Ecological communities vary in which species are present and how abundant each is. Describing community structure helps biologists compare habitats, detect change over time, and infer how interactions shape biodiversity patterns.

What ecologists mean by “community structure”

A community is the set of populations of different species that coexist and potentially interact in the same place and time. Community structure is the overall “makeup” of that community, especially which species occur and how common they are.

Community structure is not just a species list; it also includes the distribution of individuals among species (e.g., whether one species dominates or many are similarly common).

Species composition (what is there?)

Species composition identifies which species (or higher taxa) occur in a defined area. Composition can be described at different levels of resolution:

• Presence/absence data (which species occur)

• Species identity with natural history context (native vs. introduced, habitat specialists vs. generalists)

• Relative abundance categories (dominant, common, rare)

Species composition is scale-dependent: a “community” could be defined as a tide pool, a forest stand, or a whole lake shoreline, and the species detected will change with the boundaries chosen.

Relative abundance and dominance

Even if two communities contain the same number of species, they may differ strongly in structure if one has a single dominant species (very high abundance) while the other has more evenly distributed abundances. Relative abundance is commonly expressed as a proportion of the total individuals sampled for each species.

Rank–abundance (Whittaker) curves plot each species’ relative abundance (y-axis) against its rank from most to least abundant (x-axis). The horizontal extent reflects species richness, while the steepness of the decline reflects evenness (steeper = less even, more dominance by a few species). Source

Overall species diversity (how varied is it?)

The syllabus emphasises that community structure is described using overall species diversity, which captures two core features:

• Species richness: how many species are present

• Species evenness: how evenly individuals are distributed across species

Richness increases as more species are detected, but it does not indicate whether most individuals belong to one species or many.

High evenness means species have similar abundances; low evenness means a few species account for most individuals. Many ecological disturbances change evenness before they noticeably change richness, because rare species can be lost or dominant species can expand without an immediate change in the species count.

Why richness and evenness both matter

Two communities can have:

An evenness–richness scatter plot shows richness on the x-axis and an evenness measure on the y-axis, so communities can be compared on both dimensions at once. Points that share similar richness can still differ strongly in evenness (vertical separation), and points that share similar evenness can differ in richness (horizontal separation). Source

• The same richness but different evenness (one dominated by one species)

• Similar evenness but different richness (one simply has more species)

Because community structure includes “which species” and “how common,” AP Biology expects you to connect changes in structure to observable shifts in richness and/or evenness.

Measuring and describing community structure in practice

When ecologists describe community structure, they must define consistent sampling rules so comparisons are meaningful:

A quadrat placed along a transect illustrates how ecologists standardize sampling area and location when counting organisms or estimating percent cover. Repeating quadrat samples along transects (or across a site) helps generate comparable abundance data used to infer richness, evenness, and dominance patterns. Source

• Define boundaries: area, habitat type, and time window

• Choose sampling units: quadrats, transects, timed counts, or trap effort

• Standardise effort: equal area surveyed or equal sampling time across sites

• Record abundances: counts, percent cover (plants), or biomass proxies (when appropriate)

Sampling decisions affect estimates of richness and evenness because rare species are easy to miss, and patchy habitats can produce very different results depending on where samples are placed.

Interpreting differences among communities

Community structure comparisons commonly focus on:

• Turnover in species composition (some species replaced by others)

• Shifts in dominance (a formerly common species becomes rare, or vice versa)

• Changes in overall diversity (richness decreases after disturbance; evenness changes during recovery)

These patterns are used to describe communities as they vary across environments or as they change over time, while keeping the emphasis on composition and diversity rather than on detailed interaction mechanisms.