82

Community Analysis Package

17

Similarity measures

Choose the similarity measure you wish to calculate from the Similarity drop down

menu. The calculations will appear on the Similarity tab at the bottom of the

program window. For ease of use, the program will highlight sites with similarity

above a certain level. You can set this level by entering the number in the 

Set

Threshold Of

 box at the bottom of the page.

The similarity measures used.

These are simple measures of either the extent to which two habitats have

species in common (Q analysis) or species have habitats in common (R analysis

). Binary similarity coefficients use presence absence data; following the

introduction of computers more complex quantitative coefficients became

practicable. Both groups of indices can be further divided to those which take

account of the absence from both communities (double zero methods) and those

which do not. In most ecological applications it is unwise to use double zero

methods as they assign a high level of similarity to localities which both lack many

species; a problem which becomes particularly acute in habitats which have a

potentially extremely large species list such as the marine benthos. A good

account of similarity and distance measures is given in Legendre & Legendre

(1983). Because of division by zero problems for some data sets not all measures

can be calculated. When a division by zero error would occur 

CAP

 gives an index

of  99.

For measures of similarity between samples based on species presence absence

the observations can be summarised in a simple frequency table:

Sam

Species Present

Sample 2

Species Present

a

Species  Absent

c

where the number of species present in both samples is a, the number of species

present in sample 1 but missing from sample 2 is b, the number of species

missing in sample 1 but present in sample 2 is c and the number of species

missing from both samples is d. The total number of species, N, is therefore

a+b+c+d.

Binary   double zeros

Simple matching

83

Rogers_Tanimoto

83

S3

84

S4

84

S5

85

S6

85

Binary   no double zeros

Jaccards

86

Sorensen

86

S9

86

S10

87

Russel & Rao

87

Kulczynski

88

S13

88

Ochiai

89

Qualitative

Copyright 2004, PISCES CONSERVATION LTD