Madagascar subhumid forests
Madagascar. Source: WWF/ Oliver Langrand
The Madagascar subhumid forests ecoregion, covering most of the Central Highlands of Madagascar, boasts a considerable number of endemic species, found chiefly in the relict forest patches and also in some wetland areas, but the remaining habitats areagricultural areas that have almost no biological value, or eroded bare soil areas virtually devoid of vegetation.
This ecoregion is the site of some of the major extinctions of recent times, including that of the world’s largest flightless bird, Madagascar elephant bird (Aepyornis maximus); this bird stood at a height of three metres and attained a body mass of one fourth of a ton. It was hunted to extinction by native peoples, who also collected the eggs for human consumption. A number of large lemurs have also been driven to extinction by native people's activities, primarily due to slash-and-burn shifting agricultural practises. With only small fragmented areas of habitat left within this ecoregion, there is a high risk of further species extinction over the next several decades. The Madagascar subhumid forests is classified within the Tropical and Subtropical Moist Broadleaf Forests biome.
This ecoregion is an important element of the Madagascar and the Indian Ocean Islands biodiversity hotspot. Dominated by the nation of Madagascar, the fourth largest island on Earth, the hotspot also includes the independent nations of Seychelles (including Aldabra), the Comoros, Mauritius (including Rodrigues), and the French overseas departments of Réunion, Mayotte (one of the Comoros) and the Iles Esparses around Madagascar in the Indian Ocean.
Location and general description
The Madagascar subhumid forests are scattered in several biological islands of montane humid forest throughout the Central Highlands of Madagascar, the zone generally defined as above the coastal plain and escarpment contour line at 900 metres. The remaining substantial areas of forest habitat are in the Sambirano region in the northwest, portions of Amber Mountain (Montagne d’Ambre) in the north, significant areas of the northern highlands, and the middle elevational portions of certain massifs in the Central Highlands (e.g., Ankaratra and Andringitra). Also included are some wetlands and lakes (e.g., Lake Alaotra) and the "tapia" forest of the Central Highlands between 800-1600 m and in the dry southwest portion of the ecoregion. Also, a few remnant regions of forest, isolated and highly fragmented, remain scattered across the central highlands (e.g., Ambohitantely, Ambohijanahary, Isalo). In the extreme southwest of the island, the isolated mountain of Analavelona retains in the disjunctive summital area (1000-1300 m) a remnant subhumid forest surrounded by much drier vegetation. Degraded forests, huge expanses of secondary grasslands and exotic tree plantations surround these habitats. This secondary vegetation is the result of human activity; forest clearing and exploitation.
The subhumid forest ecoregion has been previously mapped as part of the eastern Madagascar regional center of endemism. To the east, these subhumid forests meet moist forests, in the lowland forest ecoregion around 800 m elevation, and to the west they merge into the dry deciduous forest ecoregion around 600 m elevation. At higher elevations (generally above 1800 to 2000 m) these habitats are replaced by ericoid thickets.
Source: World Wildlife Fund
The rainfall is approximately 1500 mm per year, although it may amount to as much as 2000 mm in the Sambirano area in the northwest and as little as 600 mm in the southwest. The temperatures at higher elevations are mainly moderate, between 15° and 25°C. There is a cool, dry season between July and September and a warmer wet season during the rest of the year.
The underlying geology of the ecoregion is mainly ancient Precambrian basement rocks that have been deformed and uplifted over millions of years. There are a few areas of more recent lava flows, and some alluvial deposits associated with wetlands.
Vast grasslands now cover much of the central highlands at elevations ranging from 1000 to 1500 metres. The majority of this upland area was formerly forested, and native peoples have affected the fauna and flora through massive deforestation. It is clear there have been very significant anthropogenic (human-caused) changes in the ecoregion. The central highlands was once home to a remarkable array of endemic species. These included several species of elephant birds (Aepyornithidae), including the world’s largest bird species (Aepyornis maximus), a giant tortoise, and several species of lemurs most of which were large bodied primates, some more massive than female gorillas, for example. All of these species have become extinct since the arrival of humans on the island approximately 2000 years ago.
Marojezy, Madagascar. Source: Olivier Langrand The secondary grasslands that cover most of the high, central highlands are composed of alien or pantropical grass and tree species. There are only three or four species of grass over vast areas, resulting in a virtually sterile landscape with extremely low species diversity (Biodiversity) and endemism. The dominant grasses are Aristida similis and A. rufescens, interspersed with a few herbaceaous species. Several species of Eucalyptus and Acacia trees have been introduced and are now the most common trees in the highlands. Some native, fire-resistant trees persist in areas of the central highlands, including the endemic palms Bismarckia nobilis and Ravenala madagscariensis and the tapia tree (Uapaca bojeri). Other native trees of the genera Sarcolaena, Tambourissa and Weinmannia are also found.
The Sambirano region in the northwest is a center of endemism and a transition zone between the species compositions (both plant and animal) of the western and the eastern regions of Madagascar. The montane forests of the Sambirano, starting at between 600 and 1000 meters, are very similar in structure to more eastern subhumid forests. Relatively little remains of the lowland forest between sea level and about 800 m, the zone that was separated as the Sambirano Domain. There are subhumid forests up to 1800 m. This habitat transitions to a more sclerophyllous forest at higher elevations. This higher ground, above 2000 m, including the Tsaratanana Massif, is included in the Ericold thicket ecoregion.
There are few remaining patches of subhumid forest on the central highlands. Small patches are found on Ankaratra Massif, and some larger forest blocks are on the slopes of Andringitra and Tsaratanana massifs. At the higher elevations, the subhumid forest, also referred to as sclerophyllous montane forest, holds canopy trees that are ten to twelve metres in height, including species from the families Rubiaceae, Lauraceae, Verbenaceae and Ericaceae. At lower elevations, from 1400 to 1600 m, the forest has a fifteen metre high canopy and includes species in the families Cunoniaceae, Araliaceae, Cornaceae, Celestraceae, Anacardiaceae, Burseraceae, Euphorbiaceae, Lauraceae and Ebenaceae.
Amber Mountain (Montagne D’Ambre) contains a significant area of humid forest. An isolated basaltic mountain with a humid microclimate above 1000 m, it is surrounded by dry deciduous forests. At sea level, the annual precipitation is approximately 980 mm, while at the peak of Amber Mountain the precipitation averages 2378 mm. The humid forest on the mountain slopes has a canopy forty metres high, dominated by plants from the families Sapotaceae, Burseraceae, Monimiaceae, Lauraceae, Flacourtiaceae, Sterculiaceae, Myrtaceae, Annonaceae, Apocynaceae, Potaliaceae and Elaeocarpaceae. In addition to the floral diversity of the forest, the faunal diversity is high with eight species of primates and nearly 80 species of birds, including the endemic Ambre Mountain Rock Thrush (Monticola erythronotus). As the natural habitats of this ecoregion experienced numerous vegetational shifts associated with Pleistocene climatic vicissitudes, many species are endemic and have very narrow altitudinal or isolated distributions.
The remaining "tapia" woodlands, in the southwest of the ecoregion are restricted in distribution. The largest intact areas of this habitat are found in the Isalo and Itremo massifs on sandstone and quartzite. They are characterized by a relatively open canopy dominated by members of the family Sarcolaenaceae and Euphorbiaceae, including the fire-resistant Uapaca bojeri and the genus Sarcolaena. The shrub layer consists of Asteraceae, Rubiaceae, and Leguminosae. There are also some endemic Kalanchoe and Aloe species.
Biodiversity features
Vasa parrot (Coracopis sp.), Madagascar. Source: WWF This ecoregion is similar to others in Madagascar in that most natural vegetation cover has been destroyed. The remaining small and isolated patches or distinctly larger blocks are biodiversity "jewels" essential for a variety endemic species. There are a total of 495 vertebrate species recorded in the Madagascar subhumid forests.
Mammals
Numerous mammalian taxa are strictly endemic to this ecoregion, including the Alaotran gentle lemur (Hapalemur griseus aloatrensis), and numerous shrews, tenrecs and rodents. A far larger number of species are near endemic, with the majority of these shared with the lowland forests to the east. At least 45 species of mammals are found only in the subhumid forest ecoregion and the lowland forest ecoregion of Madagascar and these include, for example, two species of bamboo lemurs (Hapalemur aureus and H. simus).
Diademed sifaka, Montadia. Source: C.Michael Hogan Of the endemic and near-endemic mammal species in the ecoregion, 12 species listed are on the IUCN Red List; nine species are considered vulnerable; two are endangered and one (the Alaotran gentle lemur) is critical. In the Analavelona forest a species of small mammal was recently discovered, Microgale nasoloi, that is only known from this site and the nearby Zombitse-Vohibasia Forest, the latter being classified in the Madagascar succulent woodlands ecoregion.
In addition to the large number of mammalian endemics, there are many special status mammals in the ecoregion, including the Vulnerable Aquatic tenrec (Limnogale mergulus); and the Near Threatened Aye aye (Daubentonia madagascariensis).
Avifauna
Two endemic bird species are found in the wetlands of this ecoregion, and others are confined to the subhumid forests or shared with other Madagascar ecoregions. In the wetlands, both the Alaotra little grebe (Tachybaptus rufolavatus) and the Madagascar pochard (Aythya innotata), are considered critically endangered and may be extinct. In the forests the endemic species include, for example, a new genus and species only named a few years ago called the cryptic warbler (Cryptosylvicola randrianasoloi), the yellow-browed oxylabes (Crossleyia xanthophrys), and the brown emutail (Dromaeocercus brunneus). Several other species of birds found here are limited to marshland habitats on Madagascar, including the slender-billed flufftail (Sarothrura watersi), Madagascar snipe (Gallinago macrodactyla), and Madagascar rail (Rallus madagascariensis). Further, Appert’s greenbul (Xanthomixis apperti), an endemic species with a very limited geographical distribution, is abundant on the upper reaches of the Analavelona Massif. More than 20 other bird species that occur in the subhumid forests of this ecoregion are shared only with the eastern lowland forests ecoregion.
Reptiles
There are at least 25 strictly endemic reptiles in this ecoregion. These numbers include historically described species as well as newly identified taxa. Numerous species of chameleon and dwarf chameleon only occur in this ecoregion, including Calumma oshaughnessyi ambreensis, C. tsaratananensis, Furcifer petteri, Brookesia ambreesis, B. antakarana, B. lineata, and B. lolontany in the northern and northwestern portion; and C. fallax, F. campani, and F. minor in the central and southern portions. Otpher lizard species endemic to the ecoregion include the skinks Mabuya grnadidieri, M. madagascariensis, M. nancycoutouae, Amphiglossus meva, and Androngo crenni; the geckos Lygodactylus blanci,L. decaryi and Phelsuma klemmeri, and the Plated lizard Zonosaurus ornatus. There are also a few endemic species of snakes including Pseudoxyrhopus ankafinensis, Liopholidophis grandidieri, and L. sexlineatus.
Amphibians
The Madagascar subhumid forests hold more than twenty strictly endemic amphibians. Several groups of amphibians include more than one endemic species, such as the microhylids Rhombophryne testudo, Scaphiophryne goettliebi, the mantellids Vulnerable Elegant Madagascar frog (Spinomantis elegans); Mantella crocea, M. cowani, M. eiselti, Mantidactylus domerguei, and the Near Threatened Decary's Madagascar frog (Gephiyromantis decaryi); and the rhacophorids Boophis laurenti and B. microtympanum. Other notable amphibian endemics include: Benavony stump-toed frog (Stumpffia gimmeli);
There are a number of special status amphibians in the ecoregion including the Near Threatened Ambohimitombo bright-eyed frog (Boophis majori); the Vulnerable Andoany stump-toed frog (Stumpffia pygmaea); the Endangered Andringitra Madagascar Frog (Mantidactylus madecassus); and the Near Threatened Betsileo Bright-eyed Frog (Boophis rhodoscelis).
Ecological status
Zebu grazing in secondary forest clearing east of Antananarivo. Source: C.Michael Hogan
The remaining natural habitats of the central highlands are extremely fragmented except for the zone spanning its eastern edge and the upper portion of the eastern escarpment. On the central highlands proper, the few patches of natural (Natural environment) vegetation continue to be fragmented by grassland fires. Habitats of the ecoregion are partially protected with the remaining central highland forests of Ambohijanahary and Ambohitantely, being protected areas and the eastern slopes of Andringitra and the upper slopes of Ranomafana being national parks. However, the degree to which the protected areas can maintain and manage the integrity of these habitats varies. Lack of resources, inadequate training and limited personnel, in addition to the absence of clear management plans, all contribute to the difficulty of preventing habitat destruction within the reserves.
Ecological threats
Remaining patches of forest and woodlands of the Central Highlands face continuous and intensive pressure from encroaching agriculture, increasing exploitation by growing human populations, and fire. Alien species of plants and animals are affecting habitat integrity. Nearly all of the ecoregion has been modified either directly or indirectly by humans.
The wetland areas on the central highlands are threatened by conversion to rice farming, siltation, and water pollution. Most marshlands and wetlands of Madagascar have already been degraded or converted to rice cultivation. Relatively undisturbed areas remain, including the marshlands and associated forest of the Torotorofotsy area near Andasibe and other sites scattered throughout the ecoregion. Many of these marshlands suffer water pollution with runoff from surrounding agricultural lands. Extinctions of freshwater fish have occurred in these wetlands in the last few years. Lake Alaotra, in particular, is an important wetland possessing the endemic Alaotra little grebe and Madagascar pochard. The lake is also home to an endemic primate subspecies, the Alaotra gentle lemur (Hapalemur griseus alaotrensis), and several species of endemic fish. Lake Itasy used to be a significant habitat for waterbirds, but has been degraded by intensive rice cultivation.
Justification of ecoregion delineation
The Madagascar Subhumid Forest, located in central Madagascar, is based on Cornet’s subhumid bioclimatic division, with the eastern boundary delineated at 800 meters elevation and the western boundary delineated at 600 m elevation. Although the western boundary differs from Humbert’s vegetation map, the 600 m contour better reflects climatic patterns which distinguish moist evergreen forest from dry deciduous forest. The ecoregion also includes disjunct subhumid areas such as Mt. d’Ambre in the north and the Analavelona and Isalo massifs in the southwest. The ecoregion is accorded the ecocode AT0118 by the World Wildlife Fund.
Neighbouring ecoregions
The following ecoregions have some tangency to the Madagascar subhumid forests:
- Madagascar lowland forests, to the east
- Madagascar dry deciduous forests, to the north and west
- Madagascar succulent woodlands, at the southwest
- Madagascar spiny thickets, to the south
References
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- Carleton, M. D., and S. M. Goodman. 1998. New taxa of nesomyine rodents (Muroidea: Muridae) from Madagascar's northern highlands, with taxonomic comments on previously described forms. In A floral and faunal inventory of the Réserve Spéciale d'Anjanaharibe-Sud, Madagascar: with reference to elevational variation, ed. S. M. Goodman. Fieldiana: Zoology, new series, 90: 163-200. ISBN: 9998798876
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