The major form of vegetation that supports the biodiversity of tropical estuarine ecosystems consists of intertidal forested wetlands known as mangroves. Mangroves form a small portion of the world's forested landscape, but cover 240 x 103 km2 of sheltered subtropical and tropica) coastlines (Lugo et at. 1990; Twilley et al. 1992). This vegetation dominates the intertidal zone of tropical river deltas, lagoons and estuarine coastal systems that have significant inputs of terrigenous sediments (allochthonous materials), and it can also colonize the shoreline of carbonate platforms that arc developed from calcareous sedimentary processes (with little or no influence from terrestrial runoff) (Thorn 1982; Woodroffe 1992). In each of these geomorphologically distinct regional landscapes, local variations in topography and hydrology also result in the development of distinct ecological types of mangroves such as riverine, fringe, basin and dwarf forests (Lugo and Snedaker 1974). The combination of different geomorphological settings, each with a variety of ecological types, results in a diversity of mangrove ecosystems, each with specific characteristics of structure and function (Twilley 1988, 1995). Although there are relatively few species of trees (54 true species, Tomlinson 1986) in mangrove ecosystems, the biodiversity components of these ecosystems are unique because they include structural niches and refugia for numerous fauna! and microbial species. In addition, the locations of these forested wetlands at the land-sea interface form interdependent assemblages that link the nearshore marine environment with inland terrestrial landscapes (Macnae 1968; Chapman 1976;
Odum et al. 1982; Tomlinson 5986; Gilmore and Snedaker 1993; Twilley et al. 1993).
The mosaic of mangrove habitats provides a variety of biodiversity components that are important to the function and environmental quality of tropical estuarine ecosystems. The dominant ecological function of mangroves is the maintenance of nearshore marine habitats and the concomitant provision of food and refugia to a variety of organisms at different trophic levels (Odum and Heald 1972; Thayer et al. 1987; Yanez-Arancibia et al. 1988, 1993; Rojas et al. 1992; Sasekumar et al. 1992). In addition, mangroves play a major role in maintaining water quality and shoreline stability by controlling nutrient and sediment distributions in estuarine waters (Walsh 1967; Nixon et al. 1984; Twilley 1988; Alongi et al. 1992). Coastal forested wetlands are unique in that tides allow for an exchange of water, nutrients, sediment and organisms between intertidal and coastal regions of tropical estuaries. In addition, rivers link the runoff of sediments and nutrients from upland watersheds to the productivity and biogeochcm-istry of tropical estuaries. The multiple functions of mangrove ecosystems result in the extremely high primary and secondary productivity of tropical estuaries.
This chapter will summarize information that links the biodiversity components of mangrove habitats with the functional ecology of tropical estuarine ecosystems. Although the understanding of the functional ecology of mangroves is fairly limited, there are some examples that describe the influence of specific guilds on ecological properties of mangrove ecosystems. We will present an overview of the biodiversity of mangroves using ecological classifications of landscape mosaics, and including the traditional analysis based on species diversity. Although regions of estuaries are considered depauperate in species number, these areas have always been described as one of the most productive regions of the biosphere. Information on biodiversity and ecosystem function will be presented, along with examples of the vulnerability of these coastal systems to change. Change will include land-use alterations within both the estuary and its watershed, together with projections of climate change in tropical coastal areas.
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