Mount Karthala is the active volcano that dominates the southern part of Ngazidja. Its summit is the highest point in the Comoro archipelago. The site is centred on the summit crater and comprises all the unaltered vegetation and part of the altered, but still wooded, zone below and surrounding it. The lower limit is set to include the approximate region where threatened or restricted-range bird species that do not occur down to sea-level are still common. This limit is at c.600 m in the south-west, but up to 1,000 m in the north and east. The outer slopes are nowhere very steep and ridges, ravines, valleys, and indeed any permanent surface water, are totally absent. The outer rim of the crater lies at c.2,300 m, and is 2.5–3.5 km across. The crater contains abundant evidence of volcanic activity, with many steaming vents and recent lava-flows. An inner crater, c.1 km across, contains precipitous cliffs.The lower parts of the site support underplanted forest and exotic thickets from abandoned cultivation. Above this lies intact, native mixed montane forest, which can be divided into dense, very humid forest with abundant epiphytes on the southern and western slopes, and drier, more open forest on the northern and eastern slopes. Higher still, up to and including the outer crater, lies high mountain vegetation. This zone is characterized by the ericaceous Philippia comorensis, in places forming a tree-heath taller than 3 m (montane bushland and thicket), but elsewhere lower and sparser (montane shrubland), depending on local conditions such as the age of lava-flows. The inner crater appears bare. The altitudes of the boundaries between these zones vary locally. The lower limit of intact forest is retreating upwards as clearance proceeds. In the west (Boboni) in the mid-1990s, it was at c.1,250 m, in the east (Tsinimouapanga), at least 1,200 m. In the north-east, cultivation reaches at least 1,400 m and the forests have been entirely cleared. On this basis, the unaltered forest area in the mid-1990s may have been c.6,300 ha, compared to 8,658 ha in 1983. The high mountain zone (5,500 ha) typically begins at c.1,800 m, but extends down to 1,200 m on lava-flows in the south-east. The surroundings are mostly cultivated, except to the north, along the island’s axis, where grassland dominates. In addition to agriculture in the lower areas the site is used for logging, cattle-grazing and limited collection of non-timber forest products.
See Box and Table 2 for key species. The site contains the complete community of forest-living birds of Ngazidja, which is the richest on the Comoro archipelago. Karthala supports the entire world population of three bird species, and most of the populations of two more. Of the latter, Nesillas brevicaudata is commonest on Karthala above 1,000 m but occurs lower down and also in La Grille (KM002), and the inexplicably rare Dicrurus fuscipennis is restricted to the environs of Karthala where it is uncommon in degraded forest below 1,000 m, but has also been found in coconut plantations on the coast. Six other restricted-range species occur, of which one, Columba pollenii, is near-threatened. Thirteen Ngazidja-endemic and six Comoro-endemic subspecies are present, and also the near-threatened, mainly Malagasy, Circus maillardi. No single area supports all the area’s forest species. Although several species are widespread, bird distribution is affected by both habitat (forest or high mountain vegetation) and, within the forest zone, by altitude (an ill-defined transition between upland and lowland communities exists at 800–1,000 m). The non-threatened, restricted-range species all occur below 600 m, outside this IBA, but only Foudia eminentissima and Nectarinia humbloti occur below 400 m. A few species have been found at higher density in pioneer forest on lava-flows or underplanted forests than in unaltered forest.
Non-bird biodiversity: Rich evergreen forest and high mountain flora, with many endemic and threatened species. Mammals: Miniopterus minorgriveaudi (LR/nt; Ngazidja-endemic subspecies), Rousettus obliviosus (LR/nt). Reptiles: two Ngazidja- and four Comoro-endemic species: respectively, Phelsuma comorensis, Furcifer cephalolepis, Phelsuma v-nigra, Mabuya comorensis, Lycodryas sanctijohannis, Typhlops comorensis. Butterflies: nine Ngazidja- and two Comoro-endemic species; three threatened: Papilio aristophontes (EN), Graphium levassori (EN), Amauris comorana (EN).
Pressure/threats to key biodiversity
The site is unprotected, although suggestions for a protected area (National Park, Biosphere Reserve or Resource Management Area) have been made. This must be part of the development strategy for the island, with attention to encouraging locally organized ecotourism and the reforestation of the grasslands of the central ridge of the island. The major threat is clearance for agriculture: underplanting, followed by complete clearance. This occurs on all but the poorest soils. Large trees are selectively removed for making canoes. Secondary forest in the agricultural belt is dominated by the exotic strawberry guava Psidium cattleianum, and other exotic plant species are increasing. Commercial logging is very active on a 5,000 ha concession on the south-western slopes. Previous logging activity elsewhere has been abandoned, but logged areas have been taken over by agriculture. The tree-heath zone is threatened by browsing by cattle, and by fire used to stimulate growth of palatable shoots. A plan to build a road to the crater was abandoned in 1985 before construction could begin, but if carried out this could have many damaging impacts, such as increased disturbance, exploitation and fragmentation of the forest, and penetration by exotics. Hunting occurs, especially of pigeons. Introduced rats Rattus sp(p). are abundant in the forest.