We demonstrate a contlnuoui decrease in the taxonomic dlstlnctness of a marlne assemblage along a gradient of Increasing env~ronmental contamination, In a situat~on where specles dlverslty remalns constant. Tcvo l n d~c e s have been employed, A and A ' , the f~r s t b e~n g a taxonon~ic divcrslty index empirically related to Shannon species dlvcrs~ty ( H ' ) but wlth an added component of taxonom~c separat~on, and the second a measure purely of taxonomic d~stinctness. The values of both

more »

... appear to be rather less ~nfluenccd by sample size than does H , and markedly less sample-si7c dependent than other comrnon dlverslty measures such as species richness and evenness. It 1s concluded that taxonom~c distinctness may be a more sensltlve univanate index of community perturbation than s p e c~e s d~versity We also argue that A comes closer to a 'b~odlverslty' index than H', and suggest the possibll~ty that the total genetlc complement in any biome may, w i t h~n Iimlts, remaln more or less constant but be pal-titioned differently among the h~erarchy of taxonomic unlts, accol-ding to the age or successional stage of the assemblage. K E Y WORDS. Taxonomic distinctness . Hierarchical classlflcation . Blod~verslty Environmental perturbation .