Granitic Seychelles forests
Location and General Description
The Seychelles Islands are located in the middle of the Indian Ocean, about 5 degrees south of the equator and 930 kilometers (km) northeast of Madagascar. The 115 islands in the group can be divided into two types: granitic islands and low limestone islands. The 42 granitic islands are peaks on a largely submarine plateau, situated in the northeastern part of the archipelago. Mahé, the largest and tallest island in the Seychelles (145 square kilometers (km2); 905 meters (m)), is typical of the granitic islands. A mountain ridge runs the length of the island. The lower regions have been developed for residential and agricultural use, and the upper regions are still largely forested. The granitic islands have steep sides and impressive peaks, shaped by weathering and erosion. The erosion of the steepest inclines has produced large rocky outcrops, or "glacis." The islands experience a humid tropical climate with little seasonal variation in temperature. They receive heavy monsoon rains from November to February, and in the cooler months the trade winds blow steadily from the southeast. Mean annual rainfall varies with elevation, and on the granitic islands rainfall ranges from 2,300 to 5,000 millimeters (mm). The abundant rainfall and warm temperatures, along with soil enriched by guano, allowed lush palm forests to develop on the islands, most of which have now been cleared.
At elevations below 610 m, palms, pandans and hardwoods characterize the natural forests of the granitic islands. Above this elevation, there is cloud forest, rich with tree-ferns and mosses. Forest composition varies somewhat from island to island within the Seychelles, but common tree species include: Phoenicophorium borsigianum, Albizzia falcata, Pterocarpus indicus, Adenanthera pavonina, Morinda citrifolia, Phyllanthus casticum, Pisonia grandis, and introduced coconut palms. Tree-ferns, palms, orchids, and an endemic species of pitcher plant (Lalyann potao) are all also relatively common. Although the Seychelles flora boasts many interesting species, the most famous of these is the coco-de-mer palm (Lodoicea maldavica) that is found only in the Vallée De Mai on Praslin Island. This palm grows up to 30 m tall, with leaves up to 6 m long by 4 m wide. Even more striking are the nuts: the coco-de-mer produces the largest nuts in the world, with some specimens weighing more than 22 kilograms (kg).
Due to their age, geography and isolation, the Seychelles supports a variety of endemic taxa. In the plants there is one endemic family, 12 endemic genera, and 72 endemic species from a flora of about 233 native plants. The palms are a particularly unique group, with six endemics classified into six monotypic genera. The pandans are also unusually diverse, with the granitic islands hosting eight species, of which five are endemic. Unfortunately, many of the unique plants of the Seychelles have small populations and restricted distributions. For example, the jellyfish tree (Medusagyne oppositifolia) has a total population of fewer than 30 plants scattered over three hilltops on Mahé. It is the sole representative of the endemic family, Mesdusagynaceae, and is currently one of the rarest plant species in the world. It was thought to be extinct until it was rediscovered in 1970.
The flora shows affinities with that of nearby islands, Madagascar and the Mascarenes, as well as with mainland Africa and Asia. The Seychellian species of Impatiens, Psederanthemum, and Rothmannia are more closely related to species found on the African continent then they are to species that live on Madagascar and the Mascarenes. And although Asia is twice as far away as mainland Africa, several species are found only on the Seychelles and in Indo-Malaysia and Polynesia. For example, Amaracarpus pubescens grows only in the Seychelles and in Java. These strange distribution patterns are thought to be relictual, reflecting the islands’ geological history, when they were linked with Asia.
Unlike many other mid-oceanic island groups, the Seychelles are inhabited by a diverse herpetofauna. The most famous native species are the giant land tortoise (Dipsochelys arnoldi on the granitic Seychelles and Dipsochelys dussumieri on Aldabra). Giant tortoises were once abundant on both the granitic and coralline islands throughout the Seychelles, but the populations on the granitic Seychelles were decimated following human settlement. In addition to the tortoises, there are several native terrapins, 7 species of legless caecilians, 4 native frogs, 3 native snakes, an endemic chameleon, and several species of geckos and skinks. One endemic mammal species is found here, the critically endangered Seychelles sheath-tailed bat (Coleura seychellensis).
The landbird fauna of the Seychelles is particularly unique. There are 12 endemic species found here, including two species, the Seychelles Scops-owl (Otus insularis CR) and the Seychelles paradise-flycatcher (Terpsiphone corvine CR), which are confined to single islands. Other endemics are the Seychelles kestrel (Falco araea VU), Seychelles blue-pigeon (Alectroenas pulcherrima), Seychelles swiftlet (Collocalia elaphra VU), Seychelles bulbul (Hypsipetes crassirostris), Seychelles magpie-robin (Copsychus sechellarum CR), Seychelles warbler (Acrocephalus sechellensis VU), chestnut-sided white-eye (Zosterops mayottensis), Seychelles white-eye (Zosterops modestus CR), and Seychelles fody (Foudia sechellarum VU). Two endemic subspecies of note are the black parrot (Coracopsis nigra barklyi) and Seychelles turtle-dove (Streptopelia picturata rostrata).
In the past, there were more endemic bird species on the Seychelles, but these have gone extinct (one species and one subspecies). The extreme rarity of some other taxa indicates that further extinctions are possible (at least in the wild). Intensive species recovery programs have helped the survival of some birds, including the recolonization of some parts of their former range, e.g., for Seychelles Warbler and Seychelles Magpie Robin.
Most of the lowland forests of the Seychelles granitic islands have been disturbed or destroyed. Coconut, vanilla, and cinnamon plantations occupy most of the coastal plateaus. The mountain forests are certainly not pristine, however, there is still some native forest in the higher reaches of the granitic islands. The Vallée de Mai on Praslin Island provides the best example of intact native forest and has been declared a World Heritage Site. The Morne Seychellois N.P. (35 km2) contains important mountain mist forest. Other important reserves are Aride Special Reserve (0.7 km2), Cousin Special Reserve (0.3 km2), La Digue Veuve Special Reserve (0.1 km2), and Curieuse National Park (15 km2). Although extremely small the reserves do a good job of protecting critically endangered species as well as the habitats they rely upon. For example, an intensive recovery program has helped increase the total population of the Seychelles warbler from 30 to 500 individuals. Intense management efforts have helped build up the populations of the Seychelles magpie robin on Frégate Island, and to translocate it to other islands.
Types and Severity of Threats
Despite efforts to protect the flora and fauna of the Seychelles, there are still a number of threats to the native biota. Anthropogenic disturbance of native habitats is still a problem, and human-facilitated introduction of exotic species is a continuing threat. Alien species now comprise 57 percent of the total flora of the Seychelles, and this percentage is likely to increase with time. Introduced goats, pigs, and cattle inhibit regeneration of native forest; and introduced cats, dogs, common mynah (Acridotheres tristis), and tenrecs prey upon native species, particularly birds, lizards, caecilians, and invertebrates. Introduced plants also outcompete the native vegetation and provide unsuitable habitat for the endemic animals.
Aside from land clearance and introduced species, the main conservation concern in the Seychelles is the vulnerability of small populations with restricted ranges. Although many native species have probably always had small populations, the majority of them were spread over several islands. Following human settlement, one species after another has been reduced to one or two relict populations. The endemic land birds all occupy a mere fraction of their historic range. Small populations in single locations are especially vulnerable to stochastic events and have a high probability of extinction. Translocation efforts have helped mitigate the risks inherent in having spatially restricted populations, but not without their own risks. In summary, the Seychelles government, in cooperation with international agencies, is doing a good job of protecting the unique biological heritage of the islands. However, the long-term survival of many species and habitats is still far from certain.
Justification of Ecoregion Delineation
The long isolation of the granitic islands of the Seychelles Archipelago and the exceptionally high levels of endemism in both flora and fauna warranted their classification as a unique ecoregion. Endemism at higher taxonomic levels is particularly notable, including the families Medusagynaceae and Sooglossidae.
Additional Information on this Ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
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