A biological community can be described not only by which species are present but also by their relative abundances.

Species richness is simply the number of species in the community. If one site contains 20 species and another contains 10, the first has greater richness.

But richness alone does not describe how individuals are distributed among species. Evenness measures how similar those abundances are.

Consider two communities with four species and 100 individuals each:

Community A: 25, 25, 25, 25
Community B: 97, 1, 1, 1

Both have richness 4, but community A is much more even.

Measures of species diversity combine information about richness and relative abundance. Different diversity indices weight rare and common species differently, so a diversity value is meaningful only when its definition is known.

Community composition can change even when richness remains constant. If one species disappears and another arrives, the number of species may be unchanged while ecological interactions are substantially altered.

Ecologists therefore distinguish at least three questions:

1. Which species are present? — composition.
2. How many species are present? — richness.
3. How are individuals distributed among those species? — evenness.

These quantities provide the descriptive foundation for asking why competition, predation, disturbance or environmental conditions change community structure.