and high again on the lower slope (30-50 m). The lack of shelter for grazing 

 fishes probably also explains the existence of extensive algal plains occurring 

 on sand bottoms below and between reefs (e.g., Hay 1981b, Hay et al_. 1983). 



As might be expected, the impact of fish herbivory strongly influences the 

 community structure of reef algae. Hay (1981b) and Hay et a].. (1983) suggested 

 that selection for resistance to grazing may compromise competitive ability. 

 They found that, off the Caribbean coast of Panama, fishes prevent competitively 

 dominant (but highly palatable) sand-plain species from displacing competitively 

 subordinate (but grazer-resistant) reef algae. This dichotomy may act to 

 maintain between-habitat diversity in algae (Hay 1981b). 



Within-habitat algal diversity is strongly affected by territorial 

 damself ishes. As discussed in the previous section, these fishes establish and 

 maintain dense mats of mostly filamentous algae on dead coral surfaces by 

 defending small individual areas against other herbivores, including sea urchins 

 (Williams 1980, 1981). By differentially grazing these mats (Irvine 1982, Hixon 

 and Brostoff in prep.) and/or by "weeding" undesirable species (Lassuy 1980), 

 damselfishes can affect the local diversity of algae. This effect has been 

 demonstrated by three similar experiments in Guam (Lassuy 1980), Hawaii (Hixon 

 and Brostoff 1981, 1982, 1983), and Australia (Sammarco 1983). Each experiment 

 compared algal diversity on substrates exposed to three different treatments: 

 accessible to mostly damselfish grazing inside territories, accessible to intense 

 grazing by other herbivores outside territories, and protected within fish- 

 exclusion .cages outside territories. Although strict comparisons are precluded 

 by differences in experimental design and laboratory analyses, some general 

 patterns do emerge. For both damselfish species he studied, Lassuy (1980) found 

 that, of the three treatments, caged surfaces exhibited the greatest algal diver- 

 sity after 2 months. Both Hixon and Brostoff (1981, 1983) and Sammarco (1983) 

 obtained the same result from samples taken after 2 to 6 months and 3 months, 

 respectively. However, after a year both these studies found that algal 

 diversity was greatest inside damselfish territories. These data, combined with 

 the fact that Sammarco studied one of the same species as Lassuy, suggest that 

 Lassuy 1 s (1980) samples may have represented early successional stages. In any 

 event, Hixon and Brostoff (1982, 1983) further showed that grazing intensity was 

 of intermediate intensity inside territories relative to the other two 

 treatments. These results thus supported the "intermediate-disturbance 

 hypothesis" (sensu Connell 1978). At low levels of grazing disturbance within 

 cages, a few dominant competitors (especially the red alga Tolypiocladia in the 

 Hawaii study) were capable of locally excluding most other species. At high 

 levels outside territories, many local extinctions occurred. Algal diversity 

 thus peaked at intermediate grazing intensity inside damselfish territories, 

 where the coexistence of many species was maintained because their densities were 

 apparently kept below levels where resources became limiting. Note, however, 

 that not all damselfishes enhance local algal diversity; some species maintain 

 near monocultures within their territories (e.g., Montgomery 1980a, b). 



Regardless of whether or not damselfishes enhance local diversity, the greatly 

 increased standing crop of erect algae within their territories has important 

 secondary effects on reef communities. The algal mat serves as a refuge for 

 invertebrate microfauna and/or various epiphytes (Lobel 1980, Hixon and Brostoff 

 1982 and in prep.). Also, because accretion by coralline algae adds to the reef 

 framework and such algae (along with corals, see above) are overgrown by the 

 algal mat, damselfish territories are probably sites of weakened reef structure 

 (Vine 1974, Lobel 1980). Finally, damselfish territories may indirectly affect 

 nitrogen fixation on reefs, although available data are somewhat contradictory. 

 During the same study as Sammarco (1983) described above, Wilkinson and Sammarco 

 (1983) found that nitrogen fixation by blue-green algae was positively correlated 



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