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Mangrove Forest Distribution, Disturbances, and Conservation
Mangrove (mangal) forests are one of the most simply diversified ecosystems that thrive in the New and Old Tropics. Most ecosystems that persist in the tropics are highly diversified and highly productive, not to say that mangal do not subscribe to the latter. Historically mangrove ecosystems have used the Quaternary period as an environmental vessel to spread themselves throughout the globe where they are distributed presently (Woodroffe and Grindrod, 1991). In the Caribbean, where most of this paper deals with, a forest consists of at most seven, but mainly four species of mangroves. This pattern is due to several factors that make up the environmental characteristics that are essential for mangrove habitats and human and natural perturbations that affect that environment perpetually. Human-induced effects can be classified into four categories: Reclaiming land for industrialization, urbanization, and development, Extraction of mangroves for economic purposes, climate change, and pollution such as oil spills (Ellison and Farnsworth, 1997). Natural perturbations include hurricane damage, which could be change in intensity due to climatic change, and lightning strikes, while sometimes mangal can be introduced as a species and have a direct effect on the surrounding environment (Allen, 1998). Conservation methods for mangrove forests have not been the most promising and thorough, even though there are eleven international treaties that mangrove could fall under, governments have neglected to include them (Ellison and Farnsworth, 1997). This paper aims to describe how mangroves have been distributed globally, identify disturbances that have propagated themselves, and how to conserve an ecosystem that provides many goods and services to many populations. A lot of the ways in which this paper integrates all of these thoughts is by identifying issues through three categories used by Field et al (1998) to understand what is most important. They divide ecosystemic function into three categories: biogeochemical, ecological, and anthropocentric processes. What is critical in understanding these three categories is how they overlap. This requires an interdisciplinary approach to diving into all of the issues that surround mangrove distribution, disturbances, and conservation, and furthermore I only hit on the surface of some of these issues.
Introduction. At the most basic level of identifying mangroves you can split them up into three categories: Fringe, Riverine, and Basin. Of the seventy mangrove species that exist (Duke et al., 1998), at most 7-10 are found in the Caribbean and surrounding countries and of those 7-10 four are most abundant. Rhizophora Mangal (Red), Avicennia germinans (Black), and Laguncularia racemosa are most commonly found in the Caribbean and one species is found on the Bermuda Islands. R. Mangal is an example of a riverine spp. while A. germinans is an example of a fringe spp. These three species of plants can tolerate different amounts of water and soil salinity in different geoforms. In a study conducted by Lopez-Portillo and Ezcurra (1989) to test the three species for salinity tolerance in two different geoforms (interdistributary basins and mudflats) in a bay on the Gulf of Mexico. It was found that Avicennia germinans were more abundant than the other two spp. in the mudflats (more saline, closer to the saline ocean). Duke et al. studied three different global stations and found in Panama that on the Pacific coast L. racemosa were found as far out as A. germinans. This could be due to many factors including different salinities and depth of outlet near the estuary or coast, and ocean currents.
Distribution. The distribution of mangrove ecosystems has been thoroughly documented as not being able to have a transoceanic pattern of dispersal along long distances. Most scientists would agree that mangroves have a Southeast Asian center of origin. What scientists do not always agree about is how mangroves have been introduced to certain islands. Some use evidence from indigenous people to describe introductions through native use, but there was R. Mangle-type pollen found in Holocene peat samples that predated Tongan settlement (Ellison, 1990). It has been discussed that from the Southeast Asian center of origin mangrove forests have distributed themselves around the globe and have been documented to establish themselves between the tropics of Cancer and Capricorn, and rarely are they found above this latitude. Some argue that sea surface temperature (SST) is the biggest limiting factor to mangroves while others argue that extreme climatic changes are a true limiting factor (Woodroffe and Grindrod, 1991). Woodroffe and Grindrod use oxygen isotopic evidence from benthic foraminifera to identify temperature oscillations of the last 240 kyra to explain how coastal shores receive so much sediment runoff from streams that they should be able to sustain these glacial-interglacial cycles. They data they use suggests that the ocean level was as much as 130m (18,000 yra) below and 6m (124,000 kyra) above present day levels. Islands that presently have healthy populations of mangroves therefore must have gone through local extinctions and recolonization. The only way to describe this recolonization therefore is to accept transoceanic dispersal of viable propagules. Propagules are the buoyant flowers that mangroves emit to the water to reproduce and regenerate mangrove stands. Some argue that propagules are not capable of transoceanic dispersal while others argue it is. This seems the only method feasible when explaining the survival of mangroves and the global distribution throughout multiple glacial-interglacial cycles.
Disturbances. Of the many disturbances that mangrove communities face today there are two categories, natural and anthropogenic. Of the two the natural seem to outweigh the anthropogenic on a local and regional scale, but globally the scientific community seems to be more attuned to anthropogenic disturbances. Hurricanes seem to be the most documented natural (abiotic) disturbance that effect mangrove ecosystems. A study done by Smith et al. (1994) examined the affects of hurricane Andrew on two parks in Florida. What they found had very harsh implications. They took aerial photographs and found the most devastated areas and then went and sampled leaf area index, dbh, (diameter at breast height) and measured root damage to understand the damage done to the mangrove community. They found a number of things, but most notable were the types of mortality rates for the various heights and dbh. They found that the under story trees that most likely were growing up from a gap in the canopy and an increased amount of energy was reaching them from the sun. Because these young trees were now getting full sun light and the soil was not now being drained of all of the nutrients normally being cycled by the mangrove forest, it is now feared that some spots may never be able to host plants again due to the acidic nature of the soil.
Ellison and Farnsworth, (1996) studied the anthropogenic disturbances that occur in the Caribbean to mangrove communities. Some topics that they omitted were upland forestry drainage and road construction maintenance. The three they did include go from local to global in scale and described the 80Õs as the break point between past and present. They labeled four different classes of anthropogenic disturbances and gave some conservation methods and ideas. The first was extraction of mangrove trees. Some of the Caribbean states may have laws against the extraction of mangal, but it is poorly enforced. They also spoke of oil spills as a source of pollution that damages mangrove development and propagule viability; other pollutants include agricultural runoff and sewage. The last two topics climate change and reclamation of land for non-extractable reasons are of most importance due to their relation to human nature. Both the former and latter could be avoided if western influence werenÕt so materialistic. Climate change, which isnÕt as thoroughly studied could effect mangroves by soil warming which could cause increased soil respiration, peat decomposition, CH4, and H2S release, and root turnover. They also discussed the importance of using international treaties to encompass mangrove ecosystems as a conservation method that politicians and the like have omitted from policy.
The distribution of mangroves through transoceanic dispersal and the ecosystems persistence through multiple glacial-interglacial cycles says much to the durability of the community as a whole and to the importance it has in cycling nutrients through biotic and abiotic environments. We must not become careless with the importance of these types of ecosystems. The persistence of such a community may have many goods and services that we know and some we may not know that could be integral to the survival of certain life forms (Ewel et al. 1998). An approach to conservation, education, and understanding is crucial to investigating and prolong the many biomes that exist on earth.
Katherine C. Ewel; Robert R. Twilley; Jin Eong Ong. (Jan., 1998). Different Kinds of Mangrove Forests Provide Different Goods and Services
Global Ecology and Biogeography Letters, Vol. 7, No. 1, Biodiversity and Function of Mangrove Ecosystems. pp. 83-94.
Christopher B. Field; Julie G. Osborn; Laura L. Hoffman; Johanna F. Polsenberg; David D. Ackerly; Joseph A. Berry; Olle Bjorkman; Alex Held; Pamela A. Matson; Harold A. Mooney. (Jan., 1998). Mangrove Biodiversity and Ecosystem FunctionGlobal Ecology and Biogeography Letters, Vol. 7, No. 1, Biodiversity and Function of Mangrove Ecosystems. pp. 3-14.
Aaron M. Ellison; Elizabeth J. Farnsworth. (Dec., 1996). Anthropogenic Disturbance of Caribbean Mangrove Ecosystems: Past Impacts, Present Trends, and Future Predictions. Biotropica, Vol. 28, No. 4, Part A. Special Issue: Long Term Responses of Caribbean Ecosystems to Disturbances. pp. 549-565.
Norman C. Duke; Marilyn C. Ball; Joanna C. Ellison. (Jan., 1998). Factors Influencing Biodiversity and Distributional Gradients in Mangroves. Global Ecology and Biogeography Letters, Vol. 7, No. 1, Biodiversity and Function of Mangrove Ecosystems. pp. 27-47.
Colin D. Woodroffe; John Grindrod. (Sep., 1991). Mangrove Biogeography: The Role of Quaternary Environmental and Sea-Level Change. Journal of Biogeography, Vol. 18, No. 5. pp. 479-492.
James A. Allen. (Jan., 1998). Mangroves as Alien Species: The Case of Hawaii. Global Ecology and Biogeography Letters, Vol. 7, No. 1, Biodiversity and Function of Mangrove Ecosystems. pp. 61-71.
J. Lopez-Portillo; E. Ezcurra. (1989). Response of Three Mangroves to Salinity in Two Geoforms. Functional Ecology, Vol. 3, No. 3. pp. 355-361.
Thomas J. Smith, III; Michael B. Robblee; Harold R. Wanless; Thomas W. Doyle. (Apr., 1994). Mangroves, Hurricanes, and Lightning Strikes. BioScience, Vol. 44, No. 4, Hurricane Andrew's Sweep through Natural Ecosystems. pp. 256-262.
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