Measuring species diversity is critical for ecological research and biodiversity conservation. The separate assessment of within-unit diversity and unit distinctiveness in the form of endemism may Lead to biased results when evaluating the importance of a unit for regional diversity. In this paper, we adopt the additive partitioning of species diversity and propose a series of measurements decomposing the contribution of a unit into two components, one based on within-unit species diversity and the other on unit distinctiveness, for species richness and Simpson's index. We also propose a differentiation coefficient to evaluate the distribution of species diversity within and among units and to compare the relative importance of unit distinctiveness and within-unit diversity for regional diversity. Using simulations and a real data set of tree species in a community consisting of nine plots, we compared the proposed method with other ranking methods. The definition of unit-specific additive components of species diversity facilitates diversity scaling in hierarchical systems. The individual components may be used to identify the factors determining the contribution of a unit to larger-scale diversity, while avoiding typical problems associated with the number of endemic species. The ranking of units based on an integrated assessment of α and β, diversity at the unit level provides an objective foundation for determining conservation priorities.