and heavily degrade the area. In 1997, biologists compared natural and rehabilitated marshes on the Gulf Coast, and they discovered that species diversity and infauna density were severely lower in the restored marshes. Due to a process of succession, it can take decades for normal marsh functions to resume, and therefore a general decrease in productivity can be the main result of restoring wetlands.
Other scientists believe that an older solution can produce better results: aerial spraying, which uses chemical herbicides to control Phragmites. In September 1983, at the Prime Hook Wildlife Refuge in Delaware, herbicides were sprayed from a helicopter over 500 acres of wetland, and ground evaluations eight months later revealed a 98% level of success. By continuing an annual spraying and water-management regime, the Refuge has prevented the weed from returning. Aerial spraying is a technique that has been used by several wildlife refuges in the United States.
Unfortunately, although it is usually a permanent solution for a Phragmites problem, aerial herbicide spraying is not widely used because it is very expensive, and in fragile wetlands it can cause the complete destruction of the plant communities it was designed to protect.
However, there is another option for those concerned with the invasion of non-native plants: to allow them to take over the area. In recent studies, scientists observed the diversity of the animal populations in wetlands slowly taken over by Phragmites.They discovered that, in these areas, there is an abundance of invertebrates that are usually found in natural tidal marshes, as well as a diverse population of fish that feed on these organisms. These studies provide enough evidence to prove that the reed can significantly contribute to the food webs of low-salinity marshes, and that some areas dominated by non-native plants can support a wide range of creatures similar to those in areas without invasive species.
Although biologists are trying to improve the condition of America's wetlands, I soon discover that many other people take this place for granted. As I cautiously walk through the cordgrass, I can find evidence of human interference with the marsh everywhere. Plastic wrappers and cigarettes litter the high marsh, and several battered soda cans destroy the sparkling appearance of the marsh water; I even see a discarded shoe partially buried beneath a thick layer of mud. While scientists are working to create answers to the problems in these wetlands, others, such as recreational visitors, may be inadvertently undoing their work, and I realize that not everyone values our dying marshlands.
Despite the efforts of people working to restore them, America's wetlands are slowly disappearing, and negative human interaction is often the cause. Without these marshes, Earth will be an entirely different place, one lacking in the diverse spectrum of creatures that I have observed today. As I prepare to leave the marsh, I photograph and sketch several more species of plants, as well as write in my field journal about my closing thoughts on this expedition. Looking out over the marsh, I observe the field of delicate cordgrass fighting against the merciless wind, as well as the threatening shadows cast by the domineering Phragmites. The glittering, wind-whipped water reflects the blinding rays of the afternoon sun, and I must shield my eyes from the light to fully see this complex, deceivingly tranquil scene. During these final moments, I am now pondering not what the marsh will look like in coming years, but whether it will even exist at all. How will future human interaction with plant life affect other marsh inhabitants, as well as the entire ecosystem? These unique plants are a valuable part of our world, and we must work to enlighten others in order to save our wetlands from extinction.
Gallagher, John, Ph.D., University of Delaware, Marine Biology Division. Interview by Bianca Male. October 10, 2000.
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