In general, a classificatory scheme in which the final groups are characterized by indicator species derived from the data in the course of group definition. More specifically, the term refers to a polythetic divisive classificatory scheme proposed by M. O. Hill in 1975. Sites are ranked by a reciprocal averaging ordination and divided into 2 groups at the mid-point (the ‘centre of gravity’) of all the weighted data values of the ordination. Indicator species (usually 5) are then identified as those species exclusively, or most nearly so, associated with one or other side of this division (positive and negative indicators). The site-indicator scores, effectively a rough secondary ordination, determine their final classification; and the process may then be repeated within the groups identified. The indicator species form a key, enabling new sites to be added easily into the classificatory framework without excessive recalculation.
Subjects: Plant Sciences and Forestry — Ecology and Conservation.