Because richness does not show whether individuals are concentrated in one species or distributed more evenly among several species. Two communities may therefore contain the same number of species while differing in dominance patterns and overall diversity. Examining evenness alongside richness reveals whether community structure is balanced or strongly weighted toward a small number of species.
Dominance lowers evenness because a large share of individuals belongs to one species rather than being distributed more uniformly. This matters when interpreting a community with many species: high richness alone may conceal an imbalanced structure. Reporting both measures shows whether observed diversity reflects a broad set of species, balanced abundances, or both.
An index can combine the number of species with the distribution of their relative abundances, providing a summary of community diversity. Such a value is most informative when considered with its component patterns rather than treated as a complete description by itself. The separate measures help explain whether differences among communities arise from species number, abundance balance, or both.
To apply these measures, biologists first identify the species present and determine their abundances within each community being studied. They can then compare the number of species, evaluate how evenly individuals are distributed, and, where appropriate, use a diversity index that combines both dimensions. Interpreting the results together gives a fuller picture than either measure alone.
Repeated comparisons can show whether a disturbance is associated with fewer species, a shift toward dominance by some species, or both. A change in richness may indicate altered species presence, whereas a change in evenness signals altered balance among abundances. Tracking these dimensions separately helps distinguish different forms of community change.
Conservation assessments need to recognize not only how many species remain, but also whether individuals are distributed relatively evenly among them. A site can retain many species while becoming dominated by one or a few, producing a different community structure from a site with more balanced abundances. Using both measures supports clearer comparisons of conservation conditions and outcomes.