Biodiversity
The archipelago comprises three large islands plus a number of volcanic remnants in the tropics of the southwestern Indian Ocean, generally between 700 and 1500 kilometres east of Madagascar. The terrain includes a variety of reefs, atolls, and small islands. They present various topographical and edaphic regions. On the largest islands these gave rise to unusual biodiversity. The climate is oceanic and tropical. The islands differ in size, topography, age, and in proximity to the nearest major land mass, namely (Madagascar).
There are several endemic families including the Asteropeiaceae, Sarcolaenaceae and Sphaerosepalaceae (with psiloxylaceae ). The four largest families present in the Mascarene Islands are: Myrtaceae, Rubiaceae, Orchidaceae, and Euphorbiaceae, they comprise between 193 and 223 species each, for a total of 831 species, or 26.9% of the flowering plant flora. Another seven families contain 80 or more species each: Poaceae, Apocynaceae, Cyperaceae, Cunoniaceae, Rutaceae, Araliaceae, and Sapotaceae, representing an additional ca. 660 species in all, 21.3% of the angiosperm flora. Some families that have few species, but are distributed too in the Americas, showing its ancient related origin, are typically in wet areas and occur as relict species in wet Islands wideworld, with several families including Phyllanthaceae, Hernandiaceae, Lauraceae, Ebenaceae with Mauritius ebony, Diospyros tesselaria.
As is common among remote islands, the Mascarene fauna and flora display a high degree of endemism; they include over a thousand species of which several hundred are endemic. There also are many introduced species. Most of the indigenous Mascarene flora and fauna are thought to have descended originally from Madagascan and African ancestors. The Mascarenes are home to many endemic species of Dombeyoideae, the monotypic genus Psiloxylon (Psiloxylon mauritianum), and members of the family Monimiaceae, Escalloniaceae and Foetidia. Indigenous trees for example include species in the genera Ocotea, Erythrina, Sideroxylon and several species of palms in the genus Hyophorbe. Ferns are prominent components of the biotas of the islands, especially in the tropical forest. Most ferns disperse easily via ornithochory of their spores, allowing fairly frequent colonization from Madagascar and exchange among the Mascarene islands.
Until Europeans first settled the islands in the sixteenth century the Mascarenes are not known to have harboured any human populations, so much of the island's wildlife, which would have gone extinct much earlier had any native people lived there, was still flourishing during the early days of settlement.
The islands have no native mammals, except for bats as the now extinct Pteropus subniger. Sixteen endemic bird species survive on the islands. Many of the Mascarene birds evolved into flightless forms; the most famous of which was the Dodo of Mauritius, and the Rodrigues Solitaire extinct flightless pigeons. Mauritius, Réunion, and Rodrigues were also once each home to one or more species of giant tortoises, now extinct, which comprised the genus Cylindraspis. There are thirteen living endemic reptile species, including a number of species of day geckoes (genus Phelsuma).
Much of the Mascarenes' native flora and fauna has become endangered or extinct since the human settlement of the islands in the 17th century. Settlers cleared most of the forests for agriculture and grazing, and introduced many exotic species, including pigs, rats, cats, monkeys, and mongooses. As well as the tortoises and the Dodo, thirteen additional species of birds became extinct among there were extincts Réunion Flightless Ibis, Broad-billed Parrot, Red Rail, Rodrigues Rail, Rodrigues Solitaire and the related dodo (Raphus cucullatus), in Mauritius, could swallow fruits from ocotea species, as several contemporary sources state that the dodo used gizzard stones. Seed distribution of at least some Ocotea species is performed by frugivorous birds.
There exist a few reports of "solitaires" from the Mascarenes without mention of which island these came from, and the term was also used for other species with "solitary" habits, such as the Réunion Blue Swamphen and the Réunion Sacred Ibis. At one point it was even believed that Réunion was the home of not only a white dodo, but also a white solitaire. In 1786, sub-fossil bones were discovered in a cave which confirmed Leguat's descriptions, but at this time no living residents of Rodrigues remembered having seen living birds. The star constellation Turdus Solitarius was named after this bird.
The increasing disappearance of young calvaria trees, is suggested due to Cylindraspis tortoises, fruit bats or the Broad-billed Parrot could have been dispersing the seeds.
In prehistoric times the islands were covered in a diverse range of tropical moist broadleaf forest At present however, though the Mascarene islands form a distinct ecoregion, known as the Mascarene forests, the ecoregion is not homogeneous, and comprises at least five fairly distinct vegetation zones that reflect variations in altitude and in moisture regime.
The freshwater biota includes coastal wetlands and swamp forests, grading into rainforest to windward and to lowland dry forest to leeward. Dryland areas include palm savannas, montane deciduous forests and heathlands on the highest peaks of Réunion. The dry lowland forests range from sea level to elevations of some 200 metres. They occur in regions with less than 1000 mm average annual rainfall. These dry lowland forests are dominated by palms such as Latania species and Dictyosperma album, and by the palm-like screw-pines (Pandanus species). There also are sclerophyllous trees such as species in the family Combretaceae, for example Terminalia bentzoe
Semi-dry sclerophyllous forests occur between the coastal areas and an altitude of 360 metres on Mauritius and Rodrigues. On Réunion only small relict patches remain at altitudes up to 750 metres on the western slopes. This semi-dry ecosystem occurs where the average annual rainfall is about 1000–1500 mm. Characteristic families of the flora in the include Ebenaceae, with ebony species in the genus Diospyros. Other tree families are occurring such as Pleurostylia spp. (Celastraceae), Foetidia spp. (Lecythidaceae), Olea europea subsp. africana (Oleaceae), Cossinia pinnata (Sapindaceae), Dombeya spp. (Sterculiaceae), and a variety of Sapotaceae species Sideroxylon boutonianum, Sideroxylon borbonicum spp. and Mimusops. The ecosystem is also home to several spectacular endemic species of Hibiscus (Malvaceae) Zanthoxylum spp. (Rutaceae), Obetia ficifolia (Urticaceae), and Scolopia heterophylla (Flacourtiaceae).
The Lowland rainforests are characterized by dense evergreen forests, with a canopy exceeding 30 meters. With an average annual rainfall of 1500–6000 mm, occur on Mauritius island above 360 m and all over the eastern lowlands from the coast to 800–900 m and, on the western side. On Réunion island the Lowland rainforests is present from 750 to 1100 m. These forests have a canopy of tall trees up to 30 m high and represent the more diversified plant communities of the Mascarene Islands. Characteristic plants include trees in the plant family Sapotaceae e.g. Mimusops spp. Labourdonnaisia spp., Hernandiaceae Hernandia mascarenensis, Clusiaceae Calophyllum spp., and Myrtaceae Syzygium spp., Eugenia spp., Sideroxylon spp., Monimiastrum spp.; shrubs in the plant family Rubiaceae (Gaertnera spp., Chassalia spp., Bertiera spp., Coffea spp.); bamboos as Nastus borbonicas, numerous species of orchids (e.g., Angraecum spp., Bulbophyllum spp.) and ferns e.g., Asplenium spp., Hymenophyllum spp., Trichomanes spp., Elaphoglossum spp., Marattia fraxinea.
The cloud forests, a dense type of hygrophilous rainforest, occur on Réunion between 800 and 1900 m on eastern slopes with an average annual rainfall 2000–10,000 mm, and between 1100 to 2000 m on western slopes with an average annual rainfall 2000–3000 mm and are also restricted to a small area of Mauritius around the montane area "Montagne Cocotte" above 750 m on Mauritius with an average annual rainfall 4500–5500 mm. These type of forests is present on both islands with a canopy of 6 to 10 m high. They are rich in epiphytes (orchids, ferns, mosses, lichens), emergent tree ferns (Cyathea spp.), and, originally, palms (Acanthophoenix rubra), but these now survive only in areas of Réunion where poaching has not wiped them out. Untransformed cloud forests still cover large areas on Réunion with 44,000 ha in 2005. These forests are characterized by trees such as Dombeya spp., on the Réunion island only, and species in the plant family Monimiaceae (Monimia spp., Tambourissa spp.) as canopy species, with small trees and shrubs sucth as Psiadia spp. (Asteraceae) and Melicope spp. (Rutaceae) in the understory. They also include large areas of three monodominant plant communities, forests with Acacia heterophylla (Fabaceae) as canopy species that are very similar to Acacia koa forests in Hawaii, thickets dominated by Erica reunionensis (Ericaceae), or hyperhumid screw-pine forest (Pandanus montanus).
The subalpine scrub with an average annual rainfall 2000–6000 mm is above the tree line to 1800–2000 m, at elevations where frosts occur regularly in winter, dominated by shrubs in the plant families of Ericaceae (Erica spp.), Asteraceae (Hubertia spp., Psiadia spp., Stoebe passerinoides), and Rhamnaceae (Phylica nitida), with some notable endemic species suchs Heterochaenia rivalsii (Campanulaceae), Eriotrix commersonii (Asteraceae), and Cynoglossum borbonicum (Boraginaceae).
The summits of the volcanoes are covered by large mineral areas with sparse grasslands rich in endemic grasses (Poaceae, e.g., Festuca borbonica, Agrostis salaziensis, Pennisetum caffrum) and orchids (Orchidaceae, e.g., Disa borbonica), ericoid thickets, or thickets of the small tree Sophora denudata (Fabaceae), depending on substrate texture and age.
In the Mascarenes, the angiosperms with 22 species have 21 endemic. The endemic Dombeyoideae from the Mascarenes are polyphyletic and splited into nine clades. Trochetia appears monophyletic and more closely related to Eriolaena and Helmiopsis than to Dombeya. All Dombeya taxa are included in a clade together with Ruizia and Astiria, this means that Dombeya is paraphyletic. In terms of breeding systems the Malagasy Dombeyoideae are hermaphroditic, whereas those of the Mascarenes are considered dioecious. The polyphyly of the Mascarene Dombeyoideae suggests that dioecy has been acquired several times. At least five colonization events from Madagascar to the Mascarene archipelago were produced. The evolutionary history of two lineages of Mascarene Domeyoideae seems to be related to adaption to xeric habitats.
The Tambalacoque (Sideroxylon grandiflorum), often called the dodo tree, is also threatened with extinction, although this is principally as a result of unripened seed destruction by the introduced crab-eating macaque (Macaca fascicularis) rather than any connection to a reliance on the dodo to assist with seed germination after the seeds passed through the extinct bird's digestive tract.
The Mascarene Islands are surrounded by approximately 750 square km of coral reef. Rodrigues has nearly continuous fringing reefs bounding an extensive lagoon with deep channels, whereas Mauritius is surrounded by a discontinuous fringing reef and a small barrier reef. In contrast, Réunion has very short stretches of narrow fringing reefs along the western and southwestern coasts only. The islets of the Cargados Carajos Shoals, which have a very depauperate terrestrial biota owing to being so low-lying and swamped during cyclones, are bound to the east by an extensive arc of fringing reef, which accounts for ∼30% of the reefs of the Mascarene Islands. Lagoon reefs and reef flats are dominated by scleractinian corals such as branching and tabular Acropora, Porites massives, foliaceous Montipora and Pavona, and sand consolidated with beds of seagrass such Halophila spp. (Hydrocharitaceae). Among coral reef fi shes, wrasses (Labridae), damselfish (Pomacentridae), carnivorous groupers (Serranidae), and surgeonfishes (Acanthuridae) have many species.
Read more about this topic: Mascarene Islands, Nature