18 Mangrove Ecosystems Littoraria intermedia prefers trees next to freshwater creeks, whereas the polymorphic species Littoraria pallescens is found solely on leaves Meiofauna Within the mangrove mud lies a rich fauna virtually invisible to the naked eye – the meiofauna Beneath an area of 10 cm2 of mud there may be many thousands of individuals Orders of magnitude smaller than the more conspicuous macrofaunal crabs and snails are meiofaunal herbivores, detritivores, and formidable predators, with food chains probably dependent on photosynthetic cyanobacteria (‘‘blue-green algae’’) and heterotrophic bacteria Meiofauna colonize fallen leaves, and the stages of leaf breakdown are accompanied by complex interactions and successional shifts in species composition and community structure which parallel, on a microscopic scale, the processes of macroecology The numbers of meiofaunal individuals are immense, and their diversity is astonishingly high Not only are there many species but also the species show a higher level of taxonomic diversity Among the macrofauna virtually all species belong to just three phyla: arthropods, mollusks, and chordates The meiofauna from just one mangrove area in Australia, for example, yields turbellarian flatworms, nematodes, copepods, Ciliophora, Foraminifera, bivalve mollusks, oligochaete and polychaete annelids, hydrozoa, archiannelids, kinor-hynchs, tardigrades, and gastrotrichs Very little is understood about the meiofauna of mangroves, their interactions, their functional significance in the ecosystem as a whole, and the relationship between the meiofaunal and macrofaunal worlds Their small size belies their great importance and export of matter Commercially important penaeid shrimps use mangroves as nursery areas so that shrimp catches many miles away may depend critically on mangrove productivity Hard evidence for such connections between mangroves and other ecosystems, however, is sometimes elusive, and the strength of such linkages is almost impossible to quantify Mangrove diversity Mangrove diversity must be considered at a range of spatial scales, from global patterns of species richness to the pattern of distribution, at a particular location, at a scale of a few meters In considering mangrove fauna, even smaller spatial scales become relevant At all scales, diversity is affected by the past history of the area, by physical factors, and by biotic interactions, but the importance of each of these and the timescales over which they operate vary with scale (Duke et al., 1998) Global Patterns Latitudinal Range and Species Diversity Mangroves are almost exclusively tropical or subtropical This distribution is a reflection of a temperature limitation: The global distribution of mangroves correlates very closely with, for example, the winter position of the 20 1C isotherm (Figure 10) The number of mangrove species declines with increasing latitude, with the most northerly and southerly mangroves being species of Avicennia In temperate regions, mangroves are replaced by salt marsh vegetation: plants which, like mangroves, are adapted to conditions of salinity and waterlogging but which not carry the additional burden of being a tree or of producing large propagules Connections Longitudinal Differences The salient features of typical mangrove ecosystems are relatively high rates of primary productivity, much of the results of which enter decomposition pathways, either directly or after initial breakdown by leaf-eating crabs or mollusks This is true of leaves and reproductive structures and, on a more protracted timescale, of the woody components of the trees Particulate organic matter, either small leaf fragments or bacterial cells, is ingested by molluskan and crustacean deposit and filter feeders, enters meiofaunal food chains, or accumulates in the mud The ecosystem can be viewed physically as well as in terms of the flow of energy or matter Mangrove trees supply hard surfaces on which other organisms settle, and they modify (as well as respond to) the physical environment by stabilizing the soil, facilitating accretion of mud, and retarding erosion The environment is further modified by the physical activities of burrowing Crustacea and other animals Mangrove ecosystems cannot be considered in isolation They interact with adjacent habitats through the trapping of exogenous sediment or export of particulate or soluble organic matter or inorganic nutrients Animals, by moving between mangroves and other habitats, also contribute to import Within their temperature and latitudinal constraints, mangroves show interesting patterns of species distribution The principal biogeographic division is between the Indo-West Pacific (IWP) and Atlantic–Caribbean–east Pacific (ACEP) regions These two regions have broadly similar areas of mangrove habitat, but the IWP has four times more genera and six times as many species of mangrove: 17 genera compared to 4, and 40 species compared to It is apparent that none of the mangrove genera are very diverse, possibly because of a general limitation on species diversification in harsh intertidal conditions Genera occurring in the IWP, however, are slightly more speciose than those of the ACEP: 2.35 compared with 1.75 species per genus The differences between the IWP and ACEP regions are maintained by major barriers The most obvious of these is the African continent (Figure 10) Less obvious is the barrier represented by the central Pacific This results principally from dispersal limitations rather than from the absence of suitable habitat Suitable environments are present on many Pacific islands without natural mangrove populations, as shown by the success of the artificial introduction of mangrove species to Hawaii