JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 128.235.251.160 on Sun, Abstract. Most general models do not include herbivory as a major agent of successional change. Potentially, herbivores can affect succession in three ways: accelerating or decelerating the rate of succession, where the sequence of dominant species is unaltered, or deflecting succession onto a new trajectory, where the species composition of dominants becomes substantially different than during ungrazed succession. We examined these alternatives for benthic algae on a coral-reef crest off Oahu, Hawaii. In this system, exposed coral-rock surfaces naturally undergo one of two major grazing regimes: (1) relatively protected inside defended territories of the damselfish Stegastes fasciolatus (Pomacentridae), where the benthos is dominated by filamentous algae; or (2) exposed to abundant schooling parrotfishes (Scaridae) and surgeonfishes (Acanthuridae) outside territories, where the bottom is covered mostly by crustose algae. We compared the effects of this differential grazing on primary succession, relative to ungrazed succession, by distributing on the same date 1332 settling surfaces among three treatments: exposed inside damselfish territories, exposed just outside territories, and within fish-exclusion cages just outside territories. To balance the advantages and disadvantages of different settling surfaces, we used equal numbers of each of three kinds of 50-cm2 settling plates: naturally contoured coral rock, coral rock cut into flat plates, and roughly sanded PVC plastic. To follow relative successional pathways, we sampled destructively 63 plates (21 from each grazing treatment) 17 times over 1 yr. Plates placed in the field several months before and after the main experiment suggested no seasonal differences in algal colonization. A concurrent cage-control experiment involving 144 settling plates, combined with measurements of light and water motion inside vs. outside cages, indicated that the secondary effects of cages were minor compared to the primary effect of preventing fish grazing.In the absence of fish grazing within cages, algal succession over the year followed three stages: early dominance by simple green and brown filaments (such as Enteromorpha rhizoidea and Ectocarpus indicus), a midsuccessional stage dominated by thin and finely branched red filaments (such as Centroceras clavulatum and Taenioma perpusillum), and a late stage dominated by blades and coarsely branched thick filaments (especially Tolypoicladia glomerulata). Species diversity followed a unimodal pattern during ungrazed succession, declining as a few species of late-stage algae predominated. Inside damselfish territories, succession was decelera...
