The Main Points

• I spent the bulk of the lecture discussing the fundamentals of direct and indirect ordination. While the goals of the two approaches are the same, namely to locate species populations and/or species assemblages relative to physical gradients or to gradients "derived" from patterns of similarity among the samples, the physical gradients are identified in very different ways.
  1. In direct ordination sampling, one assumes the physical and /or chemical gradients that shape the distributions of the organisims are known and can be measured during the sampling procedures. For instance, one might use altitude and aspect to dictate the sample design on a mountain. The species populations are then located relative to these two values. (Study Figures 8-20 and 8-27 and the accompanying text.)

  2. In indirect ordination, one is attempting to discover the important physical or chemical gradients "after the fact." Large numbers of samples are taken and then compared to each other by some technique like the Coefficient of Community that I used in Lecture. The coefficients are then plotted along a primary ordination axis anchored on each end by the two most dissimilar samples. Additional axes are then used to spred the samples until the magnitude of the coefficients among the samples matches the scale of distance in the graph. These derived patterns or clusters of the sample points give hints to the particular physical or chemical gradients that are important in these environments. Ricklefs discusses these ideas in the first half of Chapter 26. In the last half of the chapter he addresses the issue that stimulated all this work in the first place, namely, can we think of "communities" as discrete units?

• The last point I make here and will talk about in the next lecture is that the "continuum ecologists" question the presence of plant communities by arguing that:
  1. Species act as individuals.

  2. The result is a vegetation continuum [as long as the environment is perceived as a set of gradients].

  3. Environmental discontinuities create very different assemblages of plants in close proximity.

  4. The discontinuities cause the perceived communities, not the assemblages themselves.