phylog 

 only f 

 member 

 even o 

 d i s t r i 

 s i s t s 

 i d a e a 

 catego 

 the ec 

 am phi p 



e n e t i c 

 or ill 

 s of e 

 n a mi 

 b u t i o n 

 of sev 

 nd her 

 ry. Th 

 h i n o i d 

 ods , t 



boundaries (Figure 1) and are depicted as non-overlapping 

 ustration. The organisms listed are potential Caribbean 

 ach group; the actual composition at a given locality, 

 cro-habitat level, will depend on patterns of species 



and abundance. For the present study site, Group 1 con- 

 eral fish species in the families Scaridae and Acanthur- 

 eafter will be referred to as the herbivorous fish 

 e most important herbivore in Group 2 at this locality is 



Pi ad em a a n t i 1 1 a r u m . Members of Group 3 include several 

 ana ids, syllid polychaetes and small gastropods. 

 3 



high 



3EQ. OF REGRAZING 

 SAME PATCH 



Microher bivores 



amphtpods 



syllid polychaetes 



tanaids 



sm gastropods 



Diadema 

 Mithrax sp. 

 Blenmdae 



Acanthunda 

 Kyphosldae 

 Scaridae 



PATCH SIZE CHEATED 

 PER UNIT TIME 



large 



Figure 1 . Diagramatic representation of herbivore functional groups. 

 Group members are potential Caribbean herbivores. 



Algal communities were allowed to develop under ambient backreef 

 conditions (depth, 2 m) in St. Croix, U.S. Virgin Islands on exper- 

 imental substrata consisting of 8 cm x 8 cm x 1 cm cross-sections of 

 the coral Acropora palmata . These experimental plates were randomly 

 placed in one of the following treatments: grazing by groups 1 and 3, 

 grazing by groups 2 and 3, grazing by group 3 only, no grazing and 

 control. The accessibility of plates to functional groups was con- 

 trolled using a combination of cages and suspended platforms at the 

 same depth. Each treatment was replicated once. A detailed descrip- 

 tion of the experimental design and methods is forthcoming (Carpenter, 

 ms. in prep.). Algal community primary productivity was estimated 

 monthly from changes in dissolved oxygen in well-stirred light and 

 dark plexiglass chambers on randomly chosen plates from each treat- 

 ment. From the same plates samples were taken to estimate algal bio- 

 mass (decalcified dry wt.), species composition and chlorophyll a_ 

 content . 



The amount of algal biomass removed by each functional group was 

 estimated in two ways. The difference in algal standing crop between 

 plates not grazed and those grazed by a given group gave one estimate. 

 The second, more accurate method converted the areal production rate 

 (corrected for a constant percent of dissolved organic matter excret- 

 ion (Fogg, 1976)) for a treatment to carbon equivalents, and assuming 

 a constant carbon content, this was translated into the expected dry, 

 decalcified biomass accumulation over a time period. The difference 

 between this expected amount and the actual algal standing crop was 

 assumed to be removed by herbivores. 



All data reported here were obtained at least one year after the 

 initiation of the described treatments. 



114 



