36

Community Analysis Package

that mould community structure.

There is a considerable literature on multivariate techniques. Useful texts for

ecologists are 

Legendre & Legendre (1998)

111

, 

Digby & Kempton (1987)

111

 and

Kent and Coker (1992)

111

.

7.5

TWINSPAN

This method can be used with presence absence, % cover and quantitative data.

The ability to effectively handle % cover makes the method attractive to botanists.

The Two Way INdicator SPecies ANalysis procedure (Hill 

et al.

 1975) also

produces dendrograms of the relationship between species and samples but uses

the reciprocal averaging ordination method to order the species and samples.

Thus the method is something of a hybrid between classificatory and ordination

methods. It is particularly attractive in studies where the objective is to classify

communities so that field workers can quickly assign an area to a community type

and is much favoured by botanists. This is because it identifies indicator species

characteristic of each group identified.

TWINSPAN is a useful technique when you are seeking to identify species that

can be used to characterise particular communities. It is, however, not always an

easy method to understand. A particular, oddity of the method is the concept of

 pseudospecies . Each species is divided into a number of pseudospecies which

represent the different abundance levels at which it was found.

7.6

Principal Component Analysis (PCA)

Principal Component Analysis (PCA) is the oldest and still one of the most

frequently used ordination techniques in community ecology. It is most

appropriate for full quantitative data, but can be used if abundance is classified

into a number of abundance classes. The objective of the method is to express

the relationship between the samples in a 2 or 3 dimensional space that can be

plotted and usefully visualized. This can only be achieved if many of the species

are positively or negatively correlated. Normally this will be so for a number of

reasons. First, there is the interdependence between organisms in an ecosystem

and second, because many species respond similarly to environmental variables

such as temperature and water.

General descriptions of the procedure for biologists are given

Legendre & Legendre (1998)

111

; 

Digby & Kempton (1987)

111

;

Kent & Coker (1992)

111

.

The analysis is undertaken on either the between site variance covariance or

correlation matrix. If the species vary greatly in abundance you will probably need

to transform the data by taking logarithms or using a square root transformation.

Logarithmic transformations would be excellent if it were not for the fact that zeros

cannot be handled. A frequently used procedure is to add 1 to all the

observations. This can distort the output and it is probably more appropriate to

use a square root transformation.

If you undertake a PCA on the correlation matrix you will be giving all species,

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