Evolutionary Adaptations of Mangrove Species to Their Harsh Environment

This topic submitted by Meredith Beck ( beckmn@miamioh.edu) at 1:12 AM on 5/17/08.

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Evolutionary Adaptations of Mangrove Species to Their Harsh Environment

Between the latitudes of 32 N and 38 S there are many diverse ecosystems, one of which is the coastal mangrove ecosystem. In tropical areas near the equator, the tidal regions of the coasts of many countries are protected by these mangrove buffer ecosystems. The beauty of such areas is remarkable, yet to understand their ecological value is to truly appreciate the mangroves. The mangrove trees, over time, have made significant evolutionary strides that have made the various mangrove species well adapted to the harsh coastal environment, yet the public does not often acknowledge their value.

The term mangrove can be used in three different senses, so it is essential that it be cleared what sense of the word this paper will be addressing. Mangrove can be used to refer to the entire coastal area and the ecosystem as a whole, but this can also be called a mangal for differentiation. Mangrove can also refer to the type of flora which inhabit this ecosystem and have structural similarities due to convergent evolution and not exactly due to close relation. However, there are many shrubs and trees that have the same adaptations to the salinity and anaerobic soil, so the word mangrove alone will be used to refer to the tropical tree families which make up the major components of the mangal (Lugo, Snedaker, 1961). The most identifiable mangrove tree would be the Rhizophora genus of the Rhizophoraceae family, with the most well known species being the red mangrove, or Rhizophora mangle, which is prevalent in the Costa Rican mangals.

One of the most amazing adaptations that mangrove trees, such as the red mangrove, have is the ability to use vivipary as a means of reproduction. Unlike most organisms of the plant kingdom, mangrove trees do not disperse seeds as a mechanism of reproduction, instead the seed continues to develop into a seedling while still attached to the parent plant. There is no dormant stage for seed because of the harsh environment that the propagule is dropped in to. However, the seedlings can grow to around seventy centimeters before dropping from the parent plant, so the seedlings are well equipped for survival by the time they hit the water (Tomlinson, Cox, 2000). This fascinating evolutionary adaptation is most developed in mangrove species and, as a group of Chinese scholars showed, vivipary has multiple evolutionary origins in different mangrove lineages, but they all converged to produce the same mechanism because vivipary is advantageous for their survival in the tidal regions (Shi, et al. 2005). While others previously had proposed that vivipary was necessary because of the introduction of the propagule into salty and deep water; Tomlinson and Cox (2000) concluded that, “Vivipary, at least in mangrove Rhizophoraceae…is related to a successful establishment strategy in tidally influenced habitats because it produces an elongated seedling that is also capable of long-distance dispersal.” The tide carries the seedling away so that it can take root someplace where the canopy is open, instead of in the shade of its parent tree. Then it is the specialized characteristics of the propagule, such as the ability to float and self erect when on land that are key for the survival of mangrove trees.

Once implanted however, mangrove trees have developed to survive the harsh environment. Most other plants could not survive on the coast of the ocean because of the salt levels in the water. The cells of most plants, when introduced to super concentrated salt water, lose water to their environment through osmosis, causing the plant to wilt. Mangal species are halophytes, but can also withstand the varying salinities of the brackish water that occurs where fresh water and sea water merge. Two mechanisms employed by the different mangrove species in order to withstand the salty conditions are salt secretion and ultrfiltration.

Salt secretors have specialized salt glands on their leaves that use energy to secrete sodium chloride. On the leaves, the salt evaporates and crystallizes, such that the crystals are visible on the leaf surface. In these plants the sap is one-tenth as concentrated as sea water, which means some salt is excluded at the roots. Non-secretors, however, have sap that is one-hundredth the concentration of sea water, which is still 100 times more concentrated than normal land plants (Tomlinson, 1986). The non-secretors filter the water they take in by selectively absorbing certain ions into the roots. At the same time, the surface area used for absorption is smaller in these mangrove plants due to the absence of root hairs. Some of the salt that ends up in the plant is lost at the leaf surface through transpiration, while the rest is stored in the leaves of the plant. Here the cells must increase in volume to take in the ions, thus the leaves become more succulent. Eventually the leaves will be dropped, thus the plant will be void of its highly concentrated parts (Tomlinson, 1986).

Mangrove plants also have to cope with the difficulties of the often anaerobic soil and high water levels that are encountered in the coastal areas they inhabit. The roots are the site of most of the oxygen intake of plants, so the trouble for mangroves is that the soil is often not well aerated and many times during the day the roots can be completely submerged in water. Therefore, aerial roots developed in mangrove plants becoming a prominent feature of mangals. Different species adopt different ways to aerate their roots; for this reason, prop roots, stilt roots, kneed roots, pneumatophores, and plank roots are all prevalent. Prop and stilt roots arise from the trunk and form flying buttresses at the base of the tree. Kneed roots are horizontal roots that sporadically grow vertically into a loop before continuing with horizontal growth. The loops are thickened and create distinct knobs along the ground (Tomlinson, 1986). Pneumatophores are extensions of roots that grow through the ground and stand vertically from the soil. Plank roots are horizontal roots that extend vertically along the length of the root. The aerial parts of these roots, specialized for mangrove environments, make the mangrove trees successful with respect to gas exchange. However, other parts of the root systems are specialized for anchoring the tree and absorbing nutrients in the loose soil, as well as the horizontal component which unifies the aerial and anchoring root parts (Tomlinson, 1986).

The survival method of mangrove trees is amazing and complex, yet through history humans have always been unable to see their value because they offer little direct use for mankind. The mangal areas have always been viewed with curiosity, but with little respect and often called wasteland and not seen as aesthetically pleasing (Lugo, Snedaker, 1961). As coastlines became more in demand for waterfront property, people ignored the importance of mangrove trees, seeing no problem in wiping them out. What people did not understand was that they were ruining a vital aspect of the coastal protection from the ocean and taking away a huge portion of biodiversity. “[Mangrove] ecosystems harbour 193 plant species, 397 fish, 259 crabs, 256 molluscs, 450 insects and more than 250 other associated species. Mangrove ecosystems have the highest level of productivity among natural ecosystems,” said experts from India (Upadhyay, Ranjan, Singh, 2002). On a worldwide scale, almost half of the mangrove ecosystems have be devastated.

The mangrove ecosystem may not be as appealing to the common people right now, but the long-term indirect economic value of the mangrove buffer areas is enormous. The mangrove roots keep shorelines intact, protecting from further erosion and loss of coastline when threatened by storms (Tomlinson, 1986). In the event of a hurricane, the mangrove ecosystem would take the blunt of the hit, acting as a buffer between the land, houses, and the ocean. Seeing as the mangroves hold such value in preventing property damage, they also carry an economic benefit. Mangroves can also tolerate a large amount of wastewater because of the fast rate of nutrient cycling and detritus decomposition. The protection mangrove ecosystems provide, together with the diversity of the area out weighs the developmental values the coastland could ever provide.

The mangrove ecosystem not only holds great value for humans, but also is a marvel of the way organisms can change to become adapted to new environments. From the details of mangrove tree reproduction, to the way the mangrove trees deal with salt, to the robust way the tree anchors itself and exchanges gas at the roots, all combine to create an environment that is a safe haven for other organisms. It is amazing to see how the smallest details of one species help it to survive, as well as helping to keep so many other organisms living. After all that the mangrove ecosystems provide, humans still oblivious to this fact, continue carelessly destroying tree after tree.

Works Cited
Janzen, Daniel H. (1983). Costa Rican Natural History. Chicago: The University of
Chicago Press. 273-276.
Lugo, A., Snedaker, S. (1961, July). The Ecology of Mangroves. Miami
Shi, S., Huang, Y. Zeng, K., Tan, F., He, H., Huang, J., Fu, Y. (2005). Molecular
Phylogenetic analysis of mangroves: independent evolutionary origins of
Vivipary and salt secretion. Molecular Phylogenetics and Evolution, (34)
Tomlinson, P.B. (1986). The Botany of Mangroves. Cambridge University Press.
Tomlinson, P.B., Cox, P.A. (2000) Systematic and functional anatomy of seedlings
in mangrove Rhizophoraceae: vivipary explained? Botanical Journal of the Linnean
Society, 134: 215–231.
Upadhyay, V.P., Ranjan, R. and Singh, J. S. (2002, December). Human-Mangrove
Conflicts: The Way Out. Current Science, Vol. 83, No. 11.

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