What is Mangrove Forest?
Mangrove forests are comprised of unique plant species that form the critical interface between terrestrial, estuarine, and near-shore marine ecosystems in tropical and subtropical regions. The term mangrove is generally used to describe the assemblage of trees and shrubs that grow in saline coastal habitats, normally found in the tropics and sub-tropics. Mangrove plants are not land builders but once established, play a key role in stabilization of newly accreted sediment. Mangrove tree species have developed a wide range of features, including specialized stomatal, leaf, seed, and root structures, as adaptations for living in a saline environment with high incidence of inundation. Most mangrove species are viviparous; seeds germinate while still attached to the tree. Buoyant seeds develop into mature propagules, which then drop and disperse by water. Of note is the diversity in root structures of the mangrove tree species, designed to facilitate anchorage, nutrient uptake, and gaseous exchange. Some species, such as Heritiera fomes, Sonneratia apetala, and Avicennia officinalis, have pneumataphores; upward projecting root branches that enable the gaseous exchange required for respiratory metabolism. Other species, such as Rhizophora mucronata, have “stilt” or “knee” roots for gaseous exchange, while additional features such as the “tap” roots (buttresses) of Heritiera fomes and the horizontal spread of roots in general, add to overall stability. Some plants also excrete excess salt through leaves.
Why Care about Mangroves?
The importance of mangroves for humans and a variety of coastal organisms has been well documented. They protect inland human communities from damage caused by coastal erosion and storms, provide critical habitat for a variety of terrestrial, estuarine and marine species, and serve as both a source and sink for nutrients and sediments for other inshore marine habitats including seagrass beds and coral reefs. Mangrove species that form dense and often monospecific stands are considered “foundation species” that control population and ecosystem dynamics, including fluxes of energy and nutrients, hydrology, food webs, and biodiversity. Mangroves have been widely reviewed as supporting numerous ecosystem services including flood protection, nutrient and organic matter processing, sediment control, and fisheries. Mangrove forests are the economic foundations of many tropical coastal regions providing at least US$1.6 billion per year in “ecosystem services” worldwide. It is estimated that almost 80% of global fish catches are directly or indirectly dependent on mangroves. Mangroves sequester up to 25.5 million tonnes of carbon per year (where is this statistic from?) and provide more than 10% of essential organic carbon to the global oceans. Although the economic value of mangroves can be difficult to quantify, the relatively small number of mangrove species worldwide collectively provide a wealth of services and goods while occupying only 0.12% of the world's total land area. Mangrove ecosystems are the most biologically productive ecosystem on earth. They are the keystone habitats by the sea and ecologically most diverse across the tropical belt. Briefly speaking mangroves provide six essential ecological services: (1) land maturation, (2) protection of human habitation from cyclones, (3) oxygen production, (4) waste recycling, (5) food supply, and (6) carbon cycling.
What is Happening to Mangrove Ecosystems:
Despite its enormous biodiversity significance, mangroves are the most vulnerable to anthropogenic persecution due to varying degree of misconception surrounding its values to human and other animals in tropical ecoregions. With almost half (44%) of the world's population living within 150 km of a coastline, heavily populated coastal zones have spurred the widespread clearing of mangroves for coastal development, aquaculture, or resource use. At least 40% of the animal species that are restricted to mangrove habitat and have previously been assessed under IUCN Categories and Criteria are at elevated risk of extinction due to extensive habitat loss. It is estimated that 26% of mangrove forests worldwide are degraded due to over-exploitation for fuelwood and timber production. Similarly, clearing of mangroves for shrimp culture contributes ~38% of global mangrove loss, with other aquaculture accounting for another 14%. In India alone, over 40% of mangrove area on the western coast has been converted to agriculture and urban development. However the overlapping of marine and terrestrial resources in mangroves creates tenure ambiguities that complicate management and may induce conflict between competing interests. Globally, between 20% and 35% of mangrove area has been lost since approximately 1980, and mangrove areas are disappearing at the rate of approximately 1% per year with other estimates as high as 2–8% per year. These rates may be as high as or higher than rates of losses of upland tropical wet forests and current exploitation rates are expected to continue unless mangrove forests are protected as a valuable resource. Given their accelerating rate of loss, mangrove forests may at least functionally disappear in as little as 100 years. The loss of individual mangrove species is also of great concern, especially as even pristine mangrove areas are species-poor compared with other tropical plant ecosystems. However, there is very little known about the effects of either widespread or localized mangrove area loss on individual mangrove species or populations. Additionally, the identification and implementation of conservation priorities for mangroves has largely been conducted in the absence of comprehensive species-specific information, as species-specific data have not been collated or synthesized. Species information including the presence of threatened species is important for refining conservation priorities, such as the designation of critical habitat, no-take zones, or marine protected areas, or to inform policies that regulate resource extraction or coastal development. Below we have provided graphical images of native distributions of mangrove species' richness and proportion of IUCN Red List of Threatened (Critically Endangered, Endangered, and Vulnerable) mangrove species respectively.
The Cost of Mangrove Species Loss:
The loss of individual mangroves species and associated ecosystem services has direct economic consequences for human livelihoods, especially in regions with low mangrove species diversity and low ecosystem resilience to species loss. In the Gulf of California, for example, where there are only 4 mangrove species present (Avicennia germinans, Rhizophora samoensis, Laguncularia racemosa, Conocarpus erectus), it is estimated that one linear kilometer of the species R. samoensis, listed as Near Threatened, provides up to 1 ha of essential marine habitat and provides a median annual value of US$37,000 in the fish and blue crab fisheries. Nutrients and carbon from mangrove forests provide essential support to other near shore marine ecosystems such as coral reefs and seagrass areas, and enrich coastal food webs and fishery production. Avicennia species are dominant in inland or basin mangrove forests in many parts of the world. However, 3 of 8 (38%) species in this genus are in threatened or Near Threatened categories. Loss of these species and the mangrove forests they dominate will have far reaching consequences for water quality and other near shore ecosystems in coastal communities around the globe. For example, water purification services provided by these mangrove species in the Muthurajawela Marsh, Sri Lanka are valued at more than $US 1.8 million per year. Riverine or freshwater-preferring species, such as the Endangered Heritiera fomes and Heritiera globosa, buffer coastal rivers and freshwater communities from sedimentation, erosion and excess nutrients. Heritiera globosa is a very rare species confined to western Borneo, while Heritiera fomes is more widespread in south Asia, but has experienced significant declines in many parts of its range. Localized or regional loss of these coastal or fringe mangrove species reduces protection for coastal areas from storms, erosion, tidal waves, and floods with the level of protection also dependent on the quality of remaining habitat. Two of 4 (50%) fringe mangrove species present in Southeastern Asia (Sonneratia griffithii, Aegiceras floridum) are listed in threatened or Near Threatened categories. In other areas, such as Brazil, the central Pacific islands, or West Africa, fringe mangrove forests are often comprised of only one or two species. Even though these species are globally listed as Least Concern, local and regional loss of mangroves in these areas will have devastating impacts for coastal communities. The loss of species may indeed be of greatest economic concern in rural, high-poverty areas where subsistence communities rely on mangrove areas for fishing and for direct harvesting of mangroves for fuel, construction or other economic products. Finally it is important to note that the amount of mangrove area in some countries is increasing due to reforestation and restoration efforts. Although regeneration of degraded mangrove areas is thought to be a viable option in some areas, successful regeneration is generally only achieved by the planting of monocultures of fast-growing species, such as Rhizophora or Avicenna species. Many rare and slow growing species are not replaced and many species cannot be easily replanted with success. In sum, mangrove areas may be able to be rehabilitated in some regions, but species and ecosystems cannot be effectively restored.
What is Megachiroptera?
Megachiroptera is one of the two suborders of mammalian order Chiroptera that is bats. Of the rich diversity of vertebrate fauna, bats are unique in being the only group of mammals that, like birds, have sustained flight. One of the 26 mammalian orders, the Chiroptera includes 1117 species of
bats world over in rather two unequal suborders - the Megachiroptera (consisting 186 species of Old
World fruit bats or flying fox in one family) and the Microchiroptera (consisting 931 species in 17 families). Megachiroptera also colloquially known as Megabats of the order Chiroptera comprises fruit bats also known as flying foxes. The Megabat, contrary to its name, is not always large: the smallest species is 6cm long and thus smaller than some microchiroptera or microbats. The largest reach 40cm in length and attain a wingspan of 1.7m, weighing in at up to 1.6kg. Most fruit bats have large eyes allowing them to orient themselves visually in twilight and inside caves and forests. Their sense of smell is excellent. In contrast to the microbats, the fruit bats do not use echolocation apart from one or two species. Megachiroptera are frugivorous or nectarivorous i.e., they eat fruits or lick nectar from flowers. Often the fruits are crushed and only the juices consumed. The teeth are adapted to bite through hard fruit skins like mangrove fruits. Large fruit bats must land to eat fruit, while the smaller species are able to hover with flapping wings in front of a flower or fruit. Frugivorous bats aid the distribution of plants (and therefore, forests) by carrying the fruits with them and spitting the seeds or eliminating them elsewhere. Nectarivores actually pollinate visited plants. They bear long tongues that are inserted deep into the flower; pollen passed to the bat is then transported to the next blossom visited, thereby pollinating it. This relationship between plants and bats is a form of mutualism known as chiropterophily.
Megachiroptera as Mangrove Pollinator
Megachiroptera comprising flying foxes or fruit bats are animals of extraordinary ecological and economic importance throughout forests of the Old World tropics. Nearly 200 species play an essential role as mangrove pollinators and seed dispersers, yet they are frequently misunderstood intensely persecuted and exceptionally vulnerable to extinction Their role in the propagation of numerous important mangrove plants remains virtually uninvestigated. However, my review of already available literature demonstrates that at least half of the total mangrove plant species (there are roughly 58 species of mangrove trees and plants that are identified and named so far) rely to varying degrees on large populations of flying foxes for propagation. These plants, in addition to their many ecological contributing produce some 448 economically valuable products. The fact that flying foxes are increasingly threatened and that few baseline data exist on population trends is cause for concern. Many appear to be in severe decline, and several species are already extinct. Old World phytophagous bats (Megachiroptera: Pteropodidae) number 186 species of which 79% are Asian and 21% African. Early bats were perhaps initially attracted to mangrove flowers and fruit by the insects found around them, later finding the plants themselves nutritious. Species of Megachiroptera today feed upon floral resources, fruit and leaves from a total of at least 188 plant genera in 64 families. They may effect both pollination and seed-dispersal, and both bat-flower and bat-fruit syndromes are commonly recognized. Individual species are generally catholic in their feeding, favored food varying with locality and season. Depending upon roosting habits and season, megabats may travel considerable distances each night to feed and may undertake seasonal migrations. Their feeding in essentially valuable mangrove species is now seriously threatened by habitat destruction for shrimp aquaculture farming in many South and South East Asian countries. The single Megachiropteran family, the Pteropodidae, ranges from Africa, the eastern Mediterranean, Madagascar and the Indian Ocean islands in the west, across mainland southern Asia, throughout the islands of the western Pacific from the Ryukyu Archipelago and Ogasawara-shato in the north, to coastal eastern Australia, New Caledonia and the Loyalty Islands in the south, and east to Fiji, Tonga, Samoa and the
Cook Islands. There are over 42 genera containing of over 186 species in Megachiroptera. The largest and best known genus, Pteropus, with 57 species, is primarily an island taxon, with 55 species (96.5%) having some or all of their distribution on islands that mainly mangrove dominated coastal islands. In this genus levels of endemism are extremely high, with 35 species (61.4%) confined to single islands or small island groups. Only nine species are found in continental areas (five in Asia and four in Australia), and only
two (P. lylei and P. poliocephalus) are restricted to continents. Below are two images, one shows the Megachiroptera distribution and other one shows mangrove distribution. Combined two images together, one can appreciate how closely the Megachiroptera habitats are closely fall into mangrove habitats across the globe.
There are over 50 true mangrove species with over 100 associated species so far documented and named. The most richest diversity of mangroves is in Indo West Pacific biogeograpic realm (see image below) where species diversity is as high as 40. Notably Indo-Malayan islands pose the highest mangrove diversity where endemism if islandic Megachiroptera is high and a key contributor of the mangrove pollination. Despite the lack of ecological research linking the significant correlation of mangrove pollination by the species of fruit bats or flying foxes of the Megachiroptera guild, it is evident that mangrove and fruit bats developed an interesting symbiotic relationship that posses far reaching conservation implications for both the mangroves and the flying foxes. Below is a graphical demonstration of the proportional distribution of the mangrove species in each genus and its pollinating share by Megachiroptera of fruit bats.
Mangrove habitat loss has been cited by a number of authors as a major factor contributing to declines in fruit bat populations. Although information on habitat requirements is limited for some species, it is evident that there is considerable ecological variation within the family. Deforestation, widespread in almost all tropical mangroves of the world, has had several identifiable consequences for fruit bat populations. Many species, particularly those inhabiting mangrove swamps (e.g. Pteropus vampyrus in Malaysia and Indonesia) and lowland forest, have lost critical roosting areas. Mangrove swamps are being
destroyed by the shrimp aquaculture, woodchipping industry, firewood, and coastal development, and lowland forest is felled for agriculture and timber. For example all the 8 species of mangrove genus Sonneratia are pollinated by fruit bats and Sonneratia being commercially and socio-economically valuable mangrove tree being cleared out with dramatic population decline of the fruit bats in various mangrove ecosystems in South and South East Asia.
Mohammed Ashraf, Wildlife Biologist, Graduate of Anglia Ruskin University, Cambridge, England
Founder of the Ecoblogs, www.ecosysblogs.wordpress.com