The site embraces a section of the Mayombe massif, which covers c.11,000 km² in Congo, in an area of rather steep hills. The main forest-type is evergreen or semi-evergreen rainforest with a very mixed canopy. There is some monodominant Gilbertiodendron dewevrei forest on the eastern side of the reserve, near the ecotone with the Niari savannas, and some small grassy clearings (savanes incluses), mainly in the eastern section. The southern edge of the reserve coincides with the main road and, for part of its length, the railway line connecting Brazzaville to Pointe-Noire, along which lie several large settlements (in 1987 the three main villages of Mvouti, Mpounga and Les Saras numbered 7,000 inhabitants). There is also a road running into the reserve to the villages of Dimonika and Makaba. Various stages of forest regrowth and young Musanga forest are encountered in the vicinity of villages and associated plantations of cassava and banana.
See Box and Table 2 for key species. A total of 275 species (of which 266 are likely to breed) has been recorded in this part of the Mayombe, including rare or little-known species such as Bubo shelleyi. This figure is based on rather short visits and is certainly incomplete. Of special interest is the existence of a small relict population of two montane species in the region of Mont Mbamba at an altitude of 500–600 m, Dryoscopus angolensis and Zoothera crossleyi. This area in particular deserves further investigation.
Non-bird biodiversity: Large mammals such as Loxodonta africana (EN), Syncerus caffer nanus (LR/cd), Mandrillus sphinx (LR/nt) and small arboreal monkeys have been largely hunted out, but there are still good numbers of Gorilla gorilla (EN)and Pan troglodytes (EN).
Pressure/threats to key biodiversity
The Biosphere Reserve includes a protected core area of 90,000 ha or more and, near inhabited areas, buffer zones in which the only prohibited activities are forestry operations and hunting. In fact, while forestry operations have indeed been suspended, hunting (both subsistence and commercial) continues to be widely practised and is still the main source of income. In the main zone of human influence, along the roads and railway track, several bird species have been exterminated by hunters, including Bostrychia rara, Stephanoaetus coronatus, Agelastes niger, Guttera plumifera, Corythaeola cristata and Ceratogymna atrata. All of these are found in more remote, less disturbed areas. It is regrettable that the Biosphere Reserve was not located in a section of the Mayombe with less population pressure (mean densities of 6 people/km² in Mvouti District are way above the average rural density of 2.7). The relations between the local people and the Dimonika Project are not good.