Index of similarity

Definition

A quantitative measure expressing the degree of resemblance between two biological samples, communities, or taxonomic lists, typically ranging from 0 (no shared elements) to 1 (identical composition). In entomology and ecology, these indices compare species assemblages across habitats, treatments, or geographic sites without requiring complete enumeration of all species present.

Etymology

Example

A researcher comparing ant communities in disturbed and undisturbed forest plots calculates the Morisita-Horn index of similarity, finding a value of 0.72, indicating substantial overlap in species composition despite habitat differences.

Synonyms

• similarity coefficient
• similarity index

Related Terms

• index of dissimilarity
• Jaccard index
• Sørensen-Dice coefficient
• Morisita-Horn index
• Bray-Curtis dissimilarity
• beta diversity
• species turnover
• community ecology

Usage Notes

Indices of similarity are abundance-based (weighing common species heavily, e.g., Morisita-Horn) or presence-absence based (treating all species equally, e.g., Jaccard, Sørensen). The complement (1 − similarity) yields an index of dissimilarity or distance. Choice depends on sampling method: abundance indices suit standardized trapping data, while presence-absence indices work better for incomplete inventories typical of rapid biodiversity assessments. 'Similarity' and 'dissimilarity' are mathematically interchangeable via simple transformation, but specialists prefer 'dissimilarity' when emphasizing differences between communities.