DIVERSITY & ENDEMISM
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 islands' 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 didn't arrive until 2,000 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 live 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 150-200 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.
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.
Although the hotspot has only one endemic reptile family (the Opluridae), it boasts high species diversity and endemism; 96 percent of nearly 400 reptile species are found nowhere else on earth.
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).