Direct gradient analysis is used to display the distribution of organisms along gradients of important environmental factors. By contrast, ordination and classification techniques generally start with the analysis of community data alone and, only later, use environmental data for interpretation. Direct gradient analysis is a major research approach in community ecology, forming a methodological triad with ordination and classification. Ramensky (1930) and Gause (1930) originated direct gradient analysis, but active research began around 1950 (Whittaker 1948, 1967, 1978b). The results of numerous direct gradient analyses have become the foundation for the Gaussian model of community structure, and this model has served an important role in testing and designing multivariate methods.

Basic purposes and example

Before presenting the methods and results of direct gradient analysis in detail, a typical example will be considered to introduce basic purposes and methods.

Figure 3.1 depicts the native vegetation and topography of Nelson County, North Dakota, at a hypothetical site. Prairie, meadow, and marsh occur on a rolling plain of low relief with soil drainage appearing to be the major environmental factor (Dix & Smeins 1967). As one might gather from Figure 3.1, mere reconnaissance of Nelson County would suffice to ascertain the primary importance of drainage. Likewise, it would be easy to determine the characteristic species of prairie, meadow, and marsh. Casual inspection, however, resolves only the easiest questions; a quantitative approach is required for more difficult questions.