Interspecific associations, ordination,

INTERSPECIFIC ASSOCIATION

The association of two or more than two species is known as interspecific association or species association.  In the association species must grow together, like in many areas Stipa cornata and Bouteloua gracilis grow together in Great Plains.  In British Colombia Agropyron and Poa grow together.  In the Malwa zone Dicanthium and Indigofera grow together.  This association is actually occurring due to same ecological amplitude of two or more species.  It may be similar in geographic range, climatic factors and life forms.  Many times the different life forms exclude the competition ad increase the dependence of species on one another.  This increasing dependence is indicated by association index.  This association may be due to shed, food and protection etc.  When the environmental conditions change, then the association may be changed.  When any species is growing as dominant species in one stand then it may be sub-dominant in another stand. Like in North Dacota Agropyron is associated with Muhenbergia, Carex, Eurotia, but in British Colombia Agropyron is dominant but other genera are absent.  This presence and absence indicates the favourable or unfavorable environment.

The interspecific association can be calculated by different formulae.  The simplest method is to find out the presence of 'A' species with 'B' species and following formula can be used for association index.

Association index  =  Number of samples where 'A' occurred with 'B'

                                             Number of samples for species 'A'

If species 'A' is present in 40 sample area and in 30 sampling area 'A' and 'B' are present together, then association index of species 'A' will be :

            Association index  =  30/40  =  0.75

So the maximum association index may be 1 or 100% generally this index is less than one.  This result shows that 3/4th area is indicating association of 'A' and 'B'.  The association index can be calculated by following formula -

            Ψ²  =               (a+d)² - (b+c)²          

                            (a+b) (b+c) (c+d) (a+d)  

Where,

            A  =     number of samples where species a and b both are present.

            B  =     the number of samples in which species a is present and b is absent.

            C  =     the number of samples in which species a is absent and b is present.

            D  =     the number of samples in which both the species are absent.