Species Diversity & Community Composition

Overview

Understanding what species live where and in what abundance is the foundation of community ecology. It sounds straightforward, but anyone who's tried to do it in the field knows it's harder than it looks. You need a systematic sampling design, reliable species identification, and a way to turn raw counts into meaningful numbers. Diversity indices like Shannon-Wiener (H') and Simpson's (D) compress community data into single values that let you compare across sites, zones, or experimental conditions.

This simulation puts you in the field with a quadrat frame and a transect tape. You'll collect the kind of data that community ecologists have been gathering for over a century—species identities and abundances within standardized sampling units—and learn to analyze it properly.

What You'll Do

Place quadrats or run transects in a simulated environment. Within each sampling unit, identify every species present and count individuals (or estimate percent cover for sessile organisms). Calculate species richness (S), evenness (J'), Shannon-Wiener diversity (H'), and Simpson's index (1 – D). Compare these metrics across different zones, sites, or experimental conditions.

Build species accumulation curves by plotting the cumulative number of species detected against sampling effort. This tells you whether you've sampled enough to characterize the community—if the curve is still rising steeply, you probably haven't. If it's plateauing, you're in good shape.

Learning Objectives

1. Conduct systematic quadrat and transect surveys using standardized field protocols
2. Calculate and interpret Shannon-Wiener (H') and Simpson's (1 – D) diversity indices, and understand when each is most appropriate
3. Assess sampling sufficiency using species accumulation curves and rarefaction
4. Compare community composition across environmental gradients and identify drivers of diversity patterns

Animal Systems

• Rocky Intertidal — Quadrat surveys across tidal zones; identify mussels, barnacles, anemones, sea stars, limpets, and algae. Strong zonation patterns make this ideal for comparing diversity across an environmental gradient
• Forest Floor — Leaf litter sampling for invertebrates (beetles, springtails, mites, millipedes), fungi, and mosses. High species richness and patchy distributions challenge your sampling design
• Coral Reef — Belt transects along the reef; survey fish species, coral genera, and invertebrates. Three-dimensional habitat structure adds complexity to the sampling

Background

Ecologists distinguish between alpha diversity (local species richness), beta diversity (turnover between sites), and gamma diversity (regional species pool). This experiment focuses primarily on alpha diversity, but comparing across sites within a system gives you a window into beta diversity as well.

The Shannon-Wiener index (H') accounts for both richness and evenness—a community with 10 equally common species scores higher than one with 10 species where a single species dominates. Simpson's index (1 – D) gives the probability that two randomly chosen individuals belong to different species. Both are widely used, and understanding the difference between them matters: Shannon-Wiener weights rare species more heavily, while Simpson's is more sensitive to dominant species.

Species-area relationships, first described by Arrhenius in 1921, predict that larger areas contain more species, following a power-law function. The intermediate disturbance hypothesis (Connell, 1978) predicts that diversity peaks at intermediate levels of disturbance—too little disturbance allows competitive exclusion, too much eliminates sensitive species. Both ideas provide testable frameworks you can explore with the data you collect.