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A Brief Review of Mangrove Zonation Hypotheses
The term ÒmangroveÓ is best attributed to an ecological assemblage rather than a taxonomic assemblage. Mangroves are wet saline communities that dominate the worldÕs tropical and subtropical coasts paralleling the geographic distribution of coral reef systems. These detritus based assemblages survive in substrate ranging from freshwater inlets to hypersaline ponds. Mangroves serve as pioneer species building up substrate and habitats for a variety of species; and as a mature-phase species solidifying an assemblageÕs ecological niche.
Defining a ÒTrueÓ Mangrove
Tomlinson (1986) provides a convenient definition of a ÒtrueÓ or ÒstrictÓ mangrove comprised of five basic characteristics. First, a true mangrove must only propagate in a mangrove environment. Secondly, the species must play a major role in the structure of the community and must have the ability to form a pure stand. A true mangrove must also possess certain morphological and physiological specializations that allow it to survive in the mangrove environment. Finally, a mangrove must remain taxonomically isolated from terrestrial relatives.
Distribution
It is believed that mangroves once dominated about 75% of the worldÕs tropical coastline. There are 34 distinct true mangrove species between 30 degrees North and South of the Equator. Mangroves are limited in their distribution by climate, inundation by saltwater, salinity, tidal fluctuation, sedimentation, and wave energy. The majority of mangrove species are found in the Eastern group (Asia, India, East Africa, Australia, and the Western Pacific). Eight species can be found in the Western group (West Africa, the Caribbean, Florida, South Atlantic, Pacific North, and South America. There are five species endemic to Costa Rica. The three most common species are Rhizophora6 mangle (red), Avicennia germinans (black), and Laguncularia racemosa (white).
Zonation
Mangroves have been noted for their definitive spatial variation. Spatial variation in species occurrence and abundance has been noted in differing ecosystems throughout the world. Mangroves grow in monospecific bands parallel to the shoreline, very often with seagrass bed and coral reef system adjacent to the assemblage under the sea. A general description of mangrove zonation depicts a pattern that extends from shore to inland regions (usually higher in elevation). Tidal flooding, land elevation, and salinity are often attributed as controlling factors in mangrove zonation. As a general model, mangroves in the Western group often adhere to a Red-Black-White pattern of growth. This pattern of zonation among mangrove species has led to numerous hypotheses, each attempting to explain the assemblage growth model. There are six major categories of hypotheses attempting to explain mangrove zonation. The following is a brief exploration of these hypotheses.
1. Land building and plant succession (Davis, 1940).
The view that mangrove zonation occurs as part of a successional processes where in pioneer species give way to more mature species as substrate is built up. In this model, species first grow in the lowest intertidal zones and trap sediments. Over time, as substrate is established, new mangrove species are able to out compete the colonizing species. This process continues until the land is no longer an intertidal zone, but is part of the mainland. This hypothesis rests heavily upon the ability of the colonizing species to hold sediments to build up land. The Òzonationn represents successionÓ hypotheses is one of the older explanations of zonation processes in mangrove assemblages. The classical model of ecological succession does apply to mangrove ecosystems, however, it is not the result of land building.
2. Geomorphological influences (Thom 1976)
More recent research has revealed that mangrove communities often respond to geomorphological changes rather than create them. Research by Thom et al has revealed that mangrove vegetation is directly dependent on the dynamics of sediment topography. In this model, mangroves do not supercede abiotic land building processes. Long-term stratigraphic records from past deposits show the dependence of mangrove development geomorphological factors such as sea-level stability. Periods of rapid sea-level rise cause a decrease in the size and extent of mangrove systems. These zonation hypotheses, however, do not account for biological adaptations of individual species to contrasting physiographic factors in an intertidal zone.
3. Physio-chemical gradients and zonation
Ecologists have hypothesized that a species adapts physiologically to physico-chemical gradients in the environment. Two dominant theories exist, the Òdistinct-preference hypothesisÓ and the Òsame-preference hypothesis.Ó The distinct-preference hypothesis states that each species has its optimum along the gradient which controls where the species occurs. Zonation occurs because different species have different optima. An alternative view is that species have the same optima and other factors (competition, seed dispersal, predation) cause zonation. Many environmental parameters vary across an intertidal zone. The most frequently cited is that of tidal inundation. Tidal action introduces salinity and soil waterlogging. Salinity is the most investigated environmental gradient in a mangrove, however, mangrove are not obligate halophytes. They can survive in freshwater as well as saltwater. Studies have concluded that mangroves can grow over a broad range of conditions in the intertidal zone. Data indicates that there are two groupings of mangroves based on distribution and salinity. One group is very salt tolerant, often sprouting where salinity exceeds that of seawater. The other group, often found further inland or in regions where there is heavy rainfall can tolerate a salinity not exceeding 40%. It appears as though adaptation to salinity may have occurred in mangrove species that influences distribution.
4. Propagule dispersal and zonation (Babinowitz, 1978)
The size of a mangrove speciesÕ propagule has also been hypothesized as an explanation for species zonation. Mangrove propagules have been found to be distributed from low to high intertidal zones in a manner inversely related to the size of the propagule. Species with small propagules were found in high intertidal zones. Small propagules would not get snagged while being carried inland. Large propagules were found in low intertidal zones. It is believed that the large propagules cannot breach the root structure of mangroves found at higher elevations. In this model, the propagules were sorted based on size with the influx of tidal waters.
More recent research has found contrary evidence. It is now accepted that tidal action delivers propagules of all sizes (and species) to all areas of the intertidal zones. The question of zonation is not so much how propagules are dispersed, but what factors allow for their establishment, growth, and survival.
5. Propagule predation and forest structure(Smith 1987, Mckee 1995)
Predation has also been noted as a contributing factor to mangrove zonation. Species such as grapsid crabs are heavy consumer s of propagules. Researchers have observed that predation affects regeneration and influences the distribution of certain mangrove species. For some species there appears to be an inverse relationship between the dominance of the species in the canopy and the amount of predation on propagules. While predation may play an important role in some mangrove systems, this explanation of zonation does not apply to all mangrove systems. R. mangle propagules in Florida are not subject to the same rates of predation as R. mangle propagules in Panama. Predation hypotheses can only explain a portion of the observed patterns of mangrove zonation.
6. Competition and forest structure (Ball 1980)
Studies conducted under laboratory conditions have explored the role of competitive interactions in mangrove forests. In particular, growth in a salinity gradient was compared for several species of mangrove. Competition was measured by comparing the reduction in growth of individual species in the presence of the other species to the growth of that species alone. A pattern of competition based on salinity was discovered.
Bibliography:
Ball, M.C. 1980. Patterns of secondary succession in a mangrove forest in southern Florida. Oceologica (Berlin) 44: 226-235.
Ball, M.C. 1988. Ecophysiology of mangroves. Trees 2: 129-142.
Chapin, F. Stuart, et al. 2002. Principles of terrestrial ecosystem ecology. Springer-Verlag. New York.
Davis, J.H. 1940. The ecology and geologic role of mangroves in Florida. Publications of the Carnegie Institute, Washington, DC. Publication no. 517.
Feller, Iika C. and Sitnik, Marsha. 1996. Mangrove ecology workshop manual. Smithsonian Institute.
Kricher, John. 1997. A neotropical companion. Princeton UP. Princeton.
McKee, K.L. 1993. Soil physicochemical patterns and mangrove species distribution: reciprocal effects? J of Ecology 81: 477-487.
Mckee, K.L. 1995. Seedling recruitment patterns in a Belizean mangrove forest: effects of establishment ability and physico-chemical factors. Oecologia 101: 448-460.
Rabinowitz, D. 1978. Dispersal properties of mangrove propagules. Biotropica 10: 47-57.
Smith, T.J III. 1987. Seed predation in relation to tree dominance and distribution in mangrove forests. Ecology 68: 266-273.
Thom, B.G. 1967. Mangrove ecology and deltaic geomorphology: Tabasco, Mexico. J of Ecology 55: 301-343.
Tomlinson, P.B. 1986. The botany of mangroves. Cambridge UP. Cambridge, UK.
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