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Biodiversity is the variety in living organism that can be found at a given scale. Magurran (2004) stated that biodiversity comprised of two fundamental levels, species diversity and genetic diversity, which is strictly or monotonic increasing, functions of the number of species and genotypes, and the evenness of their relative abundances. Field et al., 1998 defined biodiversity as encompassing a wide ranging scale, which may cover from genotypic diversity within a population, up to the biome diversity within continents. Biodiversity is often measured as the number of species or genotypes, although a few studies have manipulated evenness (Wilsey and Polley, 2002).
Ecosystem function comprise of three major functions, biogeochemical, ecological and anthropocentric (Daily, 1977; Field at al., 1998). Biogeochemical functions includes primary and secondary production, decomposition, nutrient cycling, hydrology, soil development and soil fertility, regulation of climate cycles, stabilization of substrates, and purification of the air and water; ecological functions includes resilience and resistance to biotic and abiotic perturbations, maintenance of food-web integrity, and provision of habitat for a range of trophic levels; and anthropogenic function includes medicinal compounds, aesthetic and recreational values, maintenance of fisheries, and management of sediments on a range of time scales (Field et al., 1998).
A more focused concept of ecosystem function is provided by Pacala and Kinzig (2002), who suggested that ecosystem function comprise of stocks of energy and materials (biomass), fluxes of energy or material processing (productivity, decomposition), and the stability of rates or stocks over time.
2.1.2 The Relationship between Biodiversity and Ecosystem Function
Both biodiversity and ecosystem function are broad concepts and may link to each other at several scales. The relationship between biodiversity and ecosystem function is complicated yet the interdependence between these two are important in order to maintain a healthy ecosystem. Conserving biodiversity is necessary to maintain the ecosystem function. There is an uncertainty about the way in which the diversity of population and species in an ecosystem is related to the functional properties of the ecosystem. It is important that biodiversity is preserved wherever possible to maintain the regulation of ecosystem function.
According to Schulze and Mooney (1983); it is unclear how and why a change in biodiversity might change the ecosystem function. It might be due to limited knowledge about the population biology, the functional properties of most species, the mechanisms which underlie the self-assembly and organization of species in communities and the effects of variations in the arrangement of components in a complex systems.
Direct and manipulative experiments can produce important findings into the role of biodiversity in ecosystem function, but they are limited when it comes to aspects of this relationship that occur over long temporal and large spatial scales (Field et al., 1998). Research on the relationship between biodiversity and ecosystem function is now focusing on the biodiversity loss at large spatial scales, which involves reduction and change in species at different trophic levels (Raffaelli 2006). The majority of manipulative experiments designed to address the relation between biodiversity and ecosystem function have focused largely on a single trophic level within a large food web (Raffaelli et al. 2002, Bulling et al. 2006).
The challenges in assessing the ecosystem function implications in biodiversity are related to scale, both temporal and spatial. Quantifying the consequences of biodiversity for ecosystem function is a challenge under any circumstances (Huston, 1997). Direct experimental studies of biodiversity effects on ecosystem function (Naeem et al. 1995; Hooper and Vitousek, 1997) use model systems that provide reasonable access to some, but not all, of the scales over which biodiversity effects could be important. Therefore, more research is needed in order to improve the understanding on this complex relationship.
2.1.3 Biodiversity and Ecosystem Function in Mangroves
The main challenge faced by biodiversity and ecosystem function research in marine ecology is dealing with the large scales of marine systems and the difficulties of trying to conduct a complex experiment like what have done in terrestrial ecology (Naeem, 2006). Mangrove ecosystems are interesting and important for studies on the role of biological diversity for ecosystem function.
Mangroves support a low diversity of the dominant higher plants (Duke, Ball and Ellison, 1998). The diversity of other life form (arthropods, molluscs, fish and birds) are much greater. The diversity of mangrove plant species changes based on distance from centres of diversification, dispersal ability, the viability of propagules prior to rooting, and the directions of the ocean currents. On a large area, mangrove plant diversity increase with precipitation and the area of watershed (Duke, 1992) and decrease with increasing latitude (Smith and Duke, 1987; Duke et. al., 1998).
Mangroves ecosystems compress a broad range of habitat variation into a compact spatial scale. The gradient from fully submerged to fully exposed sites expands the diversity of mangrove habitats. Therefore, mangrove ecosystems provide an efficient model to evaluate interactions between environmental variation and functional consequences of biodiversity (Field et al., 1998).
One of the potentially important limitations of direct experiments on the biodiversity or function relationships is the limited nature of biotic interactions in small plots operated over short periods. Biotic interactions in mangrove ecosystems are limited by other factors, including direct human impacts (Sasekumar and Chong, 1998), alterations of surrounding terrestrial habitat, and biogeographical constraints.
There are a number of studies done on quantifying ecosystem function in mangrove ecosystems. Observations on plant physiology (Ball, 1996), primary production (Ong et. al., 1982), nutrient cycling (Gong and Ong, 1990), and biotic interactions (Farnsworth and Ellison, 1993) provide complementary studies in a number of mangrove regions, yet far from being comprehensive. Currently, remote sensing tools become more appropriate on spatially extensive analysis in large and separated sites.
Other than mangrove plants, biodiversity and ecosystem function studies on other taxa has also been conducted. For example, Sasekumar and Chong (1998) focus on the major impacts of human activities on invertebrate diversity and Farnsworth (1998) reviewing the evidence for the potential role of tree species richness and net primary production in regulating the diversity of consumers. Allen (1998) stated that little is known about the role of biodiversity in maintaining services like flood protection, nutrient and organic matter processing, sediment control, and fisheries support underlie the economic foundations of many tropical regions.
Mangrove ecosystems is ideal for comparative biodiversity or function studies that take a broad view of ecosystem function, integrating biogeochemical, ecological, and anthropogenic functions across a range of time scale. Comparative studies should be in sites that are similar except for biogeographic effects on biodiversity. It is also important to think of biodiversity gradients in climate zones as replicates on a core study (Field et al., 1998).