Thyolo Mountain is the southern outlier of the Shire Highlands, which drop sharply to the Shire river plain (c.50 m a.s.l.) on the western side, from a peak at 1,462 m. Soche Mountain (MW017) is 25 km to the north, and the Mulanje massif (MW018) is 50 km to the east. Thyolo Mountain used to support a single block of mid-altitude rainforest c.6 km in length along the north–south aligned ridge. The Forest Reserve is bordered by Satemwa tea estate on the south-east (where the forest descends to 1,160 m) and heavily populated agricultural land on all other sides, where the edge of the forest is retreating gradually to 1,200–1,300 m. From 1955 to 1980, the total area of forest decreased from c.1,500 to 1,000 ha. Since 1995–1996, the remaining area has been seriously encroached upon for gardens. As with the lower parts of the Misuku Hills and Ntchisi, Thyolo is a fig-dominated forest (with mostly the large strangler Ficus sansibarica); Celtis gomphophylla, Chrysophyllum gorungosanum, Drypetes gerrardii and Macaranga capensis are other common canopy trees.
See Box and Tables 2 and 3 for key species. About 81 species have been recorded on the mountain. The forest used to hold important numbers of Alethe choloensis (with densities of c.2 pairs/10 ha), and was the second most important site in the country after Mulanje. Oriolus chlorocephalus and Apalis chariessa were both common on the lower slopes below 1,300 m. A few pairs of Zoothera guttata also occurred. The local, isolated population of Stactolaema olivacea (a fig specialist) belongs to the race belcheri (confined to Thyolo and Namuli in adjacent Mozambique). Inexplicably, Thyolo is also the only locality in the country for Columba delegorguei and Coracina caesia. The montane species Apaloderma vittatum, Bradypterus lopezi, Serinus hypostictus, Ploceus bertrandi (and of course Alethe choloensis) all reach their southern limits of distribution here or on nearby Chiperone Mountain (site MZ010) in neighbouring Mozambique.
Non-bird biodiversity: None known to BirdLife International.
Pressure/threats to key biodiversity
From repeated encroachment on both sides, a tongue of forest in the north-west had been cut off by 1980 and has probably disappeared; enormous inroads have since been made into the principal remaining block of forest with large areas completely cleared by the collection of firewood and the illegal establishment of gardens. From 1955 to 1980 c.40% of the total forest cover had already been lost and the situation has worsened since: by the year 2000, another 40% of the forest cover had been lost, with huge gardens established right up to the beacon. Much land cleared for gardens has already been abandoned and returned to bracken as the topsoil has disappeared. In the 1960s and 1970s, renewed efforts by the Forestry Department to stop the spread of gardens, realign the boundary and mark it with short-lived eucalypt plantations all met with failure. Some of the people clearing the forest are employees of the nearby tea estates. The only way to save what is left is to provide estate employees and others with land outside the reserve.