Mangroves of Florida and the Bahamas Final Draft

This topic submitted by Karla Peters ( petersk4@miamioh.edu) at 6:02 PM on 6/6/06.

Andrea and Julie smoke some of Fidel's finest. See other pictures from Costa Rica.

Tropical Field Courses -Western Program-Miami University



Karla Peters
GLG 421
Cummins
5 June 2006

Mangroves of Florida and the Bahamas
Mangroves are a vital component of tropical ecosystems around the world. There are fifty-four species of mangrove from sixteen different families; the diversity of mangrove species suggests that their unique characteristics are the result of convergent evolution in response to similar habitat dynamics, namely saline, waterlogged environments (Hogarth, 1999: 2). The mangroves we will encounter during our exploration of Florida and San Salvador are limited to three families, including: Acanthacea (genus Avicennia), Combretaceae (genera Laguncularia and Conocarpus), and Rhizophoraceae (genus Rhizophora) (Lacerda et. al, 2001: 14-15). The two genera Avicennia and Rhizophora are the most common in the Caribbean and the world, each comprised of eight species, and are part of every mangrove ecosystem on earth (wikipedia.org; Hogarth, 1999:2).
Mangroves are found at the various interfaces of terrestrial and marine habitats and must, as mentioned above, be adept at surviving saline, waterlogged conditions that most other plants cannot tolerate. Though the range of tolerable salinities and lack of oxygen (the real problem associated with waterlogged soil) varies between species, all mangroves must cope with these hostile conditions to some degree and so have developed common survival strategies. Though mangroves have evolved to tolerate a higher salt content within their tissues, they still could not survive without mediating the amount of salt in the xylem; therefore, mangroves employ several (though not all clearly understood) different methods of keeping excess salt out of the trees’ systems. Many mangrove roots are able to block the absorption of salt by manipulation the properties of osmosis: during the transpiration process mangroves are able to produce negative hydrostatic pressure, which draws water molecules into the roots while keeping salt ions out (Hogarth, 1999:12). Excess salt that nevertheless accumulates in the mangrove’s tissues can be eliminated via specialized salt glands on the leaves, roots, or bark of the tree (Hogarth, 1999:13). Black mangroves (Avicenniea germinans) can be easily identified by “the taste test” (Lodge, 2005: 82), when one licks the leaf and can taste the salt that accumulates there from the glands on the surface of the leaves. Lastly, salt that remains in the tree seems to often be limited to the plant cell’s vacuoles and is not prevalent in the cytoplasm of the cell; this keeps all of the cell’s enzymes at optimal levels, since “high salt concentrations are known to inhibit many enzymes” (Hogarth, 1999: 13).
Mangrove adaptations to low oxygen levels are what make mangrove swamps potentially hazardous places, particularly for the ungainly or clumsy: aerial roots and pneumatophores. These structures enable mangroves to acquire oxygen from the air when it is unavailable through the soil because of waterlogged conditions. Both of these structures are covered with lenticels, pores that essentially breathe for the tree, supplying oxygen to the underground roots trapped in waterlogged (and therefore oxygen deprived) soil. The mangrove species Rhizophora mangle, or red mangrove, which we will see in Florida, is well known for its aerial roots. Often called prop roots because of the way they seem to support the tree, these roots deviate from the main trunk high above the soil surface, up to two meters high (Hogarth, 1999: 5). Red mangroves rely on aerial roots because they grow at the lowest zone, closest to the shore line, and the soil in which they grow is constantly waterlogged due to incoming tides (“Florida Mangroves”). Aerial roots also help the red mangrove to remain firmly establish in the soil, which is constantly battered by incoming and outgoing tides; as Lodge (2005: 81) writes in The Everglades Handbook, red mangroves live “where survival requires a firm grip.”
Pneumatophores, unlike aerial roots, do not appear to even be part of the same tree when viewed from the surface; however, they function exactly as aerial roots do. Black mangroves (Avicenniea germinans) and white mangroves (Languncularia racemosa) of Florida utilize this type of root structure, in which roots growing horizontally and close to the soil surface produce knee-high vertical projections—pneumatophores—every fifteen to thirty centimeters (Hogarth, 1999: 8). The result is a collection of what appear to be independent sticks growing in the general vicinity of the actual tree itself. Pneumatophores can be quite numerous, with a two to three-meter black mangrove having as many as ten thousand, and can also be quite tall, the largest being around three meters high (Hogarth, 1999: 8).
The special adaptations that mangroves have developed to cope with their uncompromising environments make them particularly bothersome in terms of human coastal development: they block the view of the water, their roots get in the way of everything, and they anchor in sticky, stinking mud. Davidson (1998: 67) picks an apt quote, from a New Zealand novelist, to describe the history of human contempt for mangrove habitats: “Growing always in shallow stagnant water, filthy black mud, or rank grass, gnarled twisted, stunted and half bare of foliage, they seems like crowds of withered, trodden-down old criminals…” These aesthetic reasons account for the loss of thousands of hectares of mangrove habitat; over fifty percent of all mangroves have been destroyed by humans in the last century (Lal, 2001: 235). But mangroves are incredibly beneficial, both to the natural ecosystem in which they exist and to man-made habitats. For example, the very existence of a mangrove swamp acts to trap sediment that may otherwise be washed out to sea; this may be one reason why coral reefs are able to thrive off of shores where mangroves are allowed to grow. According to Davidson (1998: 70), “Mangroves filter sediments and pollution coming from land, providing a buffer zone between the terrestrial and marine environments.” Because corals rely on low nutrient levels to prevent algal blooms, mangroves are a key building block in building and maintaining a coral reef, the most diverse ecosystem found on the planet (Davidson, 1998: 5).
Mangroves provide a rich habitat in and of themselves in addition to the role they play with coral reefs. A vast number of fish, reptiles, birds, crustaceans, and mammals rely on mangroves for some reason or another, be it for shelter (during part or all of the life cycle, or seasonally) or sustenance (mangrove habitats are rich in both plant and animal snacks). In Florida, mangroves provide for almost two hundred species of birds (Evel et al., 1998: 89); mangrove roots form a perfect habitat for juveniles of many fish species, which are too small to survive in their adult habitat of open ocean (Hogarth, 1999: 113). These are just a few of the many examples of plants, animals, and microfauna that call mangroves home and could not survive—at least not nearly so well—without them.
In addition to aiding in the survival of countless other species, mangroves are also quite beneficial to the human species. The presence of a mangrove swamp slows the rate of incoming flood waters, protecting human development further inland, as well as marking off the boundary of the natural tide zone and the point past which man should not consider building; mangroves also prevent coastal erosion with their matrix of sediment-trapping roots. (Evel et al., 1998: 89-90). As mentioned above, mangroves provide an ideal habitat for fish and crustaceans, such as shrimp, to feed and multiply; this is beneficial to humans because the presence of mangroves tends to increase the yield of fish or shrimp caught in an area for human consumption (Hogarth, 1999: 113). In fact, Ellison and Farnsworth (1996: 553) point out that, when mangroves are clear-cut, fishery yields drop at a rate that is nearly equal to the amount of mangrove habitat that was destroyed.
It would seem that the existence of human development and mangrove habitats are mutually exclusive; however, this is not necessarily the case. Mangrove habitats can actually benefit human development if it is carefully controlled. For example, in locations where some type of sewage treatment is required, but the building of a septic system is a complicated endeavor and requires the destruction of a mangrove habitat, the mangroves themselves can be used to treat waste water. Though Evel et al. (1998: 87) argue that there are many precautions to take when considering mangroves for this use, such as preventing the pollution of human food and water sources, it is possible and “is preferable to destruction, and wastewater disposal may even be considered as a tool in mangrove restoration.”
Mangroves can also be used for direct human benefit by way of tourism; for example, the mangrove swamp of Everglades National Park is “the largest contiguous mangrove swamp in the world” (Lodge, 2005: 79) and this claim to fame helps draw tourists into the park. Mangroves are, to most people that do not live in undeveloped tropical areas, an exotic plant that is unique and exciting. Playing up their interesting history and lifestyle may help to encourage the general public that mangroves are worth protecting and keeping.
There are many consequences with which to contend when humans begin clearing mangrove habitats. For example, the clearing of mangroves on Bimini Island, Bahamas, fifty miles off the coast of Florida, is expected to cause a significant drop in the numbers of sharks—more than a dozen species—that rely on the area for shelter from the open ocean (Eilperin, 2003). Bimini Island was cleared in 2003 to make way for a golf resort, which was built to boost tourism to the small island. Though the president of the Bimini Bay Resort and Casino claims that the development will not harm the shark and other populations around the island, Dr. Samuel H. Gruber of the University of Miami (who has studied the lemon sharks around the island for the past two decades) has found that lemon shark survival rates began falling when the lighter demolition was initiated; he does not have much hope for the lemon shark population now that the island has been cleared of its productive mangroves (Eilperin, 2003). And this is only an example of what happens when mangroves are cleared; more subtle but just as hazardous human activities, such as upland development and pollution (mainly in the forms of petroleum and thermal pollution (Ellison and Farnsworth, 1996: 553-554)), slowly smother and poison mangrove habitats and all of the species that depend upon them for survival.
Global warming may also have an adverse effect on mangroves: as the polar ice caps melt as a result of the greenhouse effect, the net sea level across the globe rises. Though the effect that rising sea levels will have on mangroves depends on their individual habitats, most scholars agree that if sea levels continue to rise at the rate at which they have been, many mangrove ecosystems will be destroyed as their original habitats are flooded and the land behind them is unavailable for colonization due to human development (Lacerda et al. 2002: 44; Dodd and Rafii, 2002: 85).
Mangroves are an important part of tropical ecosystems, both terrestrial and marine, and are intricately tied to coral reefs, the most diverse ecosystems on the planet. If the destruction of mangroves continues unabated, we will certainly be destroying not only the mangroves themselves, but also dealing a fatal blow to all of the precariously balanced environments around them. Though there is some hope remaining for mangrove survival, the current outlook is rather bleak. It is my hope that the knowledge that we gain on our trip has a ripple effect that may help to change the forecast for mangroves; if we all learn and understand how important these complicated trees are to the survival of the Caribbean as we know it, perhaps our knowledge and care can be passed on to others and, together, we can change the way we treat these all too often misunderstood trees.


Works Cited
Davidson, Osha Gray The Enchanted Braid: Coming to Terms With Nature on the Coral Reef
1998 New York: John Wiley & Sons, Inc.

Dodd, Richard S. and Zara Afzal Rafii, 2002 “Evolutionary genetics of mangroves: continental
drift to recent climate changes.” Trees 16 (2-3): 80-86

Eilperin, Juliet, 2003 “Waves of Marine Species Extinctions Feared.” The Washington Post 24
August, A01

Ellison, Aaron M. and Elizabeth J. Farnsworth, 1996 “Anthropogenic Disturbance of Caribbean
Mangrove Ecosystems: Past Impacts, Present Trends, and Future Predictions.” Biotropica
28(4a): 549-565

Evel, Katherine C.; Robert R. Twilley; Jin Eong Ong, 1998 “Different Kinds of Mangrove
Forests Provide Different Goods and Services.” Global Ecology and Biogeography
Letters 7(1): 83-94

“Florida Mangroves”

Hogarth, Peter J., 1999 The Biology of Mangroves New York: Oxford University Press

Lacerda, L. D.; J.E. Conde; B. Kjerfve; R. Alvarez-León; C. Alarcón; J. Polanía 2002 “American
Mangroves.” Mangrove Ecosystems, ed. Luiz Drude de Lacerda. New York: Springer

Lal, Padma Narsey 2002 “Integrated and Adaptive Mangrove Management Framework – an
Action Oriented Option for the New Millennium.” Mangrove Ecosystems, ed. Luiz Drude
de Lacerda. New York: Springer

Lodge, Thomas E. 2005 The Everglades Handbook: understanding the ecosystem Boca Raton:
CRC Press

“Mangroves.” Wikipedia


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