The Main Points

• The process of describing the nature of the formations led ecologists directly to the problem of describing the structure of "communities." We can think of structure here as the species composition and relative abundance of these species in some defined space. But, where does one set the boundaries within which to sample?

• Two European schools approached the problems in very different ways.
  1. The botanists of the Zurich-Montpelier school of thought took an "a priori approach". In other words, they attempted to identify a "typical" piece of the vegetation in question and examine it in very precise and specific ways. One learned a lot about a very small piece of the natural vegetation. Since most of the natural European vegetation was long since gone and what was left was in relatively small patches, this method was a reasonable approach to the problem.

  2. The botanists of the Uppsala school approached the problem of the Swedish vegetation in an "a posteriori" way. That is, they first had to identify, in a quantitative way, plot sizes that would be representative of the vegetation they sought to describe. They developed species/area curves from plot data to determine the size of the area they had to sample to approximate a "typical piece of vegetation."

• While these advances in sampling techniques and ways of visualizing 2 and 3 dimensional aspects of the plant "communities" were giving ecologists new tools, the central debate during the middle decades of this century was about whether or not "communities" should be thought of as relatively closed, organismal kinds of structures. Read Chapter 26 in Ricklefs to get a nice summary of the major work and ideas that were competing from the 1930s-1980s.

• R.H. Whittaker was important in making popular the "continuum" approach to vegetation analysis and the techniques of ordination and gradient analysis. He examined the closed vs open community hypotheses (see Fig 26-3) with extensive sampling and analysis of places like the Siskiyou Mountains of northern California, the Great Smoky Mountains of eastern Tennessee and Santa Catalina Mountains of SE Arizona (see Figs 26-10 and 26-11).

• Whittaker asked the following general questions about vegetation patterns:

  1. How are species populations distributed in relation to one another and communities along an environmental gradient?

  2. How are kinds of communities in an area related to patterns of more than one environmental gradient?

  3. How are we to interpret world-wide relations of communities to climatic gradients?