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ECOM II

Delete Unmatched Columns button   Use this to remove samples that are not

present in both data sets.

Truncate Longest Dataset button   Use this to truncate both data sets to the

number of samples in the smaller data set.

OK button   Click to continue with analysis

6.4

Environmental variables from biological features

There is a paradox in gradient analysis: species respond to the environment, but they

also modify the environment. For example, vegetation itself can be considered an

environmental factor to which vegetation responds. There are a suite of variables

derived from species data which might be useful in CCA and other constrained

ordinations: maximum height of vegetation, total biomass, light penetrating through

the canopy, woody plant cover, etc.

These variables might be quite informative in an exploratory analysis, though the

ecologist must realize that it would be difficult to distinguish cause and effect.

Species derived variables should NOT be used in hypothesis testing, because the

same data would be represented in both the dependent and the independent

variables. This would lead to circular reasoning. A compromise would be to have these

vegetation derived variables considered "passive" or "supplementary"   i.e. they

would be included in diagrams, but would not otherwise influence the ordination.

An extreme case of species derived variables is to use dummy variables derived from

a classification of samples. The classification could either be a result of a subjective

procedure (e.g. the Braun Blanquet approach or something less formal) or a

Copyright 2004  PISCES Conservation Ltd