Non-Metric Ordination Methods in Ecology

SUMMARY Two basic types of ordination, metric and non-metric, are distinguished and discussed. Non-metric ordination or non-metric scaling, unfamiliar in ecology, is considered to be more powerful in various respects than metric ordination (which includes principal components analysis). Its advantages are discussed with reference to the assumptions which have to be made if ecological inferences are to be drawn. The properties of two-dimensional metric and non-metric ordination are compared in two examples. The first illustrates a long plant succession, and contrasts the linear reconstruction by an appropriate non-metric method with the familiar arch produced by a principal components method. The second shows how two independent gradients-soil base status and grazing intensity in grassland and tall-herb communities-can be satisfactorily reconstructed by a non-metric method.