Biodiversity Of Mangroves

Biodiversity is usually defined at three levels, i.e. species, populations and ecosystems (Ray and McCormick 1992). The term "biodiversity components" has been recommended as an ecological reference to these several hierarchical levels, and the idea is particularly appropriate to describe tropical estuarine ecosystems (Yanez-Arancibia et al. 1994). Biodiversity components of tropical estuaries can refer to the high diversity of species, life histories, habitats and links in food webs, or the diverse pathways of energy flow and nutrient cycles that couple terrestrial and marine ecosystems at the land-sea interface. In addition, coastal geomorphological landforms and geophysical processes represent diverse components that effectively modulate the properties of estuarine ecosystems. These fluctuating environmental conditions of estuaries result in diverse spatial and temporal patterns of habitat utilization by organisms. This is especially true in mangrove ecosystems, since they are open systems interacting with a high diversity of functional landscapes, such as borders with terrigenous freshwater, coastal ocean water, the atmosphere and the sediment-water interface. Macnac (1968) used the term "mangal" to specify the properties of the mangrove ecosystem within the coastal landscape, in contrast to "mangrove-"' which is restricted to the characteristic spermatophyte in the intertidal zone. This distinct term to describe the properties of mangrove ecosystems, while seldom used in mangrove ecology, emphasizes the need to integrate the diverse physical, chemical and biological characteristics of tropical coastal ecosystems.

The diverse landforms of coastal regions can be considered as a biodiversity component of mangrove ecosystems (Figure 13.1). These regions can be classified into distinct geomorphological units that describe the influence of

t. Rwer-donúnaWd altoctuhonous It. Tide-dominated allochihonous barcKi-hgoon (autoduhotwus)

IV. Composite: river and wave dominated V. Drowned bedrock valley

Figure 13.1 Five basic classes of geomorphological settings that influence the distribution of mangroves in the intertidal zone. Redrawn from Thorn (1982)

Define Riverine Forest
2- Riverine forest
4.- Overwash forest

5.- Dwarf forest

Figure 13.2 Ecological types of mangroves (Lugo and Snedaker 1974) and the spatial distribution of these ecological types within the intcrtidal zone (from Gilmore and Snedaker 1993, reprinted by permission of John Wiley and Sons, Inc.). The arrows and roman numerals are used to define the location and movement of seven specific spatial guilds by principal habitat association in mangroves as described by Gilmore and Snedaker 1993 - I. Sublitttoral/Littoral; II. Arboreal Canopy; III. Benthic and Infauna Community: IV. Tidal Creek and Ditch Community; V. Basin Community; VI. Upland Arboreal Community; VII. Upland Terrestrial Community geophysical processes on the ecological characteristics of mangroves (Thorn 1982). Mangroves occur within five basic groups of coastal environments depending on a combination of geophysical energies including the relative influences of rainfall, river discharge, tidal amplitude, turbidity and wave power (Figure 13.1). These five environmental settings are all influenced by inputs of terrigenous materials, while mangroves also occur on carbonate platforms where environmental settings are dominated by calcareous sedimentary processes and nutrient-poor conditions (Woodroffe 1992). The structure and function of these carbonate platform communities provide an interesting contrast to those mangroves influenced more by terrigenous materials.

The microtopographic factors of a region determine many of the hydro-logic and chemical conditions of soil that control the patterns of forest physiognomy and zonation. In addition, tidal flooding frequency of the intertidai zone can influence the distribution of propagules and species (Rabinowitz 1978), although the influence of this mechanism ("tidal sorting") on forest structure has been recently questioned (Smith 1992). Lugo and Snedaker (1974) used the local patterns of mangrove structure in the south Florida and Caribbean regions to classify mangroves into riverine, fringe, basin, hammock and dwarf forests (Figure 13.2). This ecological classification of mangroves is also influenced by biological factors such as predation on propagules (e.g. crabs), differential resource utilization by seedlings, and physiological tolerance of trees that determine the patterns in physiognomy and zonation of mangrove trees (Davis 1940; Ball 1980; Lugo 1980; Snedaker 1982; Smith 1992). These two types of classification systems, geomorphological (Figure 13.1) and ecological (Figure 13.2), represent different levels of organization of the coastal landscape. Together they can be used to integrate the different scales of environmental factors that control the attributes of forest structure (Figure 13.3).

The species richness of trees is another biodiversity component of mangrove ecosystems (Figure 13.4), The environmental settings and biological factors described above not only influence the formation of different geomorphological and ecological types of mangrove forests, but they may also control species richness (Smith 1992). It is clear that within a continental area, changes in rainfall, temperature and tidal range may be important to the diversity of mangrove trees (Smith and Duke 1987). However, there are biogeographic factors that have resulted in an unbalanced global distribution of species richness (Tomlinson 1986). The diversity of mangrove tree species in the western hemisphere (11 species) is less compared with the eastern hemisphere (over 30 species) (Figure 13.4). This also results in much more complex zonation patterns along the inter-tidal zone of Old World continents as compared with the simpler patterns in the neotropics (such as those in Watson, 1928, compared with Davis, 1940;

Mangrove Ecosystem Structure
Figure 13.3 Hierarchical classification system to describe diverse patterns of mangrove structure and function based on global, geomorphological (regional) and ecological (local) factors that control the concentration of nutrient resources and stressors in soil

see Chapman 1976). At present, general conceptual models have improved to explain the development of zonation and forest structure within specific continental regions (Smith 1992; Gilmorc and Snedaker 1993), but the development of specific ecological models to project change in species richness and ecological types of mangroves in response to land-use or global-climate changes is still limited by a lack of understanding of the manifold routes of coastal forest development (Twilley 1995).

Mangrove ecosystems support a variety of marine and estuarine food webs involving an extraordinarily large number of animal species (Macnae 1968; Odum and Heald 1972; Yanez-Araneibia et al. 1988; Robertson and

Figure 13.4 Generalized global distribution of mangroves including approximate limits of all species (upper panel) and histogram showing approximate number of spccies of mangroves per 15e of longitude (lower panel), (from Tomlinson 1986. reprinted with the permission of Cambridge University Press)

120 90 60 30 WOE 30 60 90 120 150 Degrees longitude from meridian

Western mangroves Eastern mangroves

Figure 13.4 Generalized global distribution of mangroves including approximate limits of all species (upper panel) and histogram showing approximate number of spccies of mangroves per 15e of longitude (lower panel), (from Tomlinson 1986. reprinted with the permission of Cambridge University Press)

120 90 60 30 WOE 30 60 90 120 150 Degrees longitude from meridian

Western mangroves Eastern mangroves

Duke 1990). The export of particulate organic matter (POM) supports food webs originating with particulate feeders, whereas the sometime larger export of soluble (dissolved) organic matter (DOM) forms the basis of the nearshore heterotrophic microorganism food web (Odum 1971; Alongi 1988; Snedaker 1989; Robertson et al. 1992). Many of the species of finfish and invertebrates that utilize the mangrove habitat and its organic resources are also components of offshore areas, a phenomenon that suggests intricate patterns of diel and seasonal migrations (cf. Thayer et al. 1987; Yanez-Arancibia et al. 1988; Sasekumar et al. 1992). In addition to the marine estuarine food webs and associated species, there are a relatively large number and variety of animals, that range from terrestrial insects to birds, that live in and/or feed directly on mangrove vegetation. These include sessile organisms such as oysters and tunicatcs, arboreal feeders such as foliovores and frugivores. and ground-level seed predators. In consideration of the entire resident and casual faunal population in south Florida mangroves, Gilmore and Snedaker (1993) were able to recognize four distinct spatial guilds that may have well over an estimated 200 species, many of which are as yet uncataloged. In addition, Simberloff and Wilson (1969) documented over 200 species of insects in mangroves in the Florida Keys. For reference, the Florida mangroves consist of only three major tree species and one minor species of vascular plants. Based on these considerations, one can conclude that the low species richness of mangroves in Florida supports a disproportionately rich diversity of animals, the dimensions of which arc only now being documented. This same conclusion can be applied to other parts of the Caribbean (Ruetzler and Feller 1988; Bacon 1990; Feller 1993). Even though there is a global difference in species richness of mangrove trees between the east and west hemispheres, there does not seem to be a corresponding contrast in the functional diversity of the associated fauna. One exception is that Robertson and Blaber (1992) suggested that species richness of fish communities in the tropical Atlantic Ocean region was less than in the Indo-Pacific areas.

13.3 FOREST STRUCTURE AND ECOSYSTEM FUNCTION 13.3.1 Mangrove-specific effects on nutrient dynamics

Litter produced in the canopy of mangrove forests influences the cycling of inorganic nutrients on the forest floor, and the outwclling of organic matter to adjacent coastal waters (Figure 13.5) (Odum and Heald 1972; Twiliey et at. 1986). Thus the dynamics of mangrove litter, including productivity, decomposition and export, influence the nutrient and organic matter budgets of mangrove ecosystems (Twiliey 1988). Mangroves are forested ecosystems, and many of the ccological functions of nutrient cycling described for terrestrial forests may also occur in these intcrtidal forests. The amount of litter produced and the quality of that litter, as represented by C:N ratios and concentrations of lignin and polyphenols, contributes to the nutrient dynamics of forested ecosystems (Aber and Mclillo 1982; Mclillo et at. 1982). Thus, nitrogen cycling in the forest canopy is coupled to the nutrient dynamics in forest soils, and these are influenced by the spccies-specific nutritional ecology of the trees. Studies to test the presence of these feedback mechanisms will give insights into the ecological significance of tree biodiversity to the litter and nutrient dynamics of mangrove ecosytems.

The accumulation of leaf litter on the forest floor of mangrove ecosystems can be an important site for nutrient immobilization during decomposition (Figure 13.5; see also Section 13.3.2) (Twiliey et at. 1986). The concentration of nitrogen in leaf litter usually increases during decomposition on the forest floor (Heald 1969; Rice and Tenore 1981; Twiliey et at. 1986; Day et at.


Atmosphere Exchange = AE Immobilization = IM Litter Fall = LF Rctransloeation = RT Regeneration = RG Sedimentation = SD Tidal Exchange = TE Uptake = UT

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