Count what lives here.

Species richness is the number of species in a community. Evenness describes how evenly individuals are distributed among those species. Two sites with the same richness can have different diversity if one is dominated by a single species.

Simpson’s diversity index is one way to combine richness and evenness. Sampling effort and method must be comparable before drawing conclusions from two sites. A higher index here indicates greater diversity.

INTERACTIVE MODEL

Species diversity

Simpson diversity (1 − Σn(n − 1)/[N(N − 1)]) = 0.526Species A individuals out of 2010 of 20020

Explore: Redistribute individuals among species and watch the index respond.

SDI=1-\frac{\sum_i n_i(n_i-1)}{N(N-1)}

WORKED EXAMPLE

A site contains 5 individuals of each of two species.

N = 10 and Σn(n − 1) = 5 × 4 + 5 × 4 = 40.
SDI = 1 − 40/(10 × 9) = $\frac{5}{9}$ ≈ 0.556.

Assumed knowledge

Fractions and squares.

Learning checkpoints & source

YOUR LEARNING CHECKPOINT

Distinguish richness and evenness.
Use a diversity index to compare communities.

QCAA Biology · Unit 3 · Biodiversity and populations ↗

READY TO TRY IT?

Count what lives here.

Species diversity

A site contains 5 individuals of each of two species.

Put the idea to work.