Endemic and Near-Endemic Trees In the Usambara Mountains -
Working Paper 21

Compiled from Iversen (1991) by: Stig Johansson
East Usambara Catchment Forest Project

Tanga 1996

Introduction

The word endemic means that a plant or animal is native, indigenous to a particular area, not introduced and often with a limited geographical range. The value of the word may be rather complicated depending on how a particular geographical range is defined or depending on how well the distribution of a particular plant or animal is known. Plant distribution, especially trees and vascular plants, in Tanzania is rather well known. Hence, referring to a plant being endemic to e.g. the Eastern Arc probably rather well describes the fact that it is not found outside this range. The situation is much more complicated with animals, especially other than mammals and birds, which are rather well known. Many other groups of animals are so little studied that defining an animal as endemic to an area may simply mean that studies of their distribution are limited.

Endemism often occurs in areas which because of biogeographical reasons have been isolated. The East Usambaras is one such area. The number of endemics, even considering the word in a critical perspective, is high both among flora and fauna. This presence of endemics is one of the reasons why the East Usambara forests have been considered one of the most valuable conservation areas in Africa and included among the "biodiversity hot spots" in Africa. The attempts to explain the high number of endemics in the Usambaras has focused on two aspects. According to the "neo-endemic hypothesis" the presumed long isolation of the Eastern Arch mountains chain may have offered adequate time to develop genuine endemics. Alternatively, according to the "refuge theory" these mountains have acted as refuges for formerly widespread taxa which may have become extinct elsewhere (Iversen 1991).

A very thorough study on the distribution of vascular plants (2,855 taxa) in the whole Usambaras was conducted by Iversen (1991). This study goes into depths of phytogeographical distribution of plants and different phytogeographical elements both in the West and East Usambara. One of the valuable contribution is that the study lists those tree species which are strictly endemic to the West Usambaras, the East Usambaras and to both. It also lists the tree species which are near-endemic, i.e. they are found over a range of other areas in the eastern Arch and ecological zones with close affinity. Among the total number of vascular plants Iversen (1991) found, with reservation for a number of uncertain taxa and inadequate revisions in some groups (e.g. there were still 27 suspected endemics undescribed at the time of the study), a total on 64 taxa endemic to the West Usambaras, 64 taxa endemic to the East Usambaras, and 41 taxa endemic to both blocks (Table 1).

The endemic taxa in East Usambara are predominantly (50 taxa) in the submontane forest, and notably all taxa are forest taxa. The level on endemism is for East Usambara was 3.1 % i.e. 64 taxa out of a total indigenous flora of 2,083 taxa. The total for both blocks combined was 5.9 %.