About 30 MYA, the area was covered by extensive
rainforest. During a cooler and drier period some 10 MYA, the lowland forests were converted to
savanna, leaving the mountain ranges as "islands" where the tropical forests continued to flourish. The long-term persistence of a humid climate and the isolation of each mountain range has led to a great deal of
endemism, and a very diverse flora and fauna. The Udzungwa and other Eastern Arc mountains has extremely high biodiversity with numerous endemic species (more than 25% of the vertebrate species). 10% of them are protected by the
Udzungwa Mountains National Park and the Udzungwa Scarp Nature Forest Reserve. The Udzungwa Mountains are covered with lowland rainforest, montane rainforest,
miombo woodland, grassland, and heathland. Forests extend from elevation, and vary in composition and species type with elevation and rainfall. The wetter eastern and southeastern slopes receive more rain from the Indian Ocean and support evergreen forests on the lower slopes; the drier western and northwestern slopes have deciduous
miombo forests and woodlands at lower elevations and evergreen forests only at higher elevations. Broad areas of forest on the central plateau have been cleared for agriculture and pasture. An analysis of satellite images taken between 1999 and 2003 found of the mountains were still covered in evergreen forest. The mountains are home to many
mammals, including
Abbott's duiker (Cephalophus spadix),
Kipunji (Rungwecebus kipunji), and
Udzungwa red colobus (Cercocebus galeritus).
Elephants (Loxodonta africana) are found in the forests along the southern escarpment. The
grey-faced sengi (Rhynchocyon udzungwensis) is a species of elephant shrew endemic to the mountains. The
Udzungwa forest partridge (
Xenoperdix udzungwensis) is also endemic, and its closest relatives appear to be the hill partridges of Asia. Although the two mountain groups are ecologically distinct, the Eastern Arc Mountains share many species and plant communities with the
Southern Highlands which lie to the southwest across the Makambako Gap. Both are
Afromontane regions, home to characteristic montane species and ecologically distinct from the adjacent lowlands. The Southern Highlands' climate is more influenced by
Lake Malawi than by the
Indian Ocean. Some limited-range montane species, including the Kipunji and
Kipengere seedeater (Crithagra melanochrous), inhabit both the Udzungwa Mountains and the
Kipengere Range. The
Kihansi spray toad (Nectophrynoides asperginis), which was found only around a waterfall on the Kihansi River, became extinct in the wild in 2009 when an upstream dam altered its habitat. The toads have since been reintroduced after a successful captive breeding program. ==Protected areas and conservation==