Species Composition Calculators

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Species composition is the identity and relative abundance of all species within an ecological community. It is one of the most fundamental descriptions of a community — going beyond simple species richness (how many?) to ask which species are present and how abundant each is. Differences in species composition between communities reflect underlying gradients in environment, disturbance history, dispersal limitation, and biotic interactions. Comparing species composition among sites and tracking its change over time are central tasks in community ecology, conservation biology, and environmental monitoring.

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What Is Species Composition?

Species composition encompasses two components:

  • Species identity: Which species are present — the species list for a community or site
  • Relative abundance: How many or how much of each species is present — the evenness structure of the community

Together, these define the community's structure and function. Two communities with the same species richness may have entirely different compositions — different species, or the same species at very different abundances.

Describing Species Composition

Species composition is captured in a species × site matrix, where rows are species, columns are sites, and cells contain counts, biomass, cover, or presence/absence. This matrix is the starting point for multivariate community analysis.

Comparing Composition Between Communities

Differences in species composition between two communities are quantified by dissimilarity indices:

  • Jaccard index (J): Based on presence/absence — J = c / (a + b − c), where a and b are species richness at sites 1 and 2, and c is species in common. Ranges 0–1; lower = more similar. Jaccard similarity = 1 − Jaccard distance.
  • Sørensen (Bray-Curtis for presence/absence): 2c / (a + b) — gives more weight to shared species
  • Bray-Curtis dissimilarity: Uses abundance data — Σ|aᵢ − bᵢ| / Σ(aᵢ + bᵢ). Most common for abundance-based composition comparison.

Multivariate Ordination Methods

Because species composition is inherently multivariate (multiple species per site), ordination methods reduce dimensionality and visualize compositional variation:

  • NMDS (Non-metric Multidimensional Scaling): Ranks-based ordination preserving community dissimilarity structure. Most commonly used in community ecology.
  • PCoA (Principal Coordinates Analysis): Linear ordination of any distance matrix
  • CCA/RDA: Constrained ordinations relating species composition to environmental variables

Species Composition and Ecological Succession

Species composition changes directionally during ecological succession — the process of community development after disturbance. Pioneer species colonize bare ground and are gradually replaced by later-successional species as the abiotic environment and competitive dynamics shift. Tracking compositional change over time (chronosequences or permanent plots) reveals successional trajectories and disturbance recovery rates.

Glossary

Species Composition
The identity and relative abundance of all species in an ecological community. Described by a species list plus abundance data; compared between communities using dissimilarity indices (Jaccard, Bray-Curtis) and ordination methods.
Bray-Curtis Dissimilarity
An abundance-based compositional dissimilarity index: Σ|aᵢ−bᵢ|/Σ(aᵢ+bᵢ). Ranges 0 (identical) to 1 (no shared abundance). The most common metric for comparing species composition in community ecology.
NMDS (Non-metric Multidimensional Scaling)
A rank-based ordination method that positions communities in 2D or 3D space to reflect their compositional dissimilarities. The most widely used visualization tool for community composition data in ecology.

Frequently Asked Questions

Species composition describes both the identity (which species are present) and relative abundance (how many of each) of all species in a community. It goes beyond species richness (just the count) to characterize the full community structure. Species composition determines community function, as different species perform different ecological roles regardless of richness.

Dissimilarity indices quantify compositional differences. For presence/absence data, the Jaccard index (c/(a+b−c)) or Sørensen index (2c/(a+b)) measure the proportion of species not shared. For abundance data, Bray-Curtis dissimilarity (Σ|aᵢ−bᵢ|/Σ(aᵢ+bᵢ)) accounts for differences in relative abundance. All range from 0 (identical) to 1 (no shared species or zero shared abundance).

NMDS (Non-metric Multidimensional Scaling) is an ordination method that positions communities in low-dimensional space (typically 2D) such that their relative distances reflect the rank order of their compositional dissimilarities. Sites with similar species compositions plot close together; dissimilar sites plot far apart. Environmental variables can be overlaid as vectors. NMDS is the most widely used ordination method for visualizing community composition data in ecology.

During succession, species composition changes directionally from pioneer species (opportunistic, fast-growing, stress-tolerant) to late-successional species (competitive, long-lived, shade-tolerant) as the environment is modified by the organisms themselves. Each successional stage creates conditions that favor different species. Primary succession (on bare rock or sand) takes centuries; secondary succession (after disturbance on soil-bearing ground) can reach climax community structure in decades.