Biological diversity in Madagascar and the Indian Ocean Islands
Biological diversity in Madagascar and the Indian Ocean Islands can be defined as the biodiversity on a series of islands scattered in the western Indian Ocean off the southeast coast of Africa; this region is also known as 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.
Because Madagascar and the continental Seychelles broke off from the Gondwanaland supercontinent more than 160 million years ago, the hotspot is a living example of species evolution in isolation. Despite close proximity to Africa, the islands do not share any of the typical animal groups of nearby Africa. Instead, they have evolved an exquisitely unique assemblage of species, with high levels of genus- and family-level endemism, in only 1.9 percent of the land area of continental Africa.
The natural vegetation of this hotspot is quite diverse. On Madagascar, tropical rainforests along the eastern escarpment and in the eastern lowlands give way to western dry deciduous forests along the western coast. A unique spiny desert covers the extreme south. The island is also host to several high mountain ecosystems such as Tsaratanana and Andringitra massifs, which are characterized by forest with moss and lichens. The Sambirano region, a northern transition zone between the western dry forest and the eastern rainforest, has many endemic species.
The Madagascar Subhumid Forests form an important ecoregion that is scattered in several "islands" of montane humid forest throughout the central highlands of Madagascar, the zone generally defined as above the coastal plain and escarpment starting at 900 metres. The remaining large areas of the 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).
The lowland forests of Madagascar include a narrow strip of humid forests along the east coast, low-elevation forests (from sea level up to 800 meters). The forests extend from Marojejy in the north to the extreme southeastern corner of the island. At the northern edge of the ecoregion, around Vohémar, the moist forest changes to a transitional dry forest. This area has localized climatic conditions that lead to different vegetation, one of many such patches across Madagascar. At the ecoregion's southern limit, in the rain shadow of the Anosyennes Mountains, the moist forest changes in a very short distance from a dry transitional forest to spiny forest. At the ecoregion's western edge, the forest grades into subhumid forests around the 800 m contour on the central highlands.
The Indian Ocean islands are composed of a range of relatively recent volcanic islands (the Mascarenes and the Comoros), fragments of continental material (the main group of the Seychelles), and the coral cays of the Amirantes and the atolls of the Farquhar, Cosmoledo, and Aldabra groups, as well as the five Iles Eparses. The volcanic islands have high peaks that in the recent past were covered by dense forest; indeed, the Comoros and the Mascarenes are sometimes subjected to very high levels of rainfall (up to 6000 millimetres per year on Réunion). The highest peak in the Indian Ocean is the Piton des Neiges on Réunion (3069 metres), which received the heaviest downpour on record (4.9 metres of rain in one week in 1980). By contrast, the continental Seychelles are relatively dry with a relatively low altitude reaching only 914 meters at its highest in Mourne Seychellois National Park.
The hallmark of the flora and fauna of Madagascar and the Indian Ocean islands is not necessarily their diversity (though this is high in some groups of organisms, particularly given the island's size), but their remarkable endemism. The high level of species unique to Madagascar and its surrounding islands resulted from tens of millions of years of isolation from the African mainland and from people, who did not arrive until 2000 years ago. Endemism is marked not only at the species level, but also at higher taxonomic levels: the islands have an astounding eight plant families, five bird families, and five primate families that occur nowhere else on Earth.
Vascular plants total at least 13,000 species (and possibly as many as 16,000), of which about 90 percent are found nowhere else in the world. Incredibly, eight of at least 160 plant families found here are endemic, a level unmatched by any other hotspot: seven are confined to Madagascar and an eighth is found on the Seychelles (the Medusagynaceae). The hotspot also has at least 310 endemic genera of plants. Local endemism is high as well; some individual mountaintops have 150 to 200 plants found nowhere else on the island.
The case of the baobab, or bottle tree, illustrates the spectacular diversity and endemism of plants in this hotspot. Worldwide there are eight baobab species in the genus Adansonia, one from continental Africa, one from northwest Australia, and the remaining six from Madagascar. Grandidier's baobab (Adansonia grandidieri), the largest baobab species on the island, is pollinated by nocturnal lemurs; other Malagasy species are pollinated by fruit-eating bats. Found in the drier regions of the west and south, baobabs are well adapted to desert-like conditions. Large reserves of water are stored in their characteristic bottle shaped trunks.
Madagascar recently made headlines in the botanical world with the rediscovery of Takhtajania perrieri, the only Afro-Malagasy member of the primitive family Winteraceae, in the northeast of the country. It is fitting that Madagascar's signature endemic plant, the traveler's tree (Ravenala madagascariensis), is pollinated by the island's flagship vertebrate species, the lemurs.
The avifauna of Madagascar and the surrounding islands is characterized by low diversity but spectacular endemism. More than 300 bird species are regularly found in the hotspot, nearly 60 percent of which are found nowhere else on the planet; additionally, 42 genera and four families are endemic. The bird fauna includes some extraordinarily relict bird species on Madagascar, such as the ground-rollers, cuckoo-rollers, and mesites.
The region's birds are also seriously threatened. Over 55 endemic species are currently threatened, and 32 have already gone extinct, mainly from the Mascarenes. The wet forests of eastern Madagascar have the highest number of threatened birds, including the Madagascar serpent-eagle (Eutriorchis astur, EN) and the Madagascar red owl (Tyto soumagnei, EN). Birds endemic to the island's wetlands, which have undergone extensive conversion for rice cultivation, are faced with imminent extinction. In the east, the Madagascar pochard (Aythya innotata) has only been recorded three times since 1960; the Alaotra grebe (Tachybaptus rufolavatus, CR) has not been confirmed in the last decade. The flightless elephant bird (Aepyornis maximus), extinct more than five hundred years ago, was the heaviest bird ever to have lived at roughly 450 kilograms (nearly 1,000 pounds).
The birds of the Indian Ocean islands are similarly distressed. Réunion has witnessed the extinction of at least 10 bird species since the 1500s and all the endemic birds of Mauritius are threatened. Extinct species include the famous dodo (Raphus cucullatus), which disappeared from Mauritius in the 1600s after the island was colonized by humans, as well as the Réunion solitaire (Pezophaps solitaria) which became extinct in the 1760s.
As with birds, the diversity of the hotspot's mammals is relatively low, but the level of endemism is exceptional. About 90 percent of the more than 150 mammal species that live on the islands are endemic. And new species are being discovered in Madagascar at a rapid rate; for example, in the last 15 years, 22 new mammal species and subspecies have been described.
The most intriguing mammals of Madagascar are the lemurs, represented by five families of primates unique to this island. Madagascar is home to 72 kinds of lemurs (species and subspecies), representing 15 genera, making the Madagascar and Indian Ocean Islands Hotspot the world leader in primate endemism and the single highest priority for the conservation of primates. The lemurs of Madagascar vary widely, from the tiny Madame Berthe's mouse lemur (Microcebus berthae), which, at only 30 grams, is the world's smallest primate, to the indri (Indri indri, EN), which leaps from tree to tree similar to the airborne kangaroo. One of the most unusual lemur species is the aye-aye (Daubentonia madagascariensis, EN), which has huge ears, shaggy fur, continuously growing incisors (like a rodent), and a very thin middle finger on each hand, that together with its large ears are used for catching woodboring insect larvae or excavating coconuts.
Madagascar is also home to more than 15 endemic bat species, including the Madagascar flying fox (Pteropus rufus, VU), and numerous endemic rodents, like the unusual giant jumping rat (Hypogeomys antimena, EN), and carnivores, including the fossa (Cryptoprocta ferox, EN), which resembles a cross between a dog and a cat. An agile hunter on the ground and in the canopy, it is the primary predator of lemurs. The endemic tenrecs, a unique family of insect-eating mammals, occupy the ecological niche that shrews and moles occupy elsewhere.
Mammals are thinly distributed elsewhere on the Indian Ocean Islands, but include the world's largest bat, Livingstone's flying fox (Pteropus livingstonii, CR) on the Comoros.
The area is a major center for chameleon diversity, and it has recently been proposed that all the worlds' chameleons originated in this hotspot. The best-known endemic reptile in the Indian Ocean Islands is the Seychelles' Aldabra giant tortoise (Geochelone gigantean, VU), which lives only on Aldabra. Although the tortoise is relatively abundant, numbering about 150,000 individuals in the wild, it is considered threatened by development, illegal trade, and natural disasters.
There are two endemic families of amphibians: the Sooglossidae, found in the Seychelles, with its closest living relative in the Western Ghats of India, and the Mantellidae, endemic to Madagascar and Mayotte. Endemism is the most marked in amphibians, with only a single species of the 230 present (Ptychadena mascareniensis) not endemic to the hotspot.
Among the flagship amphibians are the beautiful frogs of the genera Mantella and Scaphiophryne. However, the most striking amphibian in the hotspot may be the tomato frog (Dyscophus antongili), a bright red, bullfrog-sized animal found only in a small corner of northeastern Madagascar. Interestingly, there are seven species of caecilians on the Seychelles, yet this amphibian order is not represented on Madagascar or on any of the other Indian Ocean Islands.
The hotspot has two distinct groups of freshwater fishes. The smaller islands are dominated by species with wide marine distributions, which enter both brackish and freshwater habitats. Madagascar's fish are mainly freshwater species of continental origin that have evolved on the island to include nearly 100 endemic species of fish, including 14 endemic genera and two endemic families.
Most of the invertebrate fauna on Madagascar is poorly known. However, some of the non-marine invertebrate groups that are reasonably well known include: terrestrial snails (651 species, all endemic); scorpions (40 species, all endemic); spiders (459 species, 390 endemics); dragonflies and damselflies (181 species, 132 endemics); lacewings (163 species, 119 endemics); tiger beetles (211 species, 209 endemics); scarab beetles (148 species, all endemic); true butterflies (300 species, 211 endemics); freshwater crayfish (six species, all endemic); and freshwater shrimp of the family Atyidae (26 species, 20 endemics). Overall, total species richness for macroinvertebrate groups covered in a recent review of the natural history of Madagascar is slightly more than 5,800 species, of which 86 percent are endemic to the island (although several speciose groups of invertebrates are not covered).
The invertebrate fauna of the Seychelles comprises 3,555 recorded species, with an estimated total of perhaps 5,100 species; of these, approximately 80 percent are endemic. One truly unique and amazing invertebrate flagship species is the endemic giant tenebrionid beetle (Polposipus herculeanus, CR), restricted to one small island in the Seychelles, and one of the largest terrestrial invertebrates in the world. The region also supports the largest millipede (Sechelleptus seychellarum) and populations of the world's largest terrestrial invertebrate, the coconut or robber crab (Birgus latro, DD).
Ironically, the isolation that allowed Madagascar and its neighboring islands to evolve a diverse and unique fauna and flora also contributed to its environmental degradation. Because humans did not arrive on the islands until around 1500 to 2000 years ago, native animals were naïve and easily slaughtered by the colonists. The islands' location off the coast of Africa made them important stopping off points on trade routes and havens for pirates. On the Mascarenes, there is evidence to suggest that the extinction spasm of much of the native megafauna was directly related to hunting.
The Malagasy people came to Madagascar from Africa and Asia, bringing with them agricultural methods, including irrigated rice cultivation, slash-and-burn agriculture, and cattle grazing. When applied to infertile, lateritic soils, these practices can lead to soil degradation, such as the gullies that pockmark the central highlands. This region was probably a mosaic of woodlands and savanna when people first arrived, and continues to be a dynamic agro-sylvo-pastoral landscape, including some massive plantations. Shifting cultivation continues to transform natural forests in some parts of the island, eventually yielding to more intensive farming practices, often including exotic trees. Overall, it is estimated that only about 17 percent of the original vegetation of Madagascar remains, with most remaining forests found along the eastern and southern regions. In the Comoros, which had the fourth highest deforestation rate in the world in the early 1990s (5.8 percent per annum), natural forests have been largely replaced with plantations, and the islands have lost at least 80 percent of their native vegetation. On the Seychelles, lowland forests have been cleared for timber production and agriculture, particularly for coconut plantations and cinnamon exploitation.
Mauritius has one of the highest human population densities in the world at 538 persons per square kilometer. In comparison, the nearly 18 million people who live in Madagascar today do not represent a very large number considering the land area of the island. However, the population is growing at more than 3 percent per year and is expected to double by the year 2025. In an area that is already one of the most economically disadvantaged in the world, this growth rate is putting tremendous pressure on the natural environment. In addition to agriculture, hunting and timber extraction, industrial and small-scale mining are growing threats.
On the other Indian Ocean Islands, these same threats have been exacerbated by the introduction of invasive alien species, brought as food sources, pets, or for pest control. Rats, cats, and mongooses have devastated populations of birds and small reptiles, while grazing rabbits, goats, pigs, and deer have denuded many landscapes. In addition, exotic plant species such as water hyacinth (Eichhornia crassipes) threaten the biodiversity of freshwater ecosystems in the hotspot.
In Madagascar, the government is beginning the third phase of its national Environmental Action Plan, with an ambitious five-year program of conservation and sustainable management activities. Today, about 2.7 percent of Madagascar's land area (16,131 km2) is officially protected in 46 legally protected areas, including national parks, strict nature reserves established to conserve ecosystems and special reserves designed to protect a particular species or a group of species. At the World Parks Congress in September 2003, the president of Madagascar, Marc Ravalomanana, announced plans to triple protected area coverage over the next five years and asked for $50 million in assistance from the international community to do so. In the first six months following this announcement, $22 million in commitments were pledged by international and local conservation organizations, international development agencies, multilateral development banks and national governments to a Biodiversity Trust Fund that was created in January 2005.
In 2001, Birdlife International identified 141 Important Bird Areas (IBA) covering about 54,806 km2 within the Madagascar and the Indian Ocean Islands Hotspot. More recently, Conservation International and other partners in Madagascar expanded upon this work to identify a total of 132 Key Biodiversity Areas based on the distribution of globally threatened species covering eight taxa: mammals, birds, amphibians, reptiles, freshwater fish, arthropods, gastropods and plants. Many of the Key Biodiversity Areas have been identified as potential conservation sites for tripling the protected area network in Madagascar.
These activities to identify and safeguard the hotspot's remaining natural habitats are being implemented hand-in-hand with projects that maximize and demonstrate the value of this conservation to the country. For example, in much of Madagascar the watershed value provided by conservation of the remaining forests is of enormous economic value to the surrounding countryside. In some Key Biodiversity Areas, ecotourism has provided a viable source of income for local communities, such as through the famous guides association in Andasibe, near Perinet (Analamazaotra) Special Reserve.
On the other Indian Ocean Islands, significantly less natural habitat is designated for protection, although the few protected areas that do exist represent almost the entirety of remaining natural habitat on the islands, with the exception of the Comoros. There are about 208 km2 of terrestrial protected areas in the Seychelles (46 percent of the land area, including two World Heritage Sites), while Réunion has 21 protected areas totaling 231 km2. Although the Comoros currently have no terrestrial protected areas, there is a plan under development to establish three terrestrial national parks, in Mount Karthala on Grand Comore, Ntringui in Anjouan and on Moheli, as well as two additional marine national parks to complement that already existing on Moheli.
Efforts at species-focused conservation represent important progress for the future of several unique species. A number of lemur species have been bred successfully in captivity, and, in 1997, the first lemur reintroduction program introduced captive-born black and white ruffed lemurs (Varecia variegata, EN) into the Betampona Nature Reserve. There are very successful combined captive breeding and community conservation programs for several species of tortoise. The Indian Ocean Islands also boast a number of threatened bird species that have been recovered from certain extinction: the pink pigeon (Streptopelia mayeri, EN), Mauritius parakeet (Psittacula eques, CR), Mauritius kestrel (Falco punctatus, VU), Rodrigues fody (Foudia flavicans, VU), Seychelles warbler (Acrocephalus sechellensis, VU), and Seychelles magpie-robin (Copyschus sechellarum, CR).
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