A class picture at Poas Volcano in Costa Rica, 1997.
I. Introduction to Mangroves
What are mangroves? "Mangroves are salt-tolerant trees that grow in coastal areas of tropical and subtropical regions where rivers empty into the ocean" (Kraynak & Tetrault, 2003). Mangroves, as well as estuaries and salt marshes, are coastal habitats that form where freshwater rivers empty into saltwater seas, or saltwater from the oceans force their way inland. These two actions can also occur at the same time. Most mangroves usually form in tropical salty waters, typically near the mouth of a river. All mangroves have a root system that sticks up in the air so the plant can breathe. They have either prop roots; structures that extend midway from the trunk and arch downward; or pneumatophores-structures that extend upward from the roots into the air.
Mangroves begin as a seed, called a propagule, which germinates while still attached to the tree. The seed has a long cigar-like shape that falls off of the parent tree and either sticks in the mud growing next to the parent tree, or floats off into the ocean. These seeds have a strong, protective covering that allows them to float and survive for long distances and periods of time. When the seedling finally reaches its point of destination, the roots will bury into the ground sending the seedling up into the air forming a new mangrove tree. There are many types of plants located on the earth and, "Approximately 80 species of plant are recognized as being mangroves" (Bellamy & Dugan, 1993).
Three types of mangroves dominate Florida and the Caribbean. These three types are the red mangrove (Rhizophora mangle), the black mangrove (Avicennia Germinans), and white mangroves (Laguncularia Racemosa). The red mangroves can be distinguished by the reddish color to the bark of the trunk roots. This type of mangrove is also called the "Walking Tree". The prop roots of the red mangroves arch out from the trunk and branches, producing additional roots that give the tree an appearance as if it is walking in the water. One other characteristic of this type of mangrove are the leaves. The leaves range from 1 to 2 inches, are broad and blunt at the tip, and have a shiny, deep green color with a lighter green on the underside of the leaf. This type of mangrove is most abundant in South Florida, the Caribbean, and Central and South America.
The black mangrove is the largest and tallest of the three types listed above because of their age. They are found upland to the red mangroves, located at higher elevations, and are the most cold tolerant. The bark of this tree is dark which gives it the name black mangrove. "The leaves are shiny dark green on the top, oblong and pointed at the tip. The undersides of the leaves are a dull green color with short dense hairs-glands-which excrete salt–the leaves serve as a back-up system for ridding the Black Mangrove of the salt that has not been excreted by the roots. When early settlers came to Florida they harvested salt from the Black Mangrove leaves" (Phillips, 2003).
The white mangroves are located at higher elevations than both the red and black mangroves. This type can also be identified by its leaves. The leaves are a light green color, approximately 3 inches in length, and are rounded at both ends. At the base of the leaves you will find two bumps called glands. The glands excrete the salt found in the water like a filtering system. Each type of mangrove is located at different areas along the coastline. Therefore, each tree plays a distinct role in the respective areas they are located.
II. Mangrove Community
Mangroves play a role both on land and in the water. In one way in which this plant species is a terrestrial tool is by stabilizing shorelines. The mangrove "wall" between the land and the sea protects the shoreline from erosion and minimizes destruction from powerful waves. Due to mangroves being a naturally flexible plant, they are able to withstand severe damage of winds, waves, and changing tides for thousands of years. Mangroves minimize the loss of property and human lives throughout the globe. Mangroves are such an abundant species that in some areas they form their own islands called mangles. A mangle is, "a term for an assemblage of mangroves interlocked to form patches, which can further extend seaward as a fringe around the shoreline or as a distinct island" (Kraynak & Tetrault, 2003). Mangroves live in shallow water areas and gather sediments that support the root structures. "Mangrove forests help to build up soil along tropical coastlines, buffer from storms, and at the same time provide a habitat for many popular marine organisms such as crabs, shrimps, and oysters"(Prance, 1998).
III. Mangroves as a Habitat
Mangroves provide a home for many organisms, not only aquatic. All of the different organisms that are found in the mangrove areas are all labeled as being euryhaline-able to withstand wide variations of salinity. Oysters are abundant in these areas. They attach themselves to the roots of the mangroves by hook-like projections from their shells. The raccoons of the Florida Everglades and the crown conch shell eat these oysters. Fiddler crabs run around the mangrove areas during low tide eating plant debris. When the tide returns, they run back to their burrows that they make in the mud. The males have a large claw that they use for communication and defense. The opposite partner has a relatively small claw.
The strangest creature living in the swamps are little fish called mudskippers. During low tide, these fish walk around the mud looking for prey. "Some species have suckers on their undersides that help them to climb rocks and mangrove trees"(Laurie, 1972). Their prey consists of small crabs, mollusks, worms, and insects. The mangrove swamps are also nurseries for many coral fish. The swamps provide a protective area for the coral fish to develop to the point where they can travel further out into the ocean to the coral reefs.
There are many species of birds that live in the mangrove areas. This is an ideal area for these birds to live in due to the easy access to both food and resting area. Many birds have developed special characteristics to their beaks and feet to help them adapt to this environment living off of certain prey. Pelicans and other seabirds live in the canopies of the mangrove swamps. During the breeding season, they form large nesting assemblages of adult birds and their offspring called large rookeries. "Other animals that find shelter in the branches and are adapted to mangroves include bats, Proboscis Monkeys, snakes, otters, the Fishing Cat. As many as 200,000 fruit bats may roost in a mangrove. Some small fruit bats roost in mangroves on offshore islands where it's safe from predators and commute daily to the mainland to feed. The bats also contribute to the mangrove: Short-nosed Fruit Bat (Cynopterus sphinx) is believed to be the only pollinator of key mangrove trees (Sonneratia)"(Singapore Zoological Gardens Docents, 2000).
Surprisingly, mangroves are like mammals. "Mangroves are viviparous (bringing forth live young), rather than producing dormant resting seeds like most flowering plants" (Booker, 1998). The seeds of mangroves are called propagules and they are different in shape and size for each type of mangrove. The red mangrove produces cigar shaped propagules that hang on the tree all year long; the black mangrove produces lima bean shaped propagules that develop in late summer and early fall; and the white mangroves produce green pea shaped propagules that develop in the late summer months. All of the propagules are moved and planted the same way. If they do not stick into the mud when they fall from the trees, they will float around in the currents of the ocean until they reach a shallow area where the roots can attach to the mud and send the rest of the seed up to begin the growth. The dispersal period for the red mangroves is 40 days, the black mangrove is 14 days, and the white mangrove is 5 days.
Since mangroves are found on the coastlines, they need to have characteristics that will help them to adapt to the salinity in these areas. The red mangrove trees use the roots in a complex system to excrete the salt from the water. "The survival of this tree in brackish water is a direct result of the tree's ability to adapt to its environment by using its roots to remove 99/100ths of the salt from the water it drinks. Red Mangrove tissue samples, when analyzed, revealed that the salt content of the water in those samples had approximately 1/100th of the salt found in the water in which the trees were growing" (Phillips, 2003). The black mangroves excrete salt from the water through short, dense hairs located on the underside of the leaves. The white mangroves excrete the salt through two bumps at the base of their leaves. Both the hairs for the black mangroves and the bumps for the white mangroves are called glands.
Since these plants are located in wetlands, they need to have certain characteristics to adapt to their environment for oxygen. Red mangroves are located in the lowest elevation and, therefore, are surrounded by more water. This type of mangrove uses prop roots to lift the bulk of the tree out of the water (when the water level is normal and not unusually high) giving the tree room to bring in oxygen. Black mangroves develop pneumatophores which are roots that extend up into the air bringing in oxygen much like we use a snorkel. In white mangroves, lenticels in the lower trunk obtain oxygen for aerenchyma. Each type of mangrove has its own characteristics that help it to adapt to the environment it grows in. This includes characteristics to help provide fresh water and oxygen for plant growth.
V. Human Impacts
In addition to providing protection to the shorelines and a home for many organisms, mangroves provide many different uses for humans. For instance, mangroves are cut down to burn and produce charcoal. The charcoal is used for home cooking fuel and is also used in barbecues in the urban centres and tourist resorts. "To make charcoal, the burnt trunks and branches are stacked in a mound and covered with mud or soil and vegetation to ensure that the wood smoulders slowly for several days. The longer the wood smoulders the better the quality of the charcoal" (Bellamy & Dugan, 1993). The trees are also used for construction purposes. Not only are mangroves cut down to provide land to develop along the coastline, the larger trees are used to build the buildings, and the thinner poles/branches are used for roofing purposes.
One safe use of the mangrove trees is fishing. The mangroves support the fisheries due to the many different species of fish that live within the roots of these trees. A positive feature of this use of the mangroves is that it does not require the destruction of the mangrove trees. A dangerous threat to mangroves is oil spills. Oil spills are a large cause of destruction to the many organisms that thrive in the mangroves.
VI. Conservation of Mangroves
Significant steps are being taken to conserve mangroves. "The legal status of Indonesia's mangroves is such that commercial harvest of the trees is regulated and requires harvesters to leave an undisturbed protection zone 100 times wider than the tidal range along the seaward margin and 50 metres (160 feet) wide along rivers" (Bellamy & Dugan, 1993). In other areas of the world there are projects being developed to improve management of mangrove forests. Some of these projects involve replanting by local communities, and developing sustainable use systems. Certain areas are being claimed as a protected area, such as the Florida Everglades, to help improve the conservation of mangroves. There is still much to be done, but we are off to a good start.
Mangroves are an interesting species to study, and are very important to many organisms-including ourselves. These organisms use this tree for their life support. The three main species of mangroves discussed herein are considered to be the most abundant. And, each of these species has their own unique characteristics. Approximately 80 plant species have been labeled as mangroves, so there is much more research that can be done to document the variety of mangrove species throughout the world. I look forward to the opportunity to learn more about the fundamentals of the contributions that mangroves make to our environment.
Bellamy, D. (1993). Wetlands in Danger. New York, NY: Oxford University Press.
Burton, R. (1991). Nature's Last Strongholds. New York, NY: Oxford University Press.
Spalding, M.D., Ravilious, C., & Green, E.P. (2001). World Atlas of Coral Reefs. Los Angeles, CA: University of California Press.
Newman, A. (2002). Tropical Rainforest. New York, NY: Checkmark Books.
Kraynak, J., & Tetrault, K.W. (2003). The Complete Idiot's Guide to The Oceans. Penguin Group (USA) Inc.
Laurie, A. (1973). The Living Oceans. Garden City, NY: Doubleday and Company Inc.
Prance, G.T. (1998). Rainforests of the World. New York, NY: Crown Publishers.
Types of Mangroves
Characteristics of Mangroves
Conservation of Mangroves
Life of a Mangrove
Human Impacts on Mangroves
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