The concept of diversity, including biodiversity itself as
well as the narrower concept of species diversity, is a
human construct without any unique mathematical
meaning. The simplest measure of species diversity is
species richness, but a good case can be made for giving
some weight to evenness as well. For example, the subjective
sense of tree species richness is likely to be
greater for a naturalist walking through a forest composed
of 10 species of trees, each equally represented,
than a forest of 10 species in which one species contributes
91% of the individuals and the others each 1%.