15 – Species Abundance and Diversity

Community ecology definition

The study of associations of interacting species inhabiting a defined area to understand biodiversity and environmental factors.

Species Richness

The total number of species present in a community.

Species Evenness

The relative abundance of each species; how equally individuals are distributed among the species present.

Low vs. High Evenness

A community dominated by one species has low evenness; a community where all species have equal numbers has high evenness.

Quantifying Diversity

Diversity is a measurement that combines both species richness (number) and species evenness (distribution).

Lognormal distribution plot

A graph plotting the logarithm of abundance against the number of species to show distribution patterns.

General principle of lognormal distribution

In most communities, a few species are very rare, a few are very abundant, and most are moderately abundant.

Rank-abundance plot

A visual tool used to compare species richness and evenness between different communities by plotting relative abundance against rank.

Environmental complexity and diversity

Diversity generally increases with environmental heterogeneity (spatial complexity).

MacArthur’s Warbler study

Demonstrated that five species of warblers coexist in the same trees because they use different parts of the tree (spatial complexity).

Nutrient availability and diversity

Increased nutrient availability typically leads to a decrease in species diversity.

The Paradox of the Plankton

The observation that many species of plankton coexist in seemingly simple environments despite competing for the same resources.

Explanation for the Plankton Paradox

Niches differ based on specific nutrient ratios (e.g., silicate vs. phosphate) and spatial heterogeneity in nutrient levels.

Disturbance definition

A discrete event—abiotic (fire/flood) or biotic (disease)—that disrupts ecosystem structure and changes resources or the environment.

Intermediate Disturbance Hypothesis (Connell)

The theory that species diversity is highest at intermediate levels of disturbance.

Diversity at low disturbance

Diversity is low because superior competitors exclude other species (competitive exclusion).

Diversity at high disturbance

Diversity is low because only a few stress-tolerant or highly colonizing species can survive the frequent disruptions.

Sousa’s (1979) boulder experiment

A study using boulder size as a proxy for wave disturbance; larger boulders moved less often than smaller ones.

Sousa’s experiment result

Medium-sized boulders (intermediate disturbance) supported the highest diversity of species compared to large or small boulders.