Morisitas

Morisitas refers to several indices in ecology developed by the Japanese ecologist Masahiro Morisita. The two best known are Morisita's dispersion index and the Morisita–Horn index of similarity. These measures are widely used to analyze how individuals and species are distributed across samples and how similar communities are in species composition.

Morisita's dispersion index, also called Morisita's I, quantifies how individuals of a species are distributed among sampling units. If counts xi are observed in each of n units and N is the total number of individuals, a common form is Iδ = [n Σ xi(xi - 1)] / [N(N - 1)]. Values near 1 indicate a random (Poisson) distribution, while values greater than 1 suggest aggregation or clumping and values less than 1 suggest a more uniform spread.

The Morisita–Horn index is a similarity measure between two communities based on species abundances. If species i has counts ai in community A and bi in community B (with totals A = Σ ai and B = Σ bi), the index is often expressed as C_MH = [2 Σ ai bi] / [ (A^2 / Σ ai^2) + (B^2 / Σ bi^2) ]. The index ranges from 0 to 1, with 1 indicating identical species composition and 0 indicating no shared abundance patterns.

Both indices are valued for their robustness to sample size and their applicability to abundance data, complementing other metrics such as the Jaccard or Sørensen indices. They are frequently used in biodiversity studies, population ecology, and microbial ecology to characterize dispersion and community similarity.