Community ecologists often analyze data by a methodological triad consisting of direct gradient analysis, ordination, and classification. These three methods have the common goal of organizing data for purposes of description, discussion, understanding, and management of communities. They vary in strategy. Direct gradient analysis portrays species and community variables along recognized environmental gradients. By contrast, ordination and classification techniques organize community data on species abundances exclusively, apart from environmental data, leaving environmental interpretation to a subsequent, independent step (with a few exceptions, as will be noted later). The result of ordination is the arrangement of species and samples in a low-dimensional space such that similar entities are close by and dissimilar entities far apart. The result of classification is the assignment of species and samples to classes; the classes may or may not be arranged in a hierarchy. These three approaches are complementary, as was shown by examples in the first chapter.

Work in organizing data matrices pertinent to ordination began early in this century, and substantial ordination work was done around 1950, using simple algorithms and hand calculations (for reviews, see Becking 1957 and Whittaker 1967). By 1970, computers were available to most ecologists, and a great number of ordination techniques had been developed. Ordination techniques were introduced more rapidly than they were tested, however, until the 1970s, when ordination comparisons were made leading to reliable recommendations and preferences.