Shielded from monsoon winds by the Central Highlands of Madagascar, Madagascar mangroves occupy a wide range of environmental and climatic conditions, and chiefly occur at the western coastline along the Mozambique Channel of the Indian Ocean. Although the ecoregion’s species richness is low, it is noteworthy in supporting certain endemic tree species; for example, thre are only 163 vertebrate taxa found in the entire ecoregion. The Madagascar mangroves are within the Mangroves biome of the Afrotropic Realm.
These mangroves also shelter highly diverse mollusk and crustacean communities, while capturing sediment that threatens coral reefs and seagrass beds. Dugongs, birds and sea turtles utilise mangroves, as do the native Malagasy people. Rice farming, shrimp aquaculture as well as stockpiling of construction materials are activities carried out within this coastal ecoregoion.
Location and general description
On Madagascar, mangroves are found primarily along the western coast. They occur in a wide range of environmental and climatic conditions, fostered by a low coastal platform, high tidal range, and a constant freshwater supply from numerous rivers that also bring a high silt load which is deposited along the coast. The largest mangrove stands are found at Mahajamba Bay, Bombetoka, south Mahavavy and Salala, and Maintirano. Mangroves occupy a stretch of coastline of approximately 1000 kilometres in length where they are often associated with coral reefs, which protect the mangroves from ocean swells. The mangroves, in turn, capture sediment from the interior lands that threatens both reefs and seagrass beds. The southern part of Madagascar has fewer mangroves because, in addition to having a longer dry season and lower rainfall, it is subject to intensive ocean swells and lacks the necessary alluvial sediments deposited by major river systems. This latter point is especially true of the eastern side of the island.
Water temperatures are relatively even from north to south, and rainfall varies with climatic zones that range from 2000 millimetres (mm) in the humid subequatorial north to 350 mm in the dry subtropical south. Madagascar has two seasons: a cool dry season from May through October, and a warm humid season from November through April. Salinity variation is greater along the northwest coast where rainfall is higher, ranging between 31.8% at the end of the rainy season to 35.2% at the end of the dry season. On the western coast, the tidal range may reach up to four metres (m) during the equinoctial periods, compared with 0.75 m on the east coast. Major rivers, which flow towards the west coast are the Mangoky, the Tsiribihina, and the Betsiboka.
Although up to nine mangrove tree species have been recorded, most of the Madagascar mangrove stands contain six species in four families: Rhizophoracae (Rhizopora mucronata, Bruguiera gymnorrhiza and Ceriops tagal), Avicenniaceae (Avicennia marina), Sonneratiaceae (Sonneratia alba) and Combretaceae (Lumnitzera racemosa). Other reported species are: Ceriops tagal, Xylocarpus granatum, and Heritiera littoralis. The primary colonisers are Sonneratia and Avicennia. Rhizopora and Bruguiera are found further backshore or along creeks. Finally, Bruguiera, Ceriops tagal and Xylocarpus are found in the tidally inundated areas. Other plant species found in the Madagascar mangroves are summarised in Koechlin et al.
Several of the Madagascar endemic birds occur in the coastal areas of western Madagascar, where these species utilise mangrove and associated wetland habitats. These species are: the Endangered Malagasy sacred ibis (Threskiornis bernieri), endemic to western coastal Madagascar and the Seychelles; the Madagascar Heron (Ardea humbloti, VU), Madagascar Teal (Anas bernieri, EN), Madagascar plover (Charadrius thoracicus, VU), and Madagascar fish eagle (Haliaeetus vociferoides, CR). The Malagasy kingfisher (Alcedo vintsioides) is also thought to occur in these mangroves. This habitat is important for migratory bird species, such as Common ringed plover (Charadrius hiaticula), Crab plover (Dromas ardeola), Gray plover (Charadrius squatarola), African spoonbill (Platalea alba) and Great White Egret (Egretta alba).
A number of mammalian taxa are found in the ecoregion, chiefly lemurs, tenrecs and bats. The sole terrestrial apex mammalian predator of the ecoregion is the Malagasy civet (Fossa fossana), a Madagascar endemic.
Tenrecs occurring in the ecoregion are: Large-eared tenrec (Geogale aurita), the tiniest extant tenrec; Greater hedgehog tenrec found in the Madagascar mangroves, an insectivorous mammal; Lesser hedgehog tenrec (Echinops telfairi); and Tailless tenrec (Tenrec ecaudatus). Each of these tenrecs is endemic to Madagascar, save for the Tailless tenrec, which is also found on Comoros and a few other islands in the region.
Primates found in the Madagascar consist of several lemur species: the Endangered Verreaux's sifaka (Propithecus verreauxi), endemic to western and southwestern Madagascar; the Vulnerable Black lemur (Eulemur macaco); the Vulnerable Red-fronted lemur (Eulemur rufus); the Vulnerable Sambirano Bamboo Lemur (Hapalemur occidentalis); the Endangered Coquerel's Mouse-lemur (Microcebus coquereli), a Madagascar endemic; the Vulnerable Decken's sifaka (Propithecus deckenii), a western Madagascar endemic; Sambirano Woolly Lemur (Avahi unicolor), a northwestern Madagascar endemic; Pale-forked crown lemur (Phaner pallescens), endemic to western Madagascar; Fat-tailed dwarf lemur (Cheirogaleus medius); and Grey mouse lemur (Microcebus murinus).
Bats occurring here are the Near Threatened Malagasy rousette (Rousettus madagascariensis), a cave rooster capable of navigating the airspace of rather dense intact forest; Vulnerable Madagascan fruit bat (Eidolon dupreanum); Near Threatened Commerson's roundleaf bat (Hipposideros commersonii); Near threatened long-fingered bat (Miniopterus schreibersi); Rufous trident bat (Triaenops rufus); Malagasy giant mastiff bat (Otomops madagascariensis), a Madagascar endemic; Malagasy White-bellied Free-tailed Bat (Mops leucostigma), endemic to Madagascar and the Comoros islands of Anjouan and Moheli; Malagasy slit-faced bat (Nycteris madagascariensis), a narrow endemic to the Irodo River Valley in northern Madagascar; Mauritian tomb bat (Taphozous mauritianus); Trouessart's trident bat (Triaenops furculus), endemic to Madagascar and the outer Seychelles atolls; Manavi Long-fingered Bat (Miniopterus manavi), endemic to Madagascar and Comoros; Grandidier's Free-tailed Bat (Chaerephon leucogaster); Robust yellow bat (Scotophilus robustus); Malagasy mouse-eared bat (Suncus madagascariensis); and Malagasy serotine (Neoromicia matroka). Flying foxes found in the ecoregion are: Madagascan flying fox (Pteropus rufus), an important seed disperser who mates whilst hanging upside down.
Other mammals found in the ecoregion are the Madagascan pygmy shrew (Suncus madagascariensis); The only Rodentia member in the ecoregion is the Dormouse tufted-tailed rat (Eliurus myoxinus)
There is only one amphibian species present in the Madagascar mangroves: Mascarene ridged frog (Ptychadena mascareniensis).
There are a limited number of reptilian taxa found in the ecoregion: Snake-eyed skink (Cryptoblepharus boutonii); and aquatic apex predator Nile crocodile (Crocodylus niloticus).
Marine non-fish species
Some sea turtles, primarily green turtle (Chelonia mydas, EN) and Hawksbill turtle (Eretmochelys imbricata, CR), nest along the western coast and are occasionally found in mangroves. The declining species Dugong (Dugong dugong, VU) is also found in the mangroves.
There is particularly high diversity among the fish populations, the families of which include: Mugelidae, Serranidae, Carangidae, Gerridae, Hemiramphidae, Plectrorhynchidae and Elopidae. The neighboring coral reefs that are associated with the mangroves have also been noted for extremely high fish diversity. There is also high diversity among mollusks and crustaceans.
Estimates of the extant mangrove area range from 2170 to 4000 km2, with 3270 km2 considered a best estimate. Of this, only about 50 km2 are found on the Madagascar east coast at eleven sites. In contrast, 29 mangrove areas are found on the west coast. More than half of these are found at four sites. Some mangroves are found in the existing marine park: Reserve Mananara Biosphere Reserve, that also includes coral reefs.
Types and severity of threats
Mangroves are threatened by development of urban areas, overfishing, and erosion caused by ongoing deforestation in the Central Highlands. Some mangrove areas have been converted to rice farming and salt production. The Malagasy government encourages development of shrimp aquaculture, and this coastal habitat type is being increasingly used by the private sector. Because of relatively low population densities and availability of wood from other sources, direct harvesting of the mangrove trees has been relatively low with the exception of some areas, particularly Mahajanga and Toliara. However, demographic trends and population growth suggest this situation could change in the future.
Justification of ecoregion delineation
Ecologically, the mangroves of Madagascar are very similar to those of the African mainland. However, they were separated due to their presence in a different biogeographical region. Nearly all of the mangroves in Madagascar occur along the low-lying western coast. Of these, only the larger stands have been delineated. This mangrove ecoregion is accorded the ecocode AT1404 by the World Wildlife Fund.
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