Biology 103
2000 Third Web Report
On Serendip

Why the World needs its Mangroves:A look into the natural and human relationships with mangrove forests

Jessica Hayes-Conroy

"One perceives a forest of jagged, gnarled trees protruding from the surface of the sea, roots anchored in deep, black, foul smelling mud, verdant crowns arching toward a blazing sun...Here is where land and sea intertwine, where the line dividing ocean and continent blurs, in this setting the marine biologist and the forest ecologist both must work at the extreme reaches of their discipline." (8).

The place so beautifully and truthfully described in the quote above is known in general terms as a mangal or a mangrove forest. Such ecosystems are a type of wooded coastal wetland found along the shores of the tropics all over the globe. They are at the interface between marine and terrestrial worlds, providing large benefits to both locations. Although mangroves are naturally hardy, having developed unique adaptations to survive in such an environment, recent exploitation by human processes threatens to destroy them beyond repair. In turn, this destruction would also be extremely harmful for all of the natural and human systems that are dependant on these mangroves. Thus, mangroves are not only unique ecosystems in and of themselves, but they are also uniquely linked to the many systems that surround them; for these reasons, mangroves should be protected.

Mangrove Ecology

Mangrove forests are comprised of taxonomically diverse, salt-tolerant tree and other plant species. They thrive in the intertidal zones of sheltered tropical shores, the overwash islands, and the estuaries of tropical areas worldwide. Mangroves can be found in 50 different varieties along the tropical coasts of such continents as Africa, Asia, Australia, and the Americas. The most diverse location for mangroves is the Indo-Pacific region, containing approximately 40 different species (8). Somewhat less diverse are the mangrove regions of West Africa, the Caribbean, and the Americas, containing around 8 different mangrove species. In the United States, the most prominent of mangrove species are the red mangrove (Rhizophora mangle) black mangrove (Avicennia germinans) the white mangrove (Laguncularia racemosa) and the buttonwood (Conocarpus erecta) (2)

.Zonation

These four species of mangrove occur in zonation patterns within the mangrove forest, each one occupying its own niche along the coast. The species differ in relation to their distance from the shoreline and the amounts of sediment and salinity at that distance.

Red mangroves are the most tolerant of high salinity and thus make up the first zone, closest to the water. They are easily identifiable by their tall prop roots extending from their trunks and branches (6). These prop roots are necessary for their survival in the wetland, rooting the tree shallowly within the first few centimeters of muddy soil. On these roots are lenticels--small pores that absorb oxygen when exposed to the air at low tide (7).

Next in the zonation pattern is the black mangrove. These trees are slightly smaller than the red mangroves and have different root structures. Their root system consists of short aerial roots, called pneumatophores, which surround the plant like a carpet. These root systems help the tree both with its stability and in its ability to collect nutrients in a water-filled environment(6).

White mangroves come next in the zonation pattern, upland from the red and black mangroves. These trees are generally shrubbier than the other two, although they can grow as tall as 10 meters. They also use their aerial roots in the same way as the black mangroves, to support their weight and provide them with nutrients. On the white mangrove, however, these roots are less visible (2).

Lastly, the buttonwood, an upland species, is not actually considered a true mangrove. Still, buttonwoods are often associated with mangrove forests and have similar adaptations to the levels of water and salinity(6).

Adaptations

All mangrove species have a number of different adaptations that help them to thrive in the salty coastal environment. As discussed above, their intricate root systems allow for fixation in loose soil, as well as for respiration and aeration. In addition to this, mangroves have specialized and unique dispersal mechanisms, being the only true viviparous plant (6). The seed of the mangrove remains attached to the parent plant until it germinates into a propagule or embryo. Then, as it falls from the tree, it often drifts with the current of the water it until finally encounters an appropriate place to take root (2). This floating property of the propagules is thought to be the reason why mangroves have spread out over the continents.

In waters of such high salinity, mangroves need special adaptations to rid themselves of the salt. In general, they can be considered facultative halophytes, meaning that they do not require salt water to live but are able to tolerate it(1). They do so, thereby out-competing freshwater plants, by the processes of microfiltration and salt excretion. Their intricate root systems are covered with tiny pores that filter out salt from the water that surrounds them. At the same time, any of the salt that finds its way into the plant is excreted through salt glands located on the base of mangrove leaves. These processes work so well that a thirsty traveler could cut the root of a mangrove and obtain fresh, drinkable water (8). The Relationship of Mangrove Forests to Other Important Marine Systems

These uniquely adapted mangrove forests have not evolved in such an environment on their own. Other marine ecosystems, namely coral reefs and sea grass beds, have evolved in a dependent relationship with them.

Coral reefs are believed to be the most biologically diverse marine ecosystems on earth, second only to terrestrial rainforests overall. They are very delicately balanced systems, depending on the interaction of hard and soft corals, sponges, anemones, snails, rays, crabs, lobsters, turtles, dolphins, and other sea life (10). The nearby mangroves are the nursery and breeding grounds for such marine life. They provide shelter and nutrients to many species, including most juvenile reef fish. Mangroves trap and produce nutrients, providing food to these animals. In addition, the mangroves' root systems protect the reefs from terrestrial sediment and other forms of pollution. In return, the reefs serve as wave breakers, helping to protect the mangroves from forceful impacts (7).

Seagrasses are aquatic flowering plants that make up a large part of the marine food web. Like the mangroves, they are also spawning and nursery grounds for many marine organisms that live in the reef. They too are depended on mangrove ecosystems, being unable to survive in areas of high turbidity and sedimentation. Mangroves help them by slowing down the velocity and forcefulness of the water, thereby preventing fine silt from clouding the water and blocking the sunlight (3). In this way, the seagrass is able to photosynthesize and flourish under calm, sunny conditions, allowing for perfect nursery grounds for coral reef species. Thus, this process of seagrass protection affects the reefs that depend on the young marine organisms and, consequently, the mangrove itself, which depends on the coral reef (9).

Other Important Mangrove Functions

Mangroves are important to many local coastal species, both terrestrial and aquatic. For many organisms, mangrove forests serve as the starting place for their food web. Its detritus (fallen leaves and organic material) serves as a nutrient source for planktonic and epiphytic algal food webs. These microorganisms and macroinvertebrates then supply the remaining members of the food web with tremendous amounts of nutrients and energy (1).

As mentioned above, mangroves also serve as a nursery and breeding ground for many reef organisms. In fact, their intricate root systems provide shelter for many marine and terrestrial animals, protecting them from ocean currents and strong winds. Many endangered species can be found living in mangrove forests, including the Florida manatee, the bald eagle, the brown pelican, and the Barbados yellow warbler (6).

In addition to the sediment pollution, discussed in relation to seagrass, mangroves also help to control other forms of pollution, including excess amounts of nitrogen and phosphorous, petroleum products, and halogenated compounds (1). Mangroves stop these contaminants from polluting the ocean waters through a process called rhizofiltration. The lenticels that are present on the mangroves' root systems, as mentioned earlier, allow the area directly around the root to remain aerobic even in anaerobic, saturated soils. Microorganisms that can break down such pollutants thrive in these environments. They use enzymes to break down and make stable the potentially dangerous substances, thus treating the effluent that runs through the mangrove system (7).

Direct Human Importance

In addition to benefiting the natural ecosystems of the surrounding area, mangroves are also extremely important to human communities as well. Traditionally, they have been sustainably used for food production, medicines, fuel wood, and construction materials. Many indigenous coastal residents rely on mangroves to sustain their traditional cultures. In this way, the mangroves' ability to act as habitat to many possible food sources, as well as it's ability to remain stable while growing tall and strong, are very important to human communities as well(8).

In addition to this, mangrove forests also act as a buffer zone between the open ocean and the land. This not only protects the shores from damage, but also its many inhabitants-including humans. Mangroves protect the coastal land areas from life threatening erosion and siltation problems, preventing a great deal of property damage and sometimes even human death(3).

Finally, the mangroves' ability to treat effluent, discussed above, is also very important for the local communities. Most of the substances that the mangroves treat are human made. Thus, the mangroves are acting as a filter system for the local communities, keeping their ocean waters free of pollution and thus their fish and other food sources free of contaminants(1).

All of these "eco-services" that the mangroves provide, free of charge to the local communities, have a tremendous economic value for all who are dependent on them. Unfortunately, although the hardy mangroves have withstood fierce storms and heavy winds for thousands of years, they are now being devastated by human business and industry(8).

Destruction of Mangrove Forests

Historically, mangrove forests have been perceived as desolate, unproductive regions along coastal areas. Ironically, local communities now see mangrove ecosystems as an impediment to their economic practices. As a result, mangrove forests are now among the most threatened habitats in the world (8). Countries like Vietnam and Ecuador have already lost close to fifty percent of their mangrove forests; others, like Java and Thailand, have lost even more. Sadly, destruction in these countries continues even today.

Industrialized nations supposedly have stricter laws protecting mangroves, although destruction continues in many of these countries as well. Mangrove forests in Australia and the United States are protected from direct exploitation, yet are still damaged by nearby development, dumping, and pollution. The mangroves' lenticels, although very helpful in providing oxygen to the root systems, are highly susceptible to clogging. Crude oil, attacks by parasites, and prolonged flooding by artificial dikes can kill large numbers of mangrove trees(8).

Developing nations also destruct mangroves through heavy pollution, although they are even more damaging in their direct exploitation. Often, in these countries, mangrove forests are completely destroyed in order to provide places for residential, commercial, and industrial development. Many mangroves have been cut down to provide ocean-side land for local housing and tourist hotels. The most destructive process, however, has been the shrimp aquaculture industry(8).

In Thailand, the shrimp industry can account for most of the mangrove destruction. Furthermore, although the shrimp industry argues that tourist areas are equally to blame, the unregulated expansion of this industry has destroyed the mangroves at such an alarming rate, that it threatens to wipeout all the mangrove forests in Thailand over the next 70 years. This fast rate of destruction occurs because the shrimp industry doesn't just destroy the forests, but uses them and then abandons them, leaving much pollution when they move on. In one year, the industry is said to expel over 1.29 billion cubic meters of effluent-far more than the remaining mangroves can successfully treat (8).

Solutions

Sadly, less than one percent of all mangroves worldwide are sufficiently protected. At this point, mangrove destruction continues to go unchecked, with little to no public notice. It seems, then, that the first step towards creating a solution is educating the public. A boycott on Thailand's shrimp industry by an industrialized nation like the US would send the message that such destruction is not allowable. In addition, creating awareness so that developments surrounding mangroves would be more careful of their dumping habits could also help stop destruction. In any case, creating awareness within local communities should foster the idea that the residents are stewards of the mangrove areas, and thus, have a responsibility to actively work to protect them (11). Such steps toward protection require a multi-disciplinary approach, requiring the help of marine and terrestrial scientists, regional and environmental planners, local and national policy makers, and many kinds of educators.

Conclusion

Mangrove forests are a very unique and complex kind of coastal wetland. They have evolved over the centuries not only in relation to their internal systems, but also to the functioning of eco and human systems that surround them. In other words, they are an open system, and thus, are directly related to all that is around them. With this way of thinking, mangrove forests become even more complex and, at the same time, more difficult to protect. Perhaps, as some environmental engineers suggest, information technology can help to stop mangrove destruction (10).. With increasing technological advances, we can begin to better understand the delicate relationships between the marine and terrestrial worlds of mangrove forests. Computer technology can help advance multi-disciplinary research by making it possible to visualize the complex systems of a mangrove. This would allow all disciplines to better understand the many processes that must be taken into account in the management of these complex mangrove forests.

WWW Sources

1) Ecological Importance of Mangroves , this website is a small informational page with links to bigger pages

2) Florida's Mangroves: Walking Trees , a basic site about Florida's mangroves

3) Currents and Sediment Transport in Mangrove Forests , A paper by Keita Furukawa and Eric Wolanski

4) Sedimentation in Mangrove Forests , Another paper by Keita Furukawa and Eric Wolanski

5) Visualization of Oceanographic and Fisheries Biology Data for Scientists and Managers , A paper by Duncan Galloway

6) Mangroves , A website on mangrove adaptations and zonation patterns

7) Mitsch, William J., and James G Gosselink, Wetlands, Third Edition, John Wiley and Sons, Inc. New York, NY, 2000

8) Mangrove Action Project , A website with detailed information on mangrove forests

9) The Importance of Mangrove Flocs in Sheltering Seagrass in Turbid Coastal Waters , A paper by Eric Walanski et al

10) Dispersion in Coral Reefs and Mangroves , A paper by Eric Wolanski and Joe Sarsenski

11) Yad Fon's Way: Thailand's Community Forest Project: The Fishers that Rescued the Sea , A small documentary paper from Mangrove Action Project




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