A swim in a tropical stream, Corcovado Natl Park.
As far as plants and animals are concerned mangroves are considered to be a long time native of Florida. The term “mangrove” refers to halophytic species of trees and shrubs. The word halophytic can then be further broken down to the word halophyte, meaning plants growing in saline soils (Odum, McIvor, Smith, 1985). This loose definition is what then allows the word mangrove to be a catch-all term.
In the state of Florida it is estimated that over 460,000 acres of mangroves exist (Odum, McIvor, Smith, 1985). And according to recent surveys ninety percent of the mangroves are located in four counties located in southern portion of the state (Odum, McIvor, Smith, 1985). The two counties of Monroe and Dade make up 68% percent of that total while at the same time containing one of the largest cities in the Unite States. But unfortunately those numbers are decreasing as more people force the development of previously uninhabited lands. Over the past decade the state of Florida has seen one of the fastest growing populations in the country. Fortunately most of the area that is covered by the mangroves is owned by the Federal government which allow for some form of protection. Today mangroves can be found throughout a majority of Florida’s coastline ranging up to Cedar Key on the West Coast to St. Augustine on the East Coast. The range of the mangroves has remained much the same over the past 50 years with the largest changes coming in the form of density and continuity. It has been noticed that large areas around Tampa Bay and Cape Canaveral have disappeared due to a variety of reasons (Florida’s Mangroves 1999).
The vast areas of mangrove forest that still exist today in the state are credited with preserving the overall health of the states coastal zones. Mangrove systems allow for important materials such as chemical elements and nutrients to be trapped and then cycled. The root systems of the mangroves not only promote population growth of marine organisms but they also can help in the preservation of coastal areas during tidal surges and other severe storm events.
Types of Mangroves:
Worldwide there are over 50 species of mangroves that exist. Of those 50 species only three exist in the state of Florida. The species include the red mangrove Rhisophora mangle, black mangrove Avicennia germinans, and the white mangrove Languncularia racemosa. Each of the three species can be found in different areas or zones of the coastal area due to their adaptations. The succession of the three species of mangroves follows in order, from the waters edge inland as followed; the red mangrove, black, and then the white (Odum, McIvor, Smith, 1985).
Due to its close proximity to the waters edge most people are probably most familiar with the red mangrove. Rhisophora mangle is one of more than 70 species of in 17 genera in the family Rhizophoraceae. The tree can reach a height of 80 feet. It has thin grey bark covering dark red wood. The leaves of the tree are usually 1 to 5 inches in length and bluntly pointed at the apex or tip. Leaves are also shiny, with a deep green color on top and duller green tint on their underside. Its root system is perhaps its most identifiable feature. The roots called “prop roots” arise from the trunk and branches. These roots are what form the intricate maze that many marine organisms use as a nursery in juvenile stages of there life. To some these roots have earned the mangroves the titles of walking trees. These prop roots due provide more than just support for the tree, they allow for the tree to obtain oxygen in the harsh anaerobic environment. The roots of the red mangrove extend above the substrate surface over a meter which allows them to obtain oxygen. These roots contain many small pores called lenticels which allow the oxygen to enter the roots and then diffuse into the plant down to the underground roots (Odum, McIvor, Smith, 1985). This process only takes place during low tide events and due to the highly hydrophobic nature of the lenticles water is blocked during high tide. To deal with the high saline conditions of their surrounding the red mangrove has developed a way to exclude it by a system referred to as “reverse osmosis”. Roots are able to exclude the salt from the water by a ultra-filtration system that is powered by the negative pressure produced by transpiration on the surface of the leaves. It has been found that the water within the tree has a concentration of salt 1/70 that of the surrounding salt water.
The black mangrove Avicennia germinans occupy the second zone moving up away from the waters edge (Odum, McIvor, Smith, 1985). Avicennia germinans is a member of the family Avicenniaceae. The tree can reach a height of 64 feet and has dark scaly bark. The leaves of the tree are 2 to 4 inches in length and are narrowly elliptic, shiny green on the top side while covered with dense hairs on the bottom. In many cases the leaves are covered with salt. Black mangroves are most easily identified by the numerous projections, called pneumatophores, that protrude from the substrate around the base of the tree. During low tide, air enters the pneumatophores and then makes its way into the living root tissue (Odum, McIvor, Smith, 1985). To help combat the highly saline conditions the black mangrove must also get read of the salt. Black magroves secrete the salt rather than excluding it like the red mangroves. In this process salt is excreted through glands on the surface of the leaves. This process is thought to be enzymatic rather than physical such as in the red mangroves. Salt secreting mangroves tend to have a internal salt content 10 times higher than salt excluding species such as red mangroves.
The third species of mangroves that exist in Florida are the white mangroves, Languncularia racemosa that exist in the upper most zone (Odum, McIvor, Smith, 1985). White mangroves are just one of 450 species of plants in 18 genera of the family Conbretaceae. Languncularia racemosa can grow in the shape of a tree or a shrub depending on its location. It can reach a maximum height of 49 feet and has flat oval leaves up to three inches long and rounded at both ends. This species of mangroves does have large prop roots or pneumatophores to facilitate the uptake of oxygen but rather a series of lenticles on the lower portion of the trunk. To aid in the removal of salt from the plant the white mangroves use the same technique as the black mangroves. Again salt is actively excreted through the special glands on the surface of the leaves.
Importance of Mangroves:
The relationship that exists between mangroves and associated marine organisms is extremely important and complex. Roots of mangrove forests provide a haven for a multitude of marine species during their juvenile stag and in some cases then entire life. But besides acting as a safe haven for maturing organisms they can also protect other even more intricate systems such as coral reefs and sea grass beds. Aside from the direct linkage that mangroves have with the species that live within them is the sometimes less obvious relationship that exists between them and humans.
Multiple studies have proven the advantages that mangrove forests offer when organisms are in a vulnerable juvenile stage. It is assumed that mangroves function as nurseries for a number of reef fish species. The benefits of the mangroves include greater food availability to lower predation rates. Studies have suggested that the reduction of predation rates in nursery habitats is related to the greater structural complexity of the habitat and also the increased turbidity caused by the structures. It has been suggested that fish hiding in the shade of cover can more easily spot predators while at the same time decreasing their detect ability (Moriniere, Nagelkerken 2004). Other studies have shown that the reduction of predation is perhaps more important than the sometimes increased food availability offered by the mangroves. One such study used AMUs (artificial mangrove units) in which differing amounts of pvc pipe were attached in a fixed pattern and the amount of fish was counted at each of the different treatments (Mumby, Alasdair 2004). The results concluded that as the number of pvc piping increased so did the number of associated fish (3). Data has also showed the direct relationship between reef fish and the use of the mangroves. Studies in Belize have showed that the dependency that reef fish have on mangroves even effects those populations where reefs are located many miles away across much deeper water (Mumby, Alasdair 2004). This same study suggests that mangrove may enhance fish biomass in two ways. First the efflux of detritus and nutrients may increase the primary production ability of neighboring systems. Secondly, as mentioned earlier the additional refuge reduces predation (Moriniere, Nagelkerken 2004).
Mangrove systems also provide food and cover for other organism than fish. Wading birds, spiny lobsters, and white tailed deer also benefit from the existence of mangrove forests. Many different birds such as egrets, wood stork, white ibis, and herons regularly nest within the different zones of the mangrove system. So it can be realized that mangroves actually offer some type of economic value. This holds true because many people visit the state of Florida to bird watch and enjoy the other organism that inhabit the forests. Some estimates indicate that the economic value of mangroves are approximately $4000-$30000 per acre (Florida’s Mangroves 1999).
Perhaps a less apparent but just as important role of mangroves are there ability to protect the coastline against erosion and storm surges. In many areas around the world it has been well documented that mangroves of all species play an important role in flood control. Along India’s coast the areas of Mangroves that still exist have been accredited with saving thousands of lives. Local scientist insist that even though many acres of mangroves can be destroyed during a single tsunami, the amount of wave energy they can dicipate is substantial. Both scientists and government officials have begun to work together in attempts to reduce mangrove destruction and promote their growth. They hope that increasing the amount of coastline inhabited by mangroves can help reduce the impacts of future tsunamis and other similar events. The ability for the mangrove systems in Florida to trap, hold on, to in some cases stabilize the inter tidal zone has allowed them to be labeled “land builders” (Kremmer 2005). Also worth mentioning in this category is the ability of the mangroves in absorbing many of the chemicals and toxins that humans emit on a regular basis. As mentioned in the Enchanted Braid this action can prevent many chemicals from reaching more complex systems such as coral reefs that are located miles off shore.
Problems Faced by Mangroves:
As with most natural systems susceptibility to disease, predation, and destruction are inevitable. Mangrove systems across the earth face many of the same perils just perhaps in different amounts. Perhaps the most important area to discuss refers to the mostly negative impact that human activities have the mangroves. Either through direct contact or by indirect we have caused a severe reduction in the amount of mangroves present today. In most instances it is our desire to take other wise prestine land and turn it into something that is more “profitable”. Through the building of homes and businesses our impacts are increasing daily.
Within the state of Florida, as mentioned earlier, certain locations have seen drastic changes in development and therefore mangrove loss. Tampa Bay, located on the west coast, has probably seen the worst of it over the past 100 years. The port in Tampa is now one of the largest ports in country (Cardozo, Hirsch 1991). It has been documented that just within the bay there has been a 44% loss in coastal wetland area which of course includes mangroves. Even areas such as Charlotte Harbor, a much less urbanized area, have seen over a 50% decrease in mangrove acreage (Cardozo, Hirsch 1991).
Situations such as these are of course not isolated and also come in different forms. One study in Queensland, Australia found that habitat destruction was not the problem but rather the harmful practices of humans miles away from the destruction. This case of mangrove mortality involved the release of a toxic herbicide into waterways containing the mangroves. The chemical Diuron, used as a anti-fouling paint in boats, was released in the water for years before the mangrove die-back was large enough to be noticed (Shearer 2004).
Herbivory and disease also account for some of the annual mangrove loss in the state. Direct herbivory of mangrove leaves and buds is highly variable between areas but can be damaging. Organisms including white-tailed deer, mangrove tree crab, and insect larvae are the main culprits (Odum, McIvor, Smith, 1985). While exact calculations of the total damage that herbivory poses to mangroves is not know many feel that current numbers underestimate the total impact. Scientist site that the larval development of the olethreutid moth can cause the entire loss of leaf. Mangrove diseases are rare and therefore not well documented. Reports have shown that the black mangrove has been affected by pathogenic fungi (Odum, McIvor, Smith, 1985). These fungi can cause lesions on the leaves as well as the formation of galls on the stems.
Mangroves are an interesting and an essential part of Florida’s coastal ecosystem. These plants through their unique characteristics are able to survive in habitats that few other plants can exist. The three species that are found in the state of Florida have each evolved different ways of dealing with the lack or oxygen and the saline conditions. With there dense and complex root systems many fish use the area as a nursery during the early stages of their life. Structure and cover that is provided by this system of roots is perhaps the most important function of the mangrove systems.
But as mentioned in the paper these mangrove systems are in constant threat both by human induced activities as well as natural activities. Increasing population growth has lead to increased amounts of contamination reaching the water of the mangroves as well as the actual physical removal of them. Many areas along the coast significant losses in mangroves has been witnessed over the past century. In areas such as Tampa over 50% of the original mangrove areas has been destroyed or loss.
1.) Cardozo, Y., Hirsch, B. Hurricane Andrew and mangrove forests. Sea Frontiers Aug 1991, Vol. 37 Issue 4 pg.32
2.) Kremmer, Janaki. A natural, low-tech solution to tsunamis: mangroves. Christian Science Monitor, 2005 Vol. 97, Issue 32.
3.) Moriniere, E. Cocheret, Nagelkerken, Meij. What Attracts juvenile coral reef fish to mangroves: habitat complexity or shade? Marine Biology, Vol. 144 2004. 139-145.
4.) Mumby, Peter, Alasdair, J.E. Mangroves enhance the biomass of coral reef fish communities in the Caribbean. Nature, 2004 Vol. 427, pg. 533-561.
5.) Odum, W.E., C.C. McIvor, and T.J. Smith, III. The ecology of the mangroves of south Florida: a community profile. U.S. Fish and Wildlife Service, Office of Biological Services. 1985 144pp.
6.)Shearer, Heather. Herbicides threaten north Queensland’s coastal mangroves. ECOS
119 Apr-Jun 2004 pg. 32-33.
7.) “What are Mangroves?” Florida’s Mangroves. 1999. http://www.floridaplants.com/horticulture/mangrove.html (5 June 2005).
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