Shielded from monsoon winds by the central mountains of Madagascar, Madagascar mangroves occupy a wide range of environmental and climatic conditions along the western coastline along the Indian Ocean. Although the ecoregion’s species richness is low, it is unusual in supporting certain endemic tree species. The mangroves also shelter highly diverse mollusk and crustacean communities, while capturing sediment that threatens coral reefs and seagrass beds. Birds, sea turtles, and dugongs all utilize mangroves, as do the Malagasy people. Rice farming, shrimp aquaculture and construction materials are all carried out within these mangroves.
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 colonizers are Sonneratia and Avicennia. Rhizopora and Bruguiera are found behind them 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 summarized 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 Madagascar Heron (Ardea humbloti, VU), Madagascar Teal (Anas bernieri, EN), Madagascar plover (Charadrius thoracicus, VU), and Madagascar fish eagle (Haliaeetus vociferoides, CR). The Madagascar kingfisher (Alcedo vintsioides) is also believed to occur in the 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).
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), and Nile crocodile (Crocodylus niloticus) are 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 the mollusks and crustaceans.
Estimates of extant mangrove area range from 2170 to 4000 km2, with 3270 km2 considered the most likely figure. Of this, only about 50 km2 are found on the 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 tree-cutting in the highlands. Some mangrove areas have been converted to rice farming and salt production. Malagasy Government encourages development of shrimp aquaculture and this habitat type is being increasingly used by the private business 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 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.
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