Comparing and classifying communities
33
7
Comparing and classifying communities
Even a quite modest ecological survey can produce a bewildering amount of
information on the presence and abundance of species and it is frequently difficult
to identify and summarise the main features and inter relationships between
communities. This chapter describes the different approaches and techniques
you can use. The aim is to describe the ways in which your data can be
presented and explored and little attention is given to the theoretical background
of the techniques.
 Useful sources for information about the mathematical background to these
techniques are 
Legendre & Legendre (1998)
111
 and 
Kent & Coker (1992)
111
.
Organising your data for analysis
33
Searching for similarity
34
Multivariate analysis
35
Cluster analysis
35
TWINSPAN
36
Principal Component Analysis (PCA)
36
Non metric multidimensional scaling (NMDS)
37
Reciprocal averaging (RA & DECORANA)
37
7.1
Organising your data for analysis
Ecological data sets can often be arranged as a two dimensional matrix
consisting of n samples (or stations) forming the columns and the S species
forming the rows. This matrix or grid can hold either the observed abundance of
each species, an abundance score or presence absence information. Presence 
absence data are just recorded as 1 if it was found in the sample and 0 if not. The
most convenient way to store and organise these data is with the use of a
spreadsheet program such as Excel or Lotus 1 2 3. Most statistical and
multivariate software can import data from these spreadsheets. If the objective is
to search for species inter relationships then the S column by S row matrix of
species correlations or similarities may be formed and what is termed an R
analysis undertaken. If the objective is to identify samples with similar
communities then the n column by n row matrix of sample correlations or
similarities may be used in a Q analysis.
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Achmil
1
3
0
0
2
2
2
0
0
4
0
0
0
0
0
0
2
0
0
0
Agrsto
0
0
4
8
0
0
0
4
3
0
0
4
5
4
4
7
0
0
0
5
Airpra
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2
0
3
0
Alogen
0
2
7
2
0
0
0
5
3
0
0
8
5
0
0
4
0
0
0
0
Antodo
0
0
0
0
4
3
2
0
0
4
0
0
0
0
0
0
4
0
4
0
Belper
0
3
2
2
2
0
0
0
0
2
0
0
0
0
0
0
0
2
0
0
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