Location and General Description
This ecoregion covers the forests and former forested habitats of the Comoros Islands in the western Indian Ocean. These islands are located in the northern part of the Mozambique Channel about 300 km from northern Madagascar and about 300 km from the mainland of East Africa. There are four islands in total - Grande Comore (1,146 km2), Moheli (211 km2 ), and Anjouan (424 km2) comprise the Independent Republic of the Comoros, and Mayotte (374 km2) is a French dependent territory.
The Comoros islands have a maritime tropical climate. The rainy season is from October to April when the predominant northerly winds of the Indian Ocean bring moist, warm air to the region. The average temperature during the wet season is 25°C with temperatures reaching above 29°C in March, the hottest month. From May to September southerly winds dominate the region bringing cooler (approximately 18°C) and drier air. Rainfall and temperature vary from island to island throughout the year and even vary within islands due to the dramatic topography. The central, higher elevation areas of an island are often cooler and wetter than the coastal regions. This climatic variation results in distinct microhabitats/microecosystems on the islands with correspondingly distinct flora and fauna.
The Comoros Islands are purely volcanic in origin, ranging in age from the oldest, Mayotte, to the youngest island, Grande Comoro. Grande Comoro is dominated by the still active volcano Mount Karthala (2,355 m), which erupts every 10 to 20 years. The 1977 outpourings of basaltic lava from this mountain are already being colonized by vegetation. However, the forests as well as the soils developed over these volcanic materials are immature. Stuart et al. (1990) reported that 7 percent of the island’s surface is in pasture, 16 percent remains as native forest and woodlands, and only 43 percent of the island’s surface is cultivatable, although this figure may be low, since cultivation on Anjouan occurs on slopes greater than 60 degrees (E. Granek pers. comm). The majority of the natural forest vegetation has been cleared, and this clearance is still proceeding. Because the soil on these islands consists of laterite, which is rich in minerals but very poor in humus material, it is subject to extreme erosion when forest cover is removed.
With some exceptions the vegetation in this ecoregion is similar to that on Madagascar. The lowland and mid-elevation evergreen moist forests are the largest and most threatened habitat, occurring from sea level to approximately 1,800 m in elevation. Above 1,800 m on Mount Karthala, stands of a giant heath (Phillipia comorensis) dominate the rugged landscape. Sparse herbaceous vegetation grows on lava flows and cinder fields at the base of this active volcano. Other lowland areas host a characteristic Indo-Pacific scrub formation. There are at least 935 plant species registered on these islands, of which over 40 percent or 416 are native. Major plant families include Sapotaceae, Ebenaceae, Rubiaceae, Myrtaceae, Clusiaceae, Lauraceae, Burseraceae, Euphorbiaceae, Sterculiaceae, Pittoscoraceae, and Celastracea (White 1983).
Of the approximately 2000 native plant species, including 175 ferns and 72 species of orchids, 33 percent are endemic to the Comoros (WCMC 1993, WWF and IUCN 1994, Adjanahoun et al. 1982, Moulaert 1998). Most of the flora of these islands has affinities with those of Africa and Madagascar; however, a small percentage is more closely related to that of Asia.
The species richness of the fauna is relatively low on the Comoros, although it is higher than most other Indian Ocean Islands due to its proximity to both Madagascar and continental Africa. There are only 8 species of extant native terrestrial mammals include three species of fruit bats, 3 insectivorous bat species and two lemurs. There are 25 species of terrestrial reptiles and two species of sea turtle, including green turtle (Chelonia mydas) that nest on the beaches of these islands, and hawksbill turtles (Eretmochelys imbricata), which are seen in the waters around the islands. Species diversity in birds and amphibians is also rather low.
Although the animal species richness is relatively low on the Comoros, there is a high proportion of endemic species. Among the avifauna, 21 species are considered strict endemics to this ecoregion. Twelve species are confined to just one island and the others occur on several of the islands (Stattersfield et al. 1998, Sinclair and Langrand 1998, Adler 1994, Thibault and Guyot 1988, Louette 1988). Examples include the Anjouan sunbird (Nectarinia comorensis) and the Anjouan brush-warbler (Nesillas longicaudata) that are strictly confined to Anjouan Island, and the Comoro olive-pigeon (Columba pollenii) and Comoro blue-pigeon (Alectroenas sganzini) which occur on all the islands. Most of these species are found in the diminishing lowland forest areas, and one, the Mount Karthala white-eye (Zosterops mouroniensis VU), is found only in the higher elevation heath vegetation zone (above 1,700 m) of Mount Karthala, on Grande Comore. This mountain has four species of birds strictly confined to it, including the critically endangered Karthala scops owl (Otus pauliani), Grand Comoro flycatcher (Humblotia flavirostris, EN), and the Grand Comoro drongo (Dicrurus fuscipennis, EN), so all these species are extremely vulnerable to habitat loss. Several of the bird species have populations less than 100 individuals and are thus critically endangered. An additional two species of birds are regarded as threatened with extinction (Hilton-Taylor 2000): the Mayotte drongo (Dicrurus waldenii, EN) and the Anjouan scops owl (Otus capnodes, CR).
Of the 8 native mammals present on these islands, two species of fruit bats are endemic (Pteropus livingstonii and Rousettus obliviosus). Other native mammals include the mongoose lemur (Eulemur mongoz, VU) and a near-endemic sub-species of fruit bat (Pteropus seychellensis comorensis). Of the 34 reptiles present, 9 are strict endemics. Endemics include two species of day geckos (Phelsuma spp.), four species of shinning-skink (Cryptoblepharus spp.), and two species of chameleon (Furcifer sp.) (Henkel and Schmidt 2000). These are no species of endemic amphibian. Among the dozens of endemic Lepidopterans are two species of swallowtail butterflies Papilio aristophontes and Graphium levassori (Desegaulx de Nolet 1984).
Cyclones and volcanic activity are major ecological processes that affect biodiversity. Cyclones bring heavy winds, which can affect fruit bat populations, topple trees and cause landslides. These effects are exacerbated by recent logging activities that open forest areas increasing their exposure to wind. Volcanic activity affects biodiversity by covering intact habitat. Recent lava fields host a number of unique plant species that are adapted to these dry and sun-exposed conditions.
The conservation status of forests in this ecoregion is poor. The human population of the islands is high, with over 700,000 residents on the four islands combined (with a population density greater than 330 people/km²). Population growth hovers around 3 percent per year and is putting increasing pressure on the forests. Little good agricultural land remains because of the rugged topography on the three islands of the RFI Comores. The majority of the human population of Grande Comoro and Mayotte is concentrated in the coastal lowland areas. However, on Anjouan, and to a lesser extent Moheli, there are significant populations in mountain villages. Introduced exotic species are problematic on these and other islands, and several management programs have been enlisted to control and even eliminate some of them.
The native vegetation of all Indian Ocean islands is seriously endangered due to the extensive conversion of land to agricultural use (WWF and IUCN 1994). Lowland forests have been almost completely destroyed up to 300 or 400 m on all four of these islands, making it likely that some endemic plant species are already extinct. Little intact forest remains on Anjouan and Mayotte, while much of the remaining forests that exist on Moheli and Grande Comore are badly degraded except at higher elevations where terrain is rugged or otherwise unproductive. It is thus likely that several plant species are at risk in these areas as well (Stuart et al. 1990).
The natural forest habitats are highly fragmented due to human activities. Only some forest patches, mostly in the higher elevations, survive, and these are currently under heavy pressure. The largest remaining block of forest is on the slopes of Mount Karthala on Grande Comore. On Anjouan there are two remaining forest tracts of approximately 10 km2 in extent. This provides the only remaining habitat for the surviving population of the Anjouan scops owl (Otus capnodes) as well as the majority of habitat for the Livingstone’s fruit bat (Pteropus livingstonii). On Mayotte forests still remain on Mounts Sapéré, Bénara and Choungi. A captive breeding program for Livingstone’s fruit bat is well established with over 2 dozen individuals being studied in captivity in Bristol Zoo and the Durrell Wildlife Conservation Trust.
None of the remaining forest areas are protected. Reserves have been proposed for several areas (including Mount Karthala), but this has not yet been established. There are just three protected areas in this ecoregion, the Saziley National Park on Mayotte, Lake Dziani Boudouni, a Ramsar wetlands site, and the Moheli Marine park including the Moheli islets, both on Moheli. Creation of a Coelacanth Marine Park is currently in process on the southern end of Grande Comore.
Types and Severity of Threats
All mature forest habitats on the Comoros Islands are highly threatened by agricultural expansion. Deforestation or conversion of land along rivers further changes the microclimatic conditions in the forest ecosystem and may severely affect temperature sensitive species including bats and reptiles. The regenerating forests on the recent lava flows of Mount Karthala are not being farmed, as these lands are marginal for agriculture. Populations of other endemic species are in some cases critically small and extremely endangered, for example in several species of birds (Stattersfield et al. 1998).
Current forest conservation measures are inadequate, and there is a high risk of large-scale extinction if appropriate protective measures are not implemented. Remaining forest patches are highly fragmented and represent predominantly montane habitats. Wildlife exploitation includes poaching of green sea turtles for local consumption. Collection of day geckoes for the pet trade is a potential threat to these species. Currently, hunting of fruit bats is not a problem, however as food resources become scarcer due to growing population, the local taboos against eating bats may disappear. Exotic species present a large problem - they are responsible for denuding smaller islands (Round Island) and have adverse effects on native vegetation on all islands. Introduced carnivores such as the Javan mongoose (Herpestes javanicus) have severe effects on ground nesting bird populations, and some introduced exotic plants out-compete or severely impact native plant species.
Justification of Ecoregion Delineation
The Comoros Islands were included with the African mainland in White’s (1983) ‘coastal mosaic’ vegetation unit. However, it is considered a distinct ecoregion because the island group is separated from both Madagascar and the African mainland, and posses a high level of endemism in various taxa, particularly birds.
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Adler, G. H. 1994. Avifaunal diversity and Endemism on Tropical Indian Ocean Islands. Journal of Biogeography 21: 85-95.
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Prepared by: Jan Schipper
Reviewed by: In progress