Measuring Life's Variety
Biodiversity — the variety of life within an ecosystem — is a key indicator of environmental health. But how do you reduce the complexity of an entire community to a single number? Ecologists have developed diversity indices that combine two components: species richness (how many species are present) and evenness (how equally individuals are distributed among them). This simulator generates species abundance distributions and computes the most widely used indices in real time.
Shannon and Simpson Indices
The Shannon index H', borrowed from information theory, treats each species as a 'symbol' and measures the information content of the community. Higher values mean greater uncertainty about which species a randomly chosen individual belongs to — more diversity. The Simpson index D measures the probability that two random individuals are different species. While Shannon is sensitive to rare species, Simpson emphasizes dominant species, making the two complementary. Used together, they provide a robust diversity assessment.
Evenness and Dominance
A community of 100 species where one species accounts for 99% of individuals is less functionally diverse than a community of 10 equally abundant species. Pielou's evenness index J' quantifies this distribution, ranging from 0 (complete dominance) to 1 (perfect equality). Disturbed ecosystems — polluted rivers, overgrazed grasslands, clear-cut forests — typically show low evenness as stress-tolerant species dominate. Recovery is often marked by rising evenness before new species arrive.
Estimating Hidden Diversity
Field surveys inevitably miss rare species. The Chao1 estimator uses the frequency of singletons and doubletons to predict how many species went undetected. If a sample contains many singletons (species seen exactly once), many more species likely remain undiscovered. Modern eDNA metabarcoding can detect species from trace DNA in water or soil, revealing hidden diversity — especially among microbes, where a single gram of soil may harbor 10,000 species.