Section 2: Biogeographic Regions of Madagascar
According to White (1983) in The Vegetation of Africa, Madagascar can be divided into two major biogeographic regions — Eastern and Western (Fig. 1). The Eastern Region comprises four floristic domains: Eastern, Central, High Mountain and Sambirano. The Western Region is divided into two Domains: Western and Southern. Each Domain is characterized by specific types of vegetation. In this Action Plan, we have used descriptions of the different regions as provided in Langrand (1990) and Nicoll and Rathbun (1990).
Eastern Domain
Lowland rainforest, the natural vegetation between sea level and 800 m altitude, extends along the east coast from north of Samabava to Tolagnaro. The average annual precipitation is between 2,000–3,000 mm and there is no dry season. Height of the evergreen canopy averages between 20–30 m, with no emergent trees. This region is characterized by high species diversity and endemism. Eastern lowland forest represents one of the most endangered vegetation types in Madagascar (only the less extensive Sambirano and High Mountain Domains are more at risk), most of it having already been cleared. The largest remaining tracts are found in the northeastern and southeastern parts of the country.
Central Domain
Forests of the Central Domain parallel those of the Eastern Domain and extend westward to Madagascar's central plateau above the 800 m elevation to altitudes of 1,300 m and occasionally reach 2,000 m. Average annual rainfall exceeds 1,500 mm and there is no dry season. Species diversity is as high as in the Eastern Domain; the level of endemism, however, is higher. The forest canopy is lower, averaging between 20–25 m; the epiphytic vegetation is more plentiful and the herbaceous stratum more developed. Most rainforest has disappeared in the Central Domain, but it remains Madagascar's most extensive biogeographic formation. The chief threats to this region are slash-and-burn cultivation, tavy, and exploitation for firewood.
High Mountain Domain
This domain comprises five distinct components: Tsaratanana in the northwest, Marojejy in the northeast, Ankaratra in the central east, Andringitra in the central southeast and Andohahela in the southeast. Forests of the High Mountain Domain grow at altitudes of 2,000–2,867 m. Rainfall is substantial throughout the year and there is a marked diurnal and seasonal variation in temperature. Species diversity is lower here than in previously mentioned domains, but species endemism is high. Fire is the chief threat to this vegetation type. The two largest intact blocks remaining are those at Marojejy and Andohahela.
Sambirano Domain
This domain consists of a small enclave of seasonal moist forest in the northwest. Together with the Tsarantanana Massif, it constitutes the northern end of the island's central mountain range. Annual rainfall exceeds 2,000 mm. The Sambirano is characterized by high species diversity and a high level of endemism, and it represents something of a transition zone between Madagascar's Eastern and Western Regions. Forest canopy height is about 30 m, with some emergents. The chief threats to this vegetation type is cultivation of rice and coffee.
Western Domain
The vegetation of the Western Domain now consists of discontinuous patches of deciduous dry forests on the coastal plains and limestone plateaus, ranging from sea level to 800 m. It covers the area from Antsiranana in the north to Morombe in the southwest. Annual rainfall ranges from 500–2,000 mm, being lightest in the south and heaviest in the north. There is a marked dry season of almost seven months during which many trees of the canopy layer shed their leaves. The shrub layer is well developed and vines are common. Plant species diversity is lower than forests of the Eastern Region (while lemur species diversity is similar), but the level of endemism is higher. The principal threats to this type of vegetation are slash-and-burn cultivation, fire and uncontrolled use by livestock.
Southern Domain
The Southern Domain is characterized by deciduous thicket or thorn scrub, and is perhaps the least disturbed of Madagascar's biogeographic areas. It runs southward from Morombe along the coast, covering much of Madagascar's southern tip from sea level to 400 m. Rainfall in this region is sparse and irregular, ranging from 300–800 mm. The dry season is marked and very long. Forest height is low and the formations are usually impenetrable due to a high incidence of thorny vegetation. The Didiereaceae, an endemic plant family, and various species of Euphorbia are the dominant plant forms. Species diversity and the level of endemism are high.
The primary threats to forests of the Southern Domain are the collection of firewood and of ornamental and medicinal plants, charcoal production and the uncontrolled use of the land for livestock, especially cattle and goats. Slash-and-burn agriculture is not a significant threat due to the poor quality of the soils.
The compilers of this Action Plan recognize the fact that the conservation status of the various species and subspecies of lemurs is related to the conservation status of the different biogeographical regions and the habitats within them. For this reason, we will present information about species, protected areas and proposed actions according to forest types within Madagascar's biogeographical regions.