TWINSPAN

TWINSPAN, which stands for Two-Way Indicator Species Analysis, is a numerical method used in ecology to classify sites or plots based on their species composition. It is a divisive hierarchical procedure that aims to produce a practical, interpretable typology of ecological communities while identifying species that best distinguish each group.

The method operates on a data set of sites (rows) and species (columns), often using presence-absence or abundance data. Through a series of split procedures, TWINSPAN partitions the sites into two groups at each step in a way that maximizes the association of certain species with each resulting group. At each split, the algorithm computes indicator values for species to determine which species are most strongly characteristic of the groups. The process is repeated recursively on subdivided groups, producing a hierarchical classification and a list of indicator species for each group or node.

The typical outputs of TWINSPAN are a dendrogram showing the hierarchical divisions of sites and tables of indicator species and their strength of association for the groups. The resulting classification is widely used in vegetation science to define community types, map vegetation, and aid interpretation of ecological patterns.

Strengths of TWINSPAN include its ability to yield an interpretable, rule-based partitioning and to highlight species most diagnostic of each group. Limitations include assumptions of discrete groups, potential sensitivity to data quality and rare species, and criticism that results can be somewhat deterministic or dependent on preprocessing. Despite newer methods, TWINSPAN remains a foundational tool for exploratory classification in ecology.