Mangrove Ecosystems: Contributions and Importance

This discussion topic submitted by Travis Buck ( tb369590@oak.cats.ohiou.edu) at 1:26 pm on 6/9/00. Additions were last made on Friday, June 9, 2000.

I always knew that mangroves were important to our global ecosystem, as is every biome. How important, I did not realize until I began researching my discussion topic. What I came to realize is that without mangroves, and I think it should be noted here that mangrove populations on this planet are declining, there would be virtual breakdowns in environmental systems and economic sectors across the globe. In this discussion, I hope to illustrate the many ways in which mangrove communities contribute to life on earth, and what can and should be done to help preserve them.
The word ìmangroveî is derived from the Portuguese word for an individual mangrove tree, ìmangue,î and from ìgrove,î the English word for a group or stand of trees. A vague definition goes something like this: Mangroves are a diverse group of predominantly tropical trees, shrubs, palms, or ground ferns that generally exceed two feet in height, and grow primarily in the marine intertidal zone. However, it isnít always exactly clear what constitutes a true mangrove. The diverse group of plants that make up the mangroves have been categorized as ìtrue mangrovesî and as ìmangrove associates.î Mangroves are trees, but their form is very versatile: they are found as low, scrubby plants in harsh conditions; whereas their canopies can attain heights of over 40 meters under favorable conditions. The combination and variety of morphological and physiological adaptations found in this diverse and unique group of plants has no equal (Robertson 65). In fact, our understanding of mangrove forests lags behind that of many other ecosystems on earth (Robertson 1). When referring to the mangrove habitat, the terms mangroves or ìmangalî are used. The habitat can also be called a mangrove forest or sometimes a tidal forest.
The very earliest recorded reference to mangrove vegetation was made in 325 B.C. by a Greek explorer named Nearchus as he traveled throughout the Mediterranean Sea. Many other early explorers were certainly familiar with the mangroves, but avoided them because of the mangrovesí historically unwholesome reputation. Mangrove forests were known as tropical swamps, mosquito and fever ridden, that harbored unpleasant animals such as crocodiles. They were also extremely difficult to traverse on foot. In 1723 the French explorer Dampier summed up the mangles of vegetation with this quote: ìWhere this sort of tree grows, it is impossible to march by reason of these stakes, which grow so mixed one among another that I have, when forced to go through them, gone half a mile and never set my foot on the ground, stepping from root to root.î However, despite the awkwardness of European explorers, other peoples in history have experienced a different, sort of symbiotic relationship with mangroves. Throughout the history of Australia, the Aborigines have entered mangrove swamps to gather wood and to enjoy consuming the many available food sources associated with mangrove habitats.
Humans have always exhibited an interest in mangroves. Testimony to this fact are the several references which claim that more than 6,000 research reports and journal articles on the commercial and scientific interest in mangroves have been published. Prior to 1970, most mangrove research was out of curiosity. It wasnít until the 1970ís that environmental awareness increased and research began to be done with goals of management and conservation, due to devastating effects on mangroves caused by the third world urbanization going on at that time. Modern tropical ecologists find mangroves intriguing because they represent an interphase between two contrasting types of communities: terrestrial and marine. (Is that true Hays?). Mangroves are the border between land and sea.
A relatively simple, yet interesting topic concerning the biology of mangroves is the fact that they are mostly found in salty, or ìXericî habitats, and therefore have developed some interesting adaptations for coping with the saltiness of their homes. Mangroves can be called ìhalophytesî in the sense that they have physiological and anatomical adaptations for growing in salty environments. Some mangroves have salt-excreting glands on or near their leaves. Others exclude salt at the roots. In addition, mangroves often have to deal with stagnant water conditions. Lack of gaseous exchange in the substrate, whether it be thick water, mud, or muck, requires the mangroves to have special breathing structures on the exposed roots and/or trunk. Some have aerial prop roots bending down from either trunk or branches, while others have no elaborate physical structures. Rather, these mangroves have small air-breathing venticels on the trunk. Still others have shallow, subsurface roots with series of vertical, finger-like breathing roots, called ìpneumatophoresî that pop up out of the ground in groups around the tree. Roots are also important as support structures for larger mangroves, as their roots donít penetrate the floor material deeply, and the trees can grow up to heights of 40 meters in parts of the world. Prop root structures help hold the tree up. Trunk ìbuttresses,î which are seen in many rainforest tree species, are commonly associated with mangroves. A final interesting fact in the biology of some mangroves trees is their seedsí ability to germinate within the fruit while still on the tree, contrary to the normal act of falling to the ground first before germinating, which is exhibited in most trees. The seeds with this trait, referred to as ìviviparous propagules,î are capable of forming a new individual once separated from the original plant. In many species, the propagules are buoyant and are able to endure weeks, even months at sea. A pretty unique form of seed dispersal, indeed.
Two main groups of mangal vegetation can be recognized: Old World (Eastern Hemisphere) mangal, containing about 60 species, and New World (Western Hemisphere) mangal, which are less rich, containing only 10 species (Raffaelli 64). Of the approximately 69 species of mangrove plants in the world, 39 are notably located in Australia. Australia is the continent with the most species diversity amongst its mangroves, and for that reason is also the location for much of the mangrove research performed today. Contrary to the popular belief of those ignorant to natural science, mangroves arenít a single species. They arenít a single genera, or even a single family of plants. Rather, there is great diversity within the mangrove taxonomy, with 20 families from two plant divisions. Within these 20 families are 26 genera of ìmangrove plants or associates.î I say ìor associatesî in order for a broad interpretation, for there are 17 genera of true mangroves, and 9 other genera including non-mangrove species. You see, as mentioned earlier, it is hard to define what exactly constitutes an actual mangrove. Mangroves are better explained as a group of coastal plants with similar adaptations achieved by evolving in similar physical habitats. Two mangrove plants are not always necessarily related genetically. The types of mangroves and ìtheir associatesî at particular sites reflect the physical environments of those sites, rather than close relations between each other. It may sound confusing, but there are many genetic differences between mangal plants. Perhaps this concept can be explained in the following sentence: a plant looks like a mangrove because it grows in a mangrove habitat.
Mangrove forests are best developed on tropical shorelines where there is an extensive and suitable ìintertidal zone,î with an abundant supply of fine-grained sediment. The intertidal zone, which is the favored habitat for mangroves, spans from the area of lowest tide up to the highest tide mark, and is also called the ìlittoralî zone. In general, mangroves do best where the temperature never drops below 20 degrees Celsius, although some species can tolerate temperatures as low as 10 degrees Celsius. Mangroves also appreciate areas with high rainfall, or an abundant freshwater supply of water through run-off or river discharge. Mangrove forests do best in tropical estuaries which receive heavy rainfall evenly distributed throughout the year. Mangroves generally enjoy low energy, muddy shorelines, particularly tropical deltas. However, they can grow on a wide variety of substrates (ground material), including sand, volcanic lava, or carbonate sediments (Robertson 8). As you move up the intertidal zone towards the high tide mark, mangroves typically tend to grow larger, adding much structural complexity and biological diversity to the shore.
Many textbooks give generalized accounts of species zonation patterns in mangrove habitats. Species zonation is the change in species as you move across an environmental gradient, such as how tree species change as you go up a mountain. Really there is no simple, cut and dry zonation pertaining to mangrove habitats; often large areas are dominated by a mix of mangrove species (Raffaelli 66). Rather than recognizing particular species zones in the case of mangroves, it is probably more important to recognize major vegetation types, taking the entire habitat and all the species into account. There are six major vegetation types for mangroves: overwash forests, basin forests, fringe forests, riverine forests, dwarf mangroves, and mangrove hammocks. In a way, these are actually types of mangrove habitats, characterized by the size of the trees and where they are in relation to the water.
The basic climatic factor governing the geographical distribution of mangroves is probably air temperature (Unesco 18). Virtually all mangroves can be found at low latitudes, where the environment is warm and humid. For example, the number of mangrove species declines from about thirty in equatorial Indo-Malaysian countries to only one species north of the African Red Sea coast at 27 degrees North. Most mangroves are found between the latitudes of 25 degrees North and 25 degrees South. Also, it might be noted that more than 50% of the worldís 100,000 square kilometers of mangrove forest are found in the Eastern Hemisphere, or ìOld World.î Although mangroves grow throughout the tropics across the world, particular regions are noted for the very broad extent of mangal. Typically these are the estuaries of large rivers. Examples are at the mouth of the Ganges River in Bangladesh, the Fly River in Papua New Guinea, and the Mekong Delta in Vietnam. The Florida Everglades is a drainage basin that gradually changes from fresh water to an extensive mangal as it meets the sea. Notably the two largest tropical rivers, the Amazon, and the Congo, donít develop extensive estuarine mangal for physiographic reasons.
Mangroves provide energy for their respective ecosystems. The organic material in freshly fallen mangrove leaves can be utilized by other organisms. So can the organically rich sediments around the tree roots, which support a number of crustaceans and mollusks. Mangroves also provide shelter for organisms, both above ground and underwater. The thick tree roots provide a surface for attachment of sessile marine invertebrates, and the nooks and crannies provide hiding for fish and smaller organisms. The tree canopies support a diverse insect fauna as well as roosts and nest sites for insectivorous and piscivorous birds. Larger mammals and crocodiles are a feature at some mangrove forests, particularly in the Indo-Pacific region. Mangroves provide nurseries for juvenile fish as well, because of their high biomass of available food and the refuge the roots offer from predators. Mangroves prevent erosion with their thick, extensive root systems, by holding sediments together. Such an extensive an tangled root system that the world record for the 100 yard dash through a mangrove forest is said to be 22 minutes and 30 seconds. The thick tangles of vegetation also protect the inland from tropical storms by dampening the power of waves and blocking wind. Although they are battered and damaged by such storms, they always grow back. No man-made coastal barrier is capable of self repair as is the mangrove.
In the last 50 years, mangroves have been in decline across the globe. Their uses to humans are numerous. Hundreds of products are derived from mangrove trees, from charcoal, to construction materials, to incense, to hair oils and honey. From them come many goods. There is even an alcohol derived from the Nypa palm mangrove associate that can be turned into transport fuel. The interesting flora and fauna associated with mangroves provide opportunities for scientific study, tourism, and education, such as in this class. Mangroves are important in the maintenance of commercial offshore stocks of fishes. It has been argued that five of the six most important commercial fishery species in the U.S. may be dependent on intertidal zones, such as mangrove habitats. Unfortunately, it is human exploitation that is leading to the demise of the mangrove. In Thailand, more than 70% of mangroves have been destroyed. Destroying factors, which mostly take place in third world countries looking to make quick profits, include several forms of pollution and aqua-culture development, such as clearing the mangroves for agriculture, mariculture ponds, and salt production, and the over exploitation of mangroves for building materials and for fuel. The future of mangroves is questionable. Many believe that through research we will find better ways to manage these unique mangrove aqua-culture systems. Others believe that what needs to be given to third world countries is the money, the technology, and the education necessary for people to achieve sustainable-yield practices. In some mud flats, degraded forests, and shrimp culture areas, mangrove replanting has been initiated. Mangrove rehabilitation around the globe is also being undertaken by government agencies, backed by the research of private sectors. Local communities have even taken steps in conserving mangrove forests, as we finally begin to realize the importance of this unique ecosystem to the global web of life. These environmentally friendly practices and attitudes are ultimately what is necessary if we are to preserve mangroves for future generations.


Bibliography

1. Raffaelli, David. Intertidal Ecology, Chapman and Hall 1996
2. Robertson, A.I. Tropical Mangrove Ecosystems, American Geophysical Union, 1992
3. Saenger, P. Global Status of Mangrove Ecosystems, IUNC 1983
4. Savage, Thomas. Florida Mangroves: A Review, Marine Research Laboratory, 1972
5. Teas, H.J. Physiology and Management of Mangroves, Dr. W. Junk, 1984
6. Tomlinson, P.B. The Botany of Mangroves, 1986

Websites:
1. http://www.biol.andrews.edu/everglades/plant/plant.htm
2. http://www.earthisland.org/map/mangee.htm
3. http://www.aims.gov.au/ibm/pages/outline.html
4. http://possum.murdoch.edu.au/~mangrove/
5. http://alf.zfn.uni
bremen.de/~kontakt/projects/madam/background.htm
6. http://www.agri-aqua.ait.ac.th/mangroves/Thquality.html


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