Species diversity is more informative than a simple species list because it incorporates how individuals are distributed among species. Simpson’s Diversity Index is a standard quantitative tool for comparing communities using relative abundance data.

What Simpson’s Diversity Index measures

Simpson’s Diversity Index combines two key aspects of diversity:

• Species richness: how many different species are present

• Species evenness: how evenly individuals are distributed among those species

A community dominated by one species has lower diversity than a community where individuals are spread more evenly, even if both communities contain the same number of species.

Three example communities with different species richness and evenness (relative abundances). By counting individuals of each species, you can see how greater dominance by one species lowers diversity even when the species list is similar. This directly motivates why Simpson’s uses relative abundances rather than presence/absence alone. Source

Richness alone cannot capture dominance patterns; that is why relative abundances are essential for Simpson’s.

Data needed: relative abundances

Simpson’s relies on relative abundance, meaning each species’ contribution to the total number of individuals sampled.

• Count individuals for each species in the same sampling area/timeframe

• Use consistent criteria for what constitutes an “individual” (especially for colonial organisms)

• Treat the dataset as a single community sample when calculating a single index value

Why relative abundance matters

Using relative abundance means the index is sensitive to dominance:

A rank-abundance (Whittaker) curve plots each species’ relative abundance against its rank (most abundant to least abundant). The total number of ranks visualizes species richness, while the steepness of the decline reflects evenness (steeper curves indicate stronger dominance by a few species). This kind of plot helps you interpret why squaring relative abundances in Simpson’s index amplifies the influence of common species. Source

• If one species becomes much more common while others become rare, diversity decreases.

• If abundances become more balanced across species, diversity increases.

Equation and variables (Simpson’s Diversity Index)

Multiple algebraically related forms are used in biology courses; AP Biology commonly uses a version where higher values indicate higher diversity.

Equation diagram showing Simpson’s index in the form $1-\sum p_i^2$, where each $p_i$ is the proportional (relative) abundance of category $i$. This visual emphasizes that the calculation is built from squared proportions, which weights dominant species more strongly. It matches the common AP-style interpretation where higher values correspond to higher diversity. Source

This form emphasizes that the index is built from the squared relative abundances, which weight common species more strongly than rare species.

Interpreting Simpson’s Diversity Index values

With $D = 1 - \sum (n_i/N)^2$:

• Higher $D$ means higher diversity (greater evenness and/or more species)

• Lower $D$ means lower diversity (greater dominance by one/few species)

What changes $D$ the most?

• Large shifts in the most abundant species typically change $D$ more than small changes among rare species, because squaring relative abundance increases the influence of dominant species.

Using the index to compare communities (good practice)

To make comparisons meaningful, keep methodology consistent:

• Use the same sampling method (e.g., same quadrat size, trap type, or transect length)

• Sample at similar times (season/time of day can change detectability and abundance)

• Ensure adequate sample size so relative abundances are stable estimates

• Report the index alongside brief sampling context (area, effort, and units)

Common interpretation pitfalls

• A slightly higher $D$ does not automatically imply a “healthier” community; it only indicates higher measured diversity based on the sample.

• If two communities have similar $D$, they can still differ in which species are present; Simpson’s does not capture species identity or ecological roles.