the basis of a mathematical model (Weber 1973). Values for e that are near 

 one show a close correspondence of the field data to the theoretical model 

 with a highly equitable distribution of abundances among the collected taxa. 

 Values of e near zero show the opposite trend and a distinctively inequitable 

 distribution of abundances among the collected organisms. In the main, 

 healthy and unpolluted ecosystems tend to demonstrate a highly equitable 

 abundance distribution with index values above 0.50, while degraded and 

 disturbed ecosystems tend to show a poor equitability with index values below 

 0.50 and approaching zero. 



Most commonly, equitability numbers between 0.60 and 0.80 are obtained 

 from nondegraded streams, and higher e values near 1.00 are rarely found in 

 the real world. As a result, periphytic samples exhibiting e readings between 

 0.60 and 0.80 are definitely indicative of good environmental conditions and a 

 lack of severe pollution. In a few rare occasions, e values above 0.80 can be 

 obtained; such high indices are also suggestive of non-polluting situations, 

 although they typically refer to a natural physical stress as might be 

 subjected in a torrential stream. 



At the other end of the scale, low e numbers between 0.00 and 0.30 are 

 fairly accurately diagnostic of some types of instream disturbance that causes 

 an inequitable distribution of abundance among the taxa, and even fairly 

 slight degradations can depress a community's equitability rating to such a 

 low level (Weber 1973). In response, periphyton collections that produce poor 

 equitabilities and index values in this lower 0.00 and 0.30 range are 

 suggestive of environmental perturbations in the associated stream reach. 

 Equitabilities in the 0.30 to 0.60 range, which affords an intermediate 

 condition, are representative of borderline or marginal situations as follows: 

 values of e above 0.50 but less than 0.60 would tend to delineate the somewhat 



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