Worked Example

88

13.3

Interpreting the results

When a Canonical Correspondence Analysis is undertaken a large number of tabbed

sheets of output will be produced. In this section we will use the Dune Meadow data

to take you through the output.

Having undertaken a CCA from the drop down:

we first examine the Variance tab. The mutilple correlation species/environment

scores indicates how much of the variation in species composition is explicable from

the environmental variables. The  figure of 0.9579 for the first axis indicates that we

can account for most of the variation in species composition by taking account of the

environmental variables. The maximum and minimum values for this coefficient are

1.0 and 0 and a value close to 1 indicates that the environmental variables are having

an appreciable effect, this correlation should be viewed with great caution and can be

highly misleading see 

Problems with Species Environment Correlation

. Note also that

the first canonical axis explains about 21% of the total variability. This is quite a large

amount but may not be significant. A 

Monte Carlo test

 must be used to test for

significance.

The results of the Monte Carlo test with 1000 replicates are shown below. This

indicates that the amount of variability explained by the environmental variables is

significant for both the first and second canonical axes ( Probability < 0.05   less than

a 5% chance that the observed relationship could occur by chance)

The sample and species scores that will be plotted are presented in grids in the

Copyright 2004  PISCES Conservation Ltd