Comparing and classifying communities

37

irrespective of abundance, equal weighting whereas the analysis undertaken on

the variance   covariance matrix will reflect differences in abundance, but can

result in the dominant species determining the output. When successful, PCA will

present major features of a complex ecological community in only 2 or 3

dimensions and the ordination of sites along these new axes can be related to

underlying environmental factors that are moulding community structure. PCA

can be judged a success when the first two or three principal axes explain an

appreciable proportion of the total variability in the data set. For large ecological

data sets with > 20 species if the three largest axes can explain more than 30%

of the variance this would be good.

7.7

Non metric multidimensional scaling (NMDS)

NMDS can be viewed as a more generalised development of PCA. Benthic

ecologists working with full quantitative data from grab samples particularly favour

it. It is widely applicable and frequently produces a better ordination than PCA. It

takes more computational effort than PCA and has a potentially important feature

that should be noted. There is no guarantee that the ordination will be the best

possible and it is therefore essential to do a number of runs with different starting

conditions to be sure that you have obtained a stable ordination.

7.8

Reciprocal averaging (RA)

Reciprocal Averaging

67

 (also termed Correspondence Analysis) and an adjusted

version called Detrended Correspondence Analysis (

DECORANA

64

) are the final

types of ordination method that are frequently used. They are best used on

quantitative data although they can give good results with classed abundance

data. Both of these methods are particularly effective when it is suspected that

the sites can be arranged along an environmental gradient. Further, the method

allows the site and species ordinations to be plotted on the same figure, which

allows the influence of the species in determining the ordination of the sites to be

uncovered.

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