Biodiversity and ecosystem function are two concepts that are interlinked to one another, one refers to the variety of life-forms and the other refers to the workings of an ecosystem. As ecosystem is related to the living and non-living components of a habitat, it is unavoidable that the biodiversity in the habitat would play a major role in influencing the outcome of the ecosystem function. Magurran (2004) defined biodiversity as the variety and abundance of species in a certain habitat, whereas Field et al. (1998) noted the smallest scale of biodiversity can be found in the form of genotypic diversity and can be expanded to the biome diversity within continents. Meanwhile, ecosystem function comprise of three major functions, biogeochemical, ecological and anthropocentric (Daily, 1977; Field at al., 1998).
In this study, focus is given on mangrove ecosystems. Mangroves owe its uniqueness to its inter-tidal habitat which gives it a special characteristic of supporting many unique species. Mangrove forest has long been known to be of high ecological importance, as feeding and nursery area of commercially important fish and invertebrate species. Mangroves play a significant role in the tropical nearshore carbon cycle, protect the coastlines by stabilizing the sediment and forming a barrier against wave action, recycle nutrients and remove toxicants from the sediment as well as the water column (Chong et al., 1990; Chong and Sasekumar, 2002).
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Juvenile nekton assemblages are relatively poorly known compared to other components of the mangrove ecosystem. There is a significant difference in the standing crop biomass of forests between the Indo-West Pacific (IWP) and Atlantic-East-Pacific (AEP), which could be due to the difference in species richness of mangrove and also due to Juvenile nekton diversity. These differences may also result in different ecosystem functioning, which is yet to be explored. There is a strong need to evaluate the juvenile nekton assemblages in mangroves and the role of mangrove as nurseries to these organisms.
This study aims to assess the influence of plant diversity on key aspects of the function and services of mangroves, namely, the production and quality of organic matter produced by mangrove forests, and their role as nurseries for juvenile nekton. The study will focus on several aspects such as the relation between mangrove plant diversity and the quantity and quality of organic matter produced by different forests, the relation between mangrove plant diversity and the mineralisation rate of mangrove leaf litter and how the quantity and quality of organic matter produced and physical habitat structure associated with forests of different diversity would affect the assemblage composition and abundance of juvenile nekton.
Initial conceptual challenges include designing a study program that would combine experimental and survey data at various spatial scales (local-forest-regional-biogeographical) for testing the impact of mangrove diversity on ecosystem function. Past studies employed widely ranging methods and assumptions, which often render data sets not quite compatible for inclusion into a meta-analysis.
2.0 LITERATURE REVIEW
2.1 Biodiversity and Ecosystem Function
2.1.1 Defining Biodiversity and Ecosystem Function
Biodiversity comprise of all living things that lives together in a habitat, interacting with non-living things in order to achieve a healthy ecosystem. The interdependence between the living and non-living things is indeed important and at the same time will affect the ecosystem function via certain mechanism such as photosynthesis, decomposition of leaf, degradation of dead organisms as well as binding and mineralization of nutrients in order to achieve a healthy and balanced ecosystem.
Biodiversity can be defined in many ways. Magurran (2004) defined biodiversity as the variety and abundance of species in a certain habitat, whereas, Field et al. (1998) noted the smallest scale of biodiversity can be found in the form of genotypic diversity and can be expanded to the biome diversity within continents. Wilsey and Polley (2002) on the hand defined biodiversity as a number of species or genotypes that is often measured although few studies have manipulated evenness as a measure of biodiversity.
Ecosystem function comprise of three major functions, biogeochemical, ecological and anthropocentric (Daily, 1977; Field at al. 1998). A more focused definition 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.
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The relationship between biodiversity and ecosystem function is important to sustain a healthy and balance ecosystem. Various reviews has discussed research conducted on the relationship between biodiversity and ecosystem function such as Naeem, 2000; Loreau et al. 2001 and Bond and Chase, 2002.
2.1.2 The Relationship between Biodiversity and Ecosystem Function
The relationship between biodiversity and ecosystem function is not well understood as it is a complex relationship (Schulze and Mooney, 1983) and difficult to measure. Both terms have large and interconnected concepts. Eventhough this relationship is complicated yet the interdependence between these two is important in order to maintain a healthy ecosystem. Therefore, conserving biodiversity is necessary to maintain the regulation of ecosystem function.
Majority of experiments have been designed to address the relation between biodiversity and ecosystem function on a single trophic level within a large food web (Raffaelli et al. 2002, Bulling et al. 2006). The challenge is to conduct experimental design which would be able to relate this relationship at multi-trophic levels and provide the consequence of difference biodiversity on the ecosystem function as high diversity would also increase the variability of interaction and interdependence.
2.1.3 Biodiversity and Ecosystem Function in Mangroves
Mangrove is one example of an ecosystem that has been recognized as high in biodiversity, which provides stability as well as high productivity (Alongi, 1998). Its unique feature and adaptation for survival in the intertidal area creates a niche rich habitat which provides food source and shelter for many organisms. The relationship between mangrove and ecosystem function is indeed important as the interdependence between them will ensure a healthy and balance ecosystem.
Mangrove is an ideal area to conduct the study on the relationship between biodiversity and ecosystem function although some challenges is anticipated such as dealing with the large scales of marine systems and the difficulties of trying to conduct a complex experiment like what has been done in the terrestrial ecology (Naeem, 2006). Nevertheless, 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 ecosystem function relationships is the limited nature of biotic interactions in small area conducted in a short time. 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.
Several studies have been done on quantifying the ecosystem function in mangrove ecosystems such as decomposition rate (Ashton et al, 1999; Sanchez-Andres et al. 2010), organic matter in the trophic level (Meziane and Tsuchiya, 2000; Meziane and Tsuchiya, 2002) and remote sensing (Dahdouh-Guebas, 2002). As far as i know, there is no direct study has been done on the production of organic matter from different mangrove species on the juveniles of fish and nekton.
My proposed study is to determine how the diversity of mangrove plant, through the production and quality of organic matter affect and thus, influence the diversity and abundance of juveniles of fish and crustacean. This is due to the fact that mangrove habitat has a high productivity in terms of organic matter through primary production and the particle of organic matter provides a major food source for the animals.
2.2 Biogeography of Mangrove Plant
2.2.1 Global Diversity and Distribution of Mangrove Plant
Mangroves are found in the estuarine areas of tropical and subtropical regions. The world distribution of mangrove plant is divided into hemispheres i.e. Indo West Pacific (IWP) and Atlantic East Pacific (AEP). The IWP comprised of East Africa, Indo-Malesia and Australasia whereas the AEP comprised of West America, East America and West Africa (Duke, 2006). The number of mangrove species varies between the IWP and AEP and also between sites within the geographical region (Hogarth, 2007). The numbers of mangrove species and mangrove animals are more abundant in the IWP and less abundant in the AEP and this has been confirmed by Ricklefs and Latham (1993) and Duke (2006).
In general the world distribution of mangrove plant is dependent on the latitudinal factors and limited by low temperature (Duke, 2006) and salinity (Little, 2000). However, the distribution of mangrove plant varies between individual species, which according to Hogarth (2007), cannot be determined by physical or climatic limitations. The global pattern of mangrove plant is limited by the dispersal properties of its propagules (Duke, 2006), distribution of suitable habitats and major physical barriers (Hogarth, 2007).
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The global distribution of mangrove plant according to regions is summarized in Table 1. More genus and species of mangroves are found in the IWP compare to the AEP. The IWP contains 23 genera and 58 species whereas the AEP contains 7 genera and 12 species. Both regions have 3 genus and 1 species in common.
Table 1: Genus and species of mangrove in the West Pacific (IWP) and Atlantic East Pacific (AEP) regions (Duke, 1992)
2.2.2 Mangrove Plant Diversity and Distribution in Australia
Generally, the regional distribution of plant will be determined by the climatic factor (e.g. rainfall, temperature, moisture level etc.). According to Hogarth (2007), the geographical distribution of mangrove species is dependent on temperature (latitudinal range) and physical barriers (longitudinal range). Therefore, the mangrove species that occur in a specific location (e.g. Queensland) are representing the species that occur in a certain region (Australasia in the IWP).
The distribution of mangrove plant in Australia is dependent on the temperature and rainfall (Hogarth, 2007). The distribution of mangrove plant in Australia is summarized in Table 2. Based on table 2, Australia contains 22 genera and 41 species of mangrove. Based on the state distribution, more genus and species of mangroves are found in the Queensland (QLD), followed by Northern Territory (NT) and Western Australia (WA).
Region and State
Table 2: Genus and species of mangrove in Australia and within state (Duke, 2006)
The local distribution of mangrove plant in Australia is dependent on the influences of salinity and tides (Duke, 2006). The salinity tolerance level varies among the mangrove plants; some can tolerate to high salinity (e.g. Avicennia marina and Rhizophora stylosa), whereas some can only tolerate to low salinity (e.g. Sonneratia caseolaris and Bruguiera sexangula). The mangrove plants that can tolerate to high salinity is located at the downstream and the plant that can tolerate to low salinity is located at the upstream.
In terms of tide, mangrove plant occurrence is related to the tidal inundation which comprised of three level i.e. low, mid and high. Mangroves such as Sonneratia alba will be inundated by the low inundation level (inundated by medium high tides and flooded more than 45 times a month) whereas Xylocarpus granatum will be inundated by the high inundation level (inundated less than 20 times a month), (Duke, 2006).
2.3 Mangrove as Nurseries
2.3.1 The Importance of Mangrove as Nurseries
Mangrove forest which is located in the estuarine environment is a unique ecosystem and known to be of high ecological importance such as feeding and nursery area for commercially important fish and invertebrate species (Chong and Sasekumar, 2002). Mangrove forest has been proven to be highly productive in terms of providing good quality organic matter (Odum and Heald, 1975), where in turn it provides good food source for the marine invertebrates. Therefore, one of the important roles of mangrove is providing feeding and nursery area to juvenile nekton.
In general, mangrove forest plays an important role as nurseries where it provides protection or shelter and food source to the marine invertebrates. The production of organic matter is vital in the estuarine food webs. The net productivity of mangrove leaf litter is high in this area, therefore the decomposition of mangrove leaf litter will generate high production and good quality of organic matter in the sediment.
The fish species that occupy the mangroves comprised both the resident and transient fish. The resident species will permanently inhabit the estuaries. The transient species (marine visitors) will utilise the estuaries temporarily during some stages in their life cycle and tend to be very abundant (Haedrich, 1983). Both these permanent resident has adapted to the conditions of this environment, but often ignored in the study of nekton assemblage.
Some the transient species will enter the estuaries to seek protection and food during the high tide. During the high tide, high abundance of both resident and transient nekton will enter the estuaries (Robertson and Duke, 1990). Study done by Robertson and Duke, 1990 stated that the fish and crustacean abundance differed among the study sites, therefore indicating that estuaries differ in their nursery-ground value. This could probably due to the production and quality of organic matter in the nursery areas.
Several studies have documented the importance of mangrove as nurseries such as for commercially exploited fish stocks (Robertson and Duke, 1987).
2.3.2 The Movement of Juvenile Nekton in Estuaries
Nursery areas provide food, shelter and protection for juvenile nekton to grow. Moreover the density, survival and growth of juvenile nekton (specifically fish and crustacean) and movement to adult habitat are improved (Sheridan and Hays, 2003). The movement of juvenile nekton in the estuary is determined by the tides. The juvenile nekton will enter the estuary during high tide and some will stay in the estuary during low tide. There are some juvenile nektons that utilises the nursery without the effect of the tides where they will remain on the emergent pool water when the tide has receded (Kneib, 1997).
The enumeration of these juveniles has not been studied extensively especially in mangrove areas. Kneib (1997) conducted a study on these early life stages of resident nekton that remained on the emergent marsh surface by using a method called Simulated Aquatic Microhabitats (SAMs). This method is good as it can capture the resident nekton that remained on the emergent surface after it is exposed by the tide and could not be sampled by traditional methods. This method will be applied in my study for the enumeration of the juveniles of fish and crustacean.
2.3.3 The Importance of Mangrove Root to Juvenile Nekton
Generally, mangrove root provides shelter, protection and food (detritus) to the juvenile nekton. Juvenile nekton prefers areas that provide higher habitat complexity in terms of higher density of plant and aerial root structural heterogeneity as well as providing food resources. Several studies documented that, juveniles of many species are found more in mangrove habitats compared with adjacent habitats such as mudflats and seagrass beds (Robertson and Duke, 1990; Laegdsgaard and Johnson, 1995).
The nursery role in mangroves is well established, but it is still uncertain why mangroves attract juvenile fish to its habitat. There are three main hypotheses, which leads to this phenomenon; the structural heterogeneity where juvenile fish are attracted to the structural heterogeneity of mangroves, the predation risk where risk or predation is lower in mangroves due to increased structural complexity, and the food availability where the availability of food is greater in mangroves (Laegdsgaard and Johnson, 2001).
Mangrove plant species vary in terms of their root structural heterogeneity (type, height density, and structure of their aerial roots) (Tomlinson, 1986). Therefore, the quality of shelter provided to juvenile nekton varies within the mangrove forest, which this proposed study attempt to investigate.
2.4 Organic Matter in Mangroves
2.4.1 The role of Organic Matter in Mangroves
Organic matter comprises of dead organism or organic compounds that undergone decomposition process. The decomposition process is due to the activity which is caused by decomposers such as bacteria, fungi and other detritus feeding organisms. In mangrove habitats, the production of organic matter is dependent on factors such as the mangrove plant species, decomposition process and perhaps physical factors.
Mangrove is known to be very productive areas as it contains high percentage of organic matter. In estuarine environment, organic matter is important in the trophic levels. The production and quality of organic matter is based on primary production from autotrophs which would make inorganic nutrients be available to secondary users (Little, 2000). Mangrove-derived detritus will become the food source to the marine invertebrates. Thus, the role of organic matter in mangrove is it provides food to the living organism and energy to the ecosystem.
Odum and Heald (1975) stated in their outwelling hypotheses where mangrove litter is the main food source for higher trophic levels. However, Robertson et al., (1992) and Lee (1995) argued that the fate of the mangrove litter is uncertain in the outwelling of carbon and nutrients, whether it forms the basis of coastal food webs or accumulate in the sediments. Lee (1995) also stated that mangrove organic matter is incorporated into the coastal foodwebs in a limited quantity.
Several studies have been done on the organic matter in mangroves such as Chong et al. 2001 (contribution of mangrove detritus to juvenile prawn nutrition) and Bouillon et al. 2008 (organic matter exchange and cycling).
2.4.2 The Importance of Organic Matter to Juvenile Nekton
In mangrove, mangrove-derived detritus is an important food source for the decomposer in the food web, such as sesarmid crabs, fiddler crabs and gastropods. Organic matter that is not exported by tidal actions will enter the sediment. The decomposer such as sesarmid and fiddler crabs will consume sediment rich in organic matter. Bacteria and fungi will accumulate on the organic matter providing additional food items for juvenile nekton.
This study will attempt to link the production of organic matter to different mangrove diversity and abundance of juvenile nekton. Bouillon et al., 2008 stated that numerous studies has been done on the carbon dynamics in mangrove ecosystems but the overall ecosystem function in terms of organic matter processing is yet to be fully understood.
2.5 Decomposition of Mangrove Litter
2.5.1 Decomposition Rate of Mangrove Litter
Mangrove leaf litter has four possible fates. It may be broken down, depending on the presence and absence of a leaf consuming fauna (Odum and Heald, 1975; Leh and Sasekumar, 1985; Robertson, 1986), decomposed by microbial action, flushed out of the mangrove forest by tides, depending on the degree and frequency of tidal inundation (Twilley et al. 1986) or river flow or may accumulate within the mangrove mud. The decomposition rate of mangrove leaf litter differs among sites, habitats, season and forest types.
The decomposition of mangrove litter will be decomposed by decomposers such as fungi and bacteria, which are then consumed by organisms of higher trophic level. Therefore, decomposition is an important mechanism in the production of organic matter for the trophic level. The decomposition rate of mangrove litter differs among mangrove species. Some mangrove species are able to decompose fast such as Sonneratia alba, which decomposes faster compare to Rhizophora apiculata (study done by Ashton et al. 1999).
The decomposed litter will have a C:N ratio around 6, which is a nutritious food source for the small marine invertebrates (Robertson et al. 1992). The nutritional value of mangrove litter varies among the mangrove species. Therefore, it is anticipated that different mangrove forest will attract different assemblages of juvenile nekton, which this proposed study hope to achieve.
Mangrove litter decompose rapidly in the intertidal area when leaf litter is submerged in water as compare to the mangrove forest when it is only submerged during the high tides (Robertson, 1988). Decomposition rate also happens rapidly during the wet season (Sanchez-Andres et al. 2010).
Several studies have been conducted on leaf decomposition study such as Robertson, 1988 (decomposition study among three mangrove species), Ashton et al. 1999 (decomposition study in managed mangrove forest) and Sanchez-Andres et al. 2010 (litterfall dynamics and nutrient decomposition).
2.5.2 The Nutritional Value of Mangrove Litter
Mangrove leaf litter provides important source of carbon, nitrogen, and other nutrients for estuarine food webs (Odum and Heald, 1975; Thongtham and Kristensen, 2005), thus it is important to sustain a healthy ecosystem. The mangrove crab feeding preferences are due to the differences in leaf structure and composition of chemical components (Ashton, 2002; Skov and Hartnoll, 2002). Therefore, leaves of different mangrove species will have differences in their structure and chemical components (Tomlinson, 1986).
According to Robertson (1988), leaves of different mangrove species varies in their nitrogen and tannin contents. Therefore, the nutritional quality that may determine consumption preferences also varies (Ashton, 2002; Skov and Hartnoll, 2002; Alongi, 2009), where this proposed study hopes to proof and achieve.
Avicennia marina has been found to be the preferred species in laboratory experiments (Kwok and Lee, 1995) because of its low tannin and low C:N ratio. Study done by Alongi (2009) indicated that Avicennia marina leaves have lower tannin and higher initial nitrogen content than Rhizophora stylosa and Bruguiera sexangula leaves.
126.96.36.199 Carbon and Nitrogen Content
The decomposition rate in mangrove leaf litter will change the nutritional value of the mangrove leaves. The measurement of carbon to nitrogen (C:N ratio) is generally being used to quantify the nutritional value of mangrove leaf litter (Fell and Master, 1984). The dietary requirement of protein for most animals is 16.5 percent of the dry weight of the diet, which corresponds to a C:N intake of about 17:1 (Russell-Hunter, 1970). Therefore a good C:N ratio is below 17:1 where it is considered as a good food source. If the C:N intake is above 17:1 there is a protein deficiency in the diet (Russell-Hunter, 1970).
The C:N ratio tends to decrease with time, therefore the older the leaf, the better it is as food source (Giddins et al. 1986) which this pattern can be seen throughout the decomposition phases. A decrease in C:N ratio from senescent leaves to partially decomposed leaves of Rhizophora mangle has been observed (Fell and Master, 1980; Newell, 1984). Similar studies in various locations in Africa, Australia and USA confirm the change in C:N ratio (Steinke et al. 1983; Twilley et al. 1986; Steinke and Ward, 1987; Robertson, 1988).
When the decomposition of mangrove leaf litter happens, the leaching of the soluble materials flow into the sediment therefore the sediment will also contain good nutritional value in terms of the C:N ratio. Sediment detritus contains high nutritional value compare to leaves (Skov and Hartnoll, 2002).
2.6 The Transfer of Organic Matter in the Food Chain
2.6.1 Fatty Acid Analysis
The transfer or organic matter in the food chain in estuarine environment is important. There are two types of chemical tracer techniques that can be applied to detect this transfer process i.e. fatty acid biomarkers and stable isotopes. In general, fatty acid biomarkers are used to trace transfer of fatty acid markers in the higher trophic levels in a food chain. According to Kristensen et al. 2008, fatty acid acts as biomarkers for prokaryotes, fungi, diatoms, dinoflagellates or vascular plants.
There are several studies that had been conducted in the transfer of fatty acid markers in the various trophic levels in the estuarine food chain. Study on a single trophic transfer is widely being conducted in these environments (eg. Meziane and Tsuchiya, 2002). However, study beyond a single trophic transfer is limited. Hall et al., 2006 conducted an experiment on the transfer of FA markers across three trophic levels (mangrove leaves, sesarmine crab and swimmer crab), in an estuarine food chain.
In the estuarine environment, fatty acids are used to investigate sources of organic matter in sediments (Meziane and Tsuchiya, 2000) and also used as trophic markers to determine the transfer of matter within the food web. Fatty acids are also used to detect the diet of marine invertebrates (Kharlamenko et al. 1995; Meziane et al. 1997; Meziane and Tsuchiya, 2000, Meziane and Tsuchiya, 2002) in the estuarine environment.
Fatty acids are an important component in the tissue of living organisms. The most common saturated fatty acids (SAFAs), Palmitic acid (16:0), is the most abundant compound in the fatty acid composition of mangrove leaves (Meziane and Tsuchiya, 2002; Hall et al, 2006). The polyunsaturated fatty acids (PUFA), in particular 18:2ω6 and 18:3ω3, has been identified as useful biomarkers of mangrove leaves (Hall et al., 2006).
The measurement of C and N content on the surface sediment is used to detect the fatty acid markers from the mangrove leaves (Meziane and Tsuchiya, 2000; Meziane and Tsuchiya, 2002). Previous studies in relating the C:N ratios to the fatty acid markers have focused on surface sediment in the intertidal area (Meziane and Tsuchiya, 2000) and in an estuary which has mangrove forest that receives agricultural wastewater (Meziane and Tsuchiya, 2002).
This study will attempt to determine the transfer of fatty acid markers from different mangrove species to the juvenile nekton and to see the linkages of organic matter between these two trophic levels and also the diet of the juvenile nekton whether the fatty acid markers of the mangrove species are present.
2.6.2 Stable Isotope Analysis
Stable isotope is an important tool to detect the source and transfer of organic matter in the estuarine environment. In general, stable isotope is use to record both source and trophic level information. Sulphur and carbon is normally used to record source information whereas nitrogen is used to record trophic level information (Michener and Kaufman, 2007). According to Peterson (1999), stable isotope is used as an indicator to detect and identify sources of nutrients and tracing the flow of organic matter in coastal food webs. In the estuarine environment, carbon and nitrogen isotope is generally used to investigate the source and transfer of organic matter.
Analysis of δ13C is used to determine the dominant food source and estimate its relative contribution. By comparing δ13C values of different organisms, it is also possible to track mangrove carbon through successive stages of food chains (Hogarth, 2007). There are various indicators in the δ13C ratios such as C3 terrestrial plant material (δ13C = -23 to -30°), seagrasses (-3 to -15°), macroalgae (-8 to -27°), C3 marsh plants (-23 to -26°), C4 marsh plants (-12 to -14°), benthic algae (-10 to -20°), and marine phytoplankton (-18 to -24°) (Fry and Sherr, 1984). The δ13C ratios of mangrove plants (example: Rhizophora sp., Bruguiera sp., Sonneratia sp., and Avicennia sp.) fall in the range of -26 to -29 ° (study done by Gearing et al., 1980). These δ13C ratios of plant (consumer) will not change if it dies or being eaten.
In estuarine environment, the knowledge of the quality of the food source such as organic matter is important to understand the importance of isotopic routing (Michener and Kaufman, 2007). In the trophic level, animals that consume plants and detritus will reflect δ13C ratio of the material they consume, which is clearly shown in the study by Chong et al. 2001, (primary food source for the juvenile prawns in the upper estuaries), Newell et al. 1995 (nutrition of juvenile Penaeus merguiensis within the mangrove forest).
Isotope ratios differ among plants, animals, and microbes (Peterson and Howarth, 1987; Peterson et al., 1985). It is also known that this method has some limitations (Peterson et al., 1985) as variations in isotope ratios for a given organism are common and the ratio may not be absorbed because of further fractionation along a food chain.
My proposed study will attempt to identify the mangrove species indicator in the diet of the juvenile nekton, which will possibly give me an answer regarding the transfer of organic matter in this trophic level.