DISCUSSION 



On the scale used in this study, between-habitat differences in herbivory 

 are shown to be consistent on 3 different reefs (fig. 1). Rates of macrophyte 

 removal on reef flats or on deep reef slopes are significantly reduced relative 

 to removal rates on shallow reef slopes. In addition to the reefs studied 

 here, shallow reef flats also have been shown to function as spatial escapes 

 from herbivory in the Virgin Islands (Adey and Vassar 1975, Steneck and Adey 

 1976), the Netherlands Antilles (van den Hoek et aj_. 1978), and Panama (Hay 

 1981c, Hay et a]_. 1983). Decreased herbivory on deep reef slopes has been 

 hypothesize'd - to provide an explanation for the increased algal abundance that 

 occurs at depth on some reefs (van den Hoek et aj_. 1978), and the rate of 

 macrophyte removal recently has been shown to decrease with depth on a wide 

 variety of undisturbed Caribbean reefs (Hay 1984). 



Reef flats and deep reef slopes usually are characterized by reduced 

 topographic complexity; in areas where predatory fishes are abundant, these 

 more simplified habitats may be avoided by herviborous fishes because they 

 offer few places to hide when attacked by predators. On shallow reef slopes, 

 herbivorous fishes concentrate their grazing in areas of greater topographic 

 complexity (Hay £t jajL 1983) and, on heavily fished reefs where predatory fishes 

 are relatively rare, herbivorous fishes make increased use of deeper reef slopes 

 (Hay 1984). All of these patterns suggest that the probability of being preyed 

 upon may play a significant role in determining the spatial pattern of foraging 

 by herbivorous fishes. 



Previous studies have suggested that the evolution of herbivore resistance 

 in seaweeds involves costs that result in decreased growth rates and decreased 

 competitive ability in the absence of herbivores (Lubchenco 1980; Lubchenco and 

 Gaines 1981; Hay 1981a, c; Hay et jH . 1983). The data presented in figure 2 

 provide a partial test of this hypothesis; if characteristics that promote 

 herbivore resistance mandate costly tradeoffs, then herbivore resistance should 

 not evolve in species that occur primarily in habitats subject to low rates of 

 herbivory. Patterns exhibited by reef flat seaweeds support the hypothesis; 

 they are subject to low rates of herbivory (fig. 1) and exhibit little resistance 

 when exposure to herbivores is increased (fig. 2). 



Rates of Thalassia removal on deep (30-40 m) reef slopes were significantly 

 higher than on reef flats, but the magnitude of difference was not large--17.5% 

 versus 9.3% at Carrie Bow, and 12.8% versus 1% on Ligvhthouse (fig. 1). However, 

 differences in herbivore resistance of species from these habitats were striking 

 (fig. 2). Despite the low rate of removal of Thalassia that was documented on 

 deeper sections of the reef slope, seaweeds from these deeper areas were very 

 resistant to herbivory. Even though the Thalassia bioassay shows herbivory to 

 be relatively low in both reef-flat and deep reef-slope habitats, herbivore 

 resistance appears to have been selected for on the deep reef slope and selected 

 against on the shallow reef flat. This apparent paradox can be explained if 

 one considers rate of biomass removal by herbivores (i.e., the Thalassia 

 bioassay) relative to rate of production through photosynthesis. Seaweeds in 

 shallow waters may grow many times faster than seaweeds in deeper waters (Hay 

 1981a, b). The low rate of biomass removal that occurs on reef flats can 

 rapidly be replaced by photosynthesis since light is plentiful and turbulence 

 prohibits the formation of large diffusion gradients that would slow nutrient 

 acquisition (Doty 1971). Since production of seaweed biomass is very slow on 

 deeper reef areas, even small losses to herbivores may exceed gains and thus 

 select for increased herbivore resistance. As an example, if herbivores removed 

 equivalent amounts of plant material from deep and shallow sites, selective 



100 



