Community Analysis Package
Detrended Correspondence Analysis DECORANA
Detrended correspondence analysis was devised by Hill (1979) as an attempt to
improve upon reciprocal averaging (RA). Two problems that occur with reciprocal
averaging are termed the arch effect and end point compression . When the first
and second axes produced by RA are plotted it is often observed that the points
are arranged in an arch because of the quadratic relationship between the axes
rather than any ecological relationship. DECORANA removes this arch by a
technique termed detrending. The tendency for points at each end of the first axis
to be closer together than those in the middle is removed by segmenting the axis
and expanding the terminal segments and compressing those towards the centre.
Whereas RA scales the axes between 0 and 100 in relation to the magnitude of
the eigenvalue, DECORANA scales in units of average standard deviation of
species turnover. Therefore a change of 50% in species composition occurs in
about 1 standard deviation.
Samples or species which are very different in their composition or distribution
from others in the data set present problems for DECORANA and in general such
outliers should be removed from the data before the method is applied.
When DECORANA is selected from the ordination drop down menu the Setup for
DECORANA window is displayed. For many users the default settings can be
used by simply clicking OK.
The options are described in turn below:
Downweight rare species
. Select this option if the influence of rare species is to
be reduced. If selected, the abundance of species rarer than the frequency of the
commonest species divided by 5 are down weighted in proportion to their
Number of axis rescalings
. Input an integer number, the default of 4 should
generally be used.
Axes shorter than this value will not be re scaled. The
default is zero.
Number of segments
. This is the number of segments the axis is divided into for
re scaling. Input an integer number, the default of 30 should generally be used.
Output for DECORANA is presented under a number of tabbed components that
can each be viewed by clicking on the tab. These are described in turn below.
Species Scores DECORANA
Sample Scores DECORANA
This text window gives the values of the eigenvalues calculated for each axis.
These are estimated by iteration and this window also gives the residual term for
each iteration. The eigenvalues are calculated to an accuracy of 0.000001. If after
999 iterations this residual value is not achieved computation is halted and a
warning message produced.
The length of the gradient is proportional to the rate of species appearance and
disappearance (species turnover) along that gradient.
Copyright 2004, PISCES CONSERVATION LTD