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. The attempts to explain the high number of endemics in the Usambaras has focused on two aspects. 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.