Location and General Description
This ecoregion includes two separate geographic regions: (1) the western side of Madagascar from the Ampasindava Peninsula in the north to Belo-sur-Tsiribihina and Maromandia in the south and (2) the northern part of the island excluding Montagne d’Ambre above 1000 m. This ecoregion is contiguous with the succulent woodlands ecoregion in the southwest and the sub-humid forest ecoregion to the north and east – the latter limit largely coincides with the western edge of the central highlands around 600 m.
The climate of the ecoregion is tropical, with temperatures ranging from a mean maximum of 30° to 33oC and a mean minimum of 8° to 21oC. There is a wet and a dry season, with most of the rainfall from October to April. Precipitation declines from an annual average of around 1,500 mm in the north to around 1,000 m in the south of the region. The ecoregion occupies the rain shadow on the western side of the central highlands of Madagascar and has a relatively long, pronounced dry season.
The elevation of the ecoregion rises gradually from sea level at the western edge to around 600 m at the eastern edge, where it meets the western edge of the central highlands. There are some significant topographic variations throughout the ecoregion, such as the limestone massifs of Ankarana, Namoroka and Bemaraha, and the volcanic cone of Montagne d’Ambre.
The geology of the ecoregion is varied, being rather complex in some zones, and includes ancient Precambrian basement rocks, unconsolidated sands, and Tertiary and Mesozoic limestone (Du Puy and Moat 1996). While most of the forest on the Tertiary limestone has been destroyed, the spectacular karsts of the Mesozoic limestone and the associated forest patches are more or less intact. The ecoregion is a mosaic of dry deciduous forest, degraded secondary forests and grasslands. It is believed that prior to human colonization of Madagascar around 2000 years ago, most of the ecoregion was covered with dry deciduous forest. The secondary grasslands are a result of frequent burning of the dry forest. They are similar to the secondary grasslands of the central highlands with very low faunal and floral diversity, dominated by alien plant species. These grasslands are virtually sterile landscapes. By contrast, the largely undisturbed dry deciduous forests of western Madagascar have a high diversity of endemic plant and animal species.
The forest is essentially deciduous with most trees losing their leaves during the dry season (May to October), and consequently, relatively heavy leaf litter is a characteristic of the forest. The different soils relating to the various geological substrates is reflected in notable structural and taxonomical differences in the forests of the region. For example, the most luxuriant forests with the highest canopies (10 to 15 m) are found on the richer soils. The forests on the sandy soils have shorter canopies (10 to 12 m), and forests on the calcareous rocks and soils are stunted (Nicoll and Langrand 1989). The plant families represented in the canopy are Leguminosae, Bignoniaceae, Euphorbiaceae, Sapindaceae, and Anacardiaceae. In the scrub layer, there is a diversity of liana species of the Asclepiadaceae family and shrubs of the Leguminosae and Rubiaceae families. There is very little herb layer, though some forests have a carpet of Lissochilus orchids. Some of the distinctive plants in the forests include the flamboyant tree, Delonix regia (family Leguminosae), and several species of baobabs (Adansonia, family Bombacaceae). Distinctive Pachypodium spp. occur on the drier, calcareous soils of the west.
The tsingy massifs are often dissected by canyons where dry deciduous forest grows. Dominant canopy trees in Ankarana include Dalbergia and Cassia spp. (Leguminosae), Ficus spp. (Moraceae) and Adansonia madagascariensis. There is a diverse shrub layer but few epiphytes. The tsingy plateau inlcudes excellent habitat for drought-adapted succulents. Species found on Ankarana include Pachypodium decaryi, Adenia neohumbertii (family Passifloraceae) and several species of Euphorbia: E. ankarensis, E. pachypodiodes, and E. neohumbertii var. neohumbertii (Preston-Mafham 1991).
The Madagascar dry deciduous forest forms a major part of the western center of endemism in Madagascar (White 1983) and the dry, deciduous forests of this region have high biological importance. While the species diversity is not as high as in the moist eastern forests, the levels of endemism are higher. White (1983) estimated generic/specific plant endemism as 20 and 70 percent, respectively.
Many plant species are unusual looking as a result of adaptations to the dry climate, and hot, exposed conditions found on the tsingy formations. While succulents are found throughout arid regions of Madagascar, the dry forest species differ from the succulent species seen further south in succulent woodland and spiny thicket ecoregions. In the dry deciduous forest succulence is rarely seen in the leaves, but is more common in the main tissues. Bottle trees and bottle lianas are common, including those of the genus Adenia, and the thorny long-necked bottles of Pachypodium. Pachycaul tree species include four species of Adansonia, several Moringa, and Delonix hildebrantii (Rauh 1995). Another adaptation against dehydration can be seen in Platycerium quadridichotomum, which has the ability to completely dry up and then revive itself after rainfall. Bottle lianas and Begonia spp. respond to drought by losing most of their aerial parts, while western bamboo species completely lose their leaves in the dry season (Guillaumet 1984).
The fauna of the deciduous dry forest ecoregion has some overlap with that of the succulent woodlands, but it is mostly distinct, endemic and diverse. There is high beta diversity of lemur species across a latitudinal gradient, with five subspecies of Propithecus, three species of Lepilemur, and five species of Microcebus found throughout the ecoregion. The alpha diversity is also high within habitats in the ecoregion. In some primary dry deciduous forest sites, there are eight known sympatric species of lemurs, many of which are endemic to the region and which represent four of the five endemic and endangered families of primates in Madagascar.
Endemic mammals species to the ecoregion include the golden-crowned sifaka (Propithecus tattersalli), mongoose lemur (Eulemur mongoz), western forest rat (Nesomys lambertoni), golden-brown mouse lemur (Microcebus ravelobensis), northern rufous mouse lemur (M. tavaratra), western rufous mouse lemur (M. myoxinus), Perrier's sifaka (Propithecus diadema perrieri), Milne-Edwards’s sportive lemur (Lepilemur edwardsi), and a species of forest mouse, Macrotarsomys ingens. Lemur species, particularly the brown lemur (Eulemur fulvus), may be critical to the regeneration of the forests because they are some of the few and potentially most important seed dispersers in this diverse forest (Ganzhorn et al. 1999).
The dry deciduous forests are one of the primary habitats for the island’s largest predator, the fossa (Cryptoprocta ferox), and some of the smaller endemic Carnivora. The rivers and lakes of the ecoregion are critically important habitats for the endemic and endangered Madagascar sideneck turtle (Erymnochelys madagascariensis). This species represents a significant "Gondwanaland relic," as its closest relatives are in the Podocnemis genus of in South America. The scrubland and bamboo forests of the ecoregion are the habitat of one of the most endangered reptiles in the world, the ploughshare tortoise (Geochelone yniphora). Other critical endemic reptiles of the ecoregion include the chameleons Brookesia bonsi, B. decaryi (C. Raxworthy 2000, pers. comm.) At least three chameleon species are endemic to this ecoregion, these include Furcifer tuzetae, F. rhinoceratus, and F. angeli. The dwarf chameleons Brookesia exarmata and B. perarmata are endemic to the Tsingy of Bemaraha. The colorful arboreal snake Lycodryas ‘Stenophis" citrinus is only recorded from Tsingy de Bemaraha and Namoroka region. Several geckos are endemic to this ecoregion including Paroedura maingoka, P. vazimba, P. tanjaka, Uroplatus geuntheri, and Lygodactylus klemmeri, the latter is only known from the Tsingy de Bemaraha. Futher, the region also holds several endemic skinks species including Mabuya tandrefana, Pygomeles braconnieri, and Androngo elongatus. Recently new species of plated lizard were described from the ecoregion -- Zonosaurus bemaraha in the southern portion and Z. tsingy in the northern portion (Raselimanana et al. 2000).
The ecoregion contains important habitats for 131 of the 186 resident terrestrial bird species listed for Madagascar (Langrand 1990). Several of these species are associated with lakes and rivers of the region, such as the Manambolo, Betsiboka, Mahajamba, and their satellite lakes. These species include Bernier’s teal (Anas bernieri), Madagascar fish eagle (Haliaeetus vociferoides), Humblot’s heron (Ardea humblotii) and the Sakalava rail (Amaurornis olivieri) (Stattersfield et al. 1998). These birds are dependent on wetlands and they are becoming increasingly isolated and restricted due to habitat fragmentation and conversion to rice paddy. Some of these species also use the fringes of the mangroves on the western coast of Madagascar (see ecoregion description). Several bird species are confined to the western forests, have limited or disjunct ranges, in some cases associated with habitat fragmentation including Van Dam’s vanga (Xenopirostris damii), and white-breasted mesite (Mesitornis variegata).
Several other species of plants and animals endemic to the ecoregion are also particularly threatened due to their restricted ranges. These include two of the six Malagasy endemic baobab species (Adansonia grandidieri and A. suarezensis), two species or subspecies of primate, Perrier's sifaka and golden crowned sifaka, western forest rat, and ploughshare tortoise.
The dry deciduous forest ecoregion includes a narrow and fragmented band of coastal, or littoral, forests. Most of these littoral forests have already been degraded or destroyed, but the remaining areas are known to contain rare and locally endemic genera and species of plants (Du Puy and Moat 1996).
A majority of the western forests have been destroyed (Jenkins 1987). The area’s dry forest habitat, as mapped by White (1983), shows that the main areas of intact forest are located near the western coast, and the areas closest to the central highlands re already composed of secondary grassland. More recent surveys indicate that the remaining forest is smaller than originally thought and ranges from 12,000 to 20,000 km2 (Du Puy and Moat 1996). Most of this forest is in small fragments of 35 km2 or less, with only 55 blocks larger than 35 km2 and only 5 blocks larger than 500 km2 (Morris and Hawkins 1999). Many of the isolated blocks of remaining primary forest are found in a number of protected areas, but there are some important areas of dry forest habitat that are not protected at the present time. The largely intact forests that are found in protected areas are often insignificant and the existing mechanisms for managing and monitoring these areas may not be effective in protecting the forest (Nicoll and Langrand 1989). Key protected areas include Ankarafantsika National Park (605 km2) and Tsingy de Bemaraha World Heritage Site (1520 km2). A priority-setting workshop identified several sites in need of immediate biodiversity protection (Ganzhorn et al. 1997). In the north, these sites include Daraina and the Andavakoera and Irodo areas. Further south, the Bemaraha site is considered a priority for enlargement.
Types and Severity of Threats
The major threat to the dry, deciduous forests is destruction and fragmentation through intentional burning to clear land for grazing and agricultural lands, and through wildfires sparked by burning adjacent secondary grasslands. With an expanding rural population and increasing degradation of existing arable lands, the pressure on the remaining forest is extremely high. Selective logging and the removal of large trees pose additional threats of forest habitat degradation. It is likely that much of the remaining forest is already secondary forest that has been selectively logged and has lost the largest of its trees. These degraded forests do not support viable populations of at least 7 of the 8 species of lemur found in more intact forests (Ganzhorn 1995). Several species of diurnal lemurs are hunted for food, and this may be adversely affecting the regeneration of the forests (Ganzhorn 1995, Ganzhorn et al. 1999).
River, wetland, and lake systems are threatened with siltation resulting from deforestation of adjacent forests and soil erosion and run-off from the central highlands. Lakes and wetland habitats are also being destroyed through rice paddy cultivation, over fishing, and invasive species (e.g. water hyacinth, Eichornia crassipes).
Justification of Ecoregion Delineation
This ecoregion follows the dry bioclimatic zone of Cornet (1974) that extends inland to the 600 m contour. This is believed to provide a better reflection of original vegetation than the 950 m contour that Humbert and Cours Darne (1965) used to define their Western Domain phytochoria. The western ecoregion has its southern limit in the Belo-sur-Tsiribihina and Maromandia region and extends to the northern portion of the island (not including Mt. Ambre). There is an eastern extension of this region running from the Ampasindava Peninsula to the Vohémar area.
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Prepared by: Helen Crowley
Reviewed by: In progress