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J. Exp. Mar. Biof. &of., 1986, Vol. 100, pp. 225-269 225 Elsevier JEM 726 EXPERIME~AL SEPARATION OF EFFECTS OF CONSUMERS ON SESSILE PREY IN THE LOW ZONE OF A ROCKY SHORE IN THE BAY OF PANAMA: DIRECT AND INDIRECT CONSEQUENCES OF FOOD WEB COMPLEXITY BRUCEA. MENGE’, JANE LUBCHENCO, LINDA R. ASHKENAS* Department ofZoology. Oregon State University, Cowallis. OR 97331, U.S.A. and FRED RAMSEY Department of Statistics, Oregon State University, Corvallis, OR 97331, U.S.A. (Received 20 September 1985; revision received 14 April 1986; accepted 25 April 1986) Abstract: The effects of predation by a diverse assemblage of consumers on community structure of sessile prey was evaluated in the low rocky intertidal zone at Taboguilla Island in the Bay of Panama. Four mnctional groups of consumers were defined: (I) large fishes, (2) small fishes and crabs, (3) herbivorous molluscs, and (4) predaceous gastropods. (1) and (2) included fast-moving consumers and (3) and (4) included slow-moving consumers. Experimental treatments were: no consumers deleted (all groups present), most combinations of deletions of single groups (i.e., one group absent, three present), pairs of groups deleted (two absent, two present), trios of groups deleted (three absent, one present), and the entire consumer assemblage deleted (all groups absent). Changes in abundance (percent cover) of crustose algae, solitary sessile invertebrates, foliose algae, and colonial sessile invertebrates were quantified periodically in 2-4 plots of each treatment from February 1977 to January 1980 after the initiation of the experiment in January 1977. Space on this shore is normally dominated by crustose algae; foliose algae, solitary sessile invertebrates, and colonial sessile invertebrates are all rare. After deletion of all consumers, ephemeral green algae increased from 0 to nearly 70% cover. Thereafter, a succession of spatial dominants occurred, with peak abundances as follows: the foliose coralline alga Junia spp. by July 1977, the barnacle Balanus inexpectutus by April 1978, and the rock oyster Chama echinata by January 1980. Although no longer occupying primary rock space, Jania persisted as a dominant or co-dominant turf species (with the brown alga Giffordia mitchelliae and/or the hydrozoan Abietinariu sp.) by colonizing shells of sessile animals as they became abundant instead of the rock surface. Multivariate analysis variance (MANOVA) indicated that the effect of each group was as follows. Molluscan herbivores grazed foliose algae down to the crier-resistant, but com~titively inferior algal crusts, altered the relative abundances of the crusts, and inhibited recruitment of sessile invertebrates. Predaceous gastropods reduced the abundance of sohtary sessile animals. SmaJl fishes and crabs, and large fishes reduced the cover of solitary and colonial sessile animals and foliose algae, although they were incapable of grazing the foliose algae down to the rock surface. Many of the effects of each consumer group on prey groups or species were indirect; some effects were positive and some were negative. The variety of these indirect effects was due to both consumer-prey interactions among the consumers, and competitive or commensalistic interactions among the sessile prey. Comparison of the sum of the effects of each of the ’ To whom reprint requests should be addressed. * Present address: Department of Fisheries and Wildlife, Oregon State University, Corvalhs, OR 9733 1, U.S.A. 226 BRUCEA. MENGEH-AL. single consumer groups (i.e., the sum of the effect observed in treatments with one group absent, three present) with the total effects of all consumers (i.e., the effect observed in the treatment with all groups absent) indicates that a “keystone” consumer was not present in this community. Rather, the impacts of the consumer groups were similar but, due to dietary overlap and compensatory changes among the consumers, not readily detected in deletions of single consumer groups. The normally observed dominance of space by crustose algae is thus maintained by persistent, intense predation by a diverse assemblage of consumers on potentially dominant sessile animals and foliose algae. The large difference in structure between this and temperate intertidal communities seems due to differences in degree, not kind of ecological processes which produce the structure. Key words: community structure and organization; competition; herbivorous mollusc; indirect effect; keystone consumer; Panama; intertidal; tropics consumer; crab; fish; gastropod; predation; pseudoreplication; rocky Investigations of community dynamics have traditionally followed one of two avenues. These are (1) study of the influence of biotic interactions on the structure of the community (here termed “community regulation”), and (2) study of energy flow and nutrient cycling of the community (here termed “community or ecosystem function”). Studies of community regulation typically focus on the roles of species, and an ultimate goal is to understand how species interact with each other and the physical en~onment to produce patterns of community structure (i.e., distribution, relative abundances, species richness, size structure). Studies of ecosystem function typically focus on the roles of entire trophic levels in the flow of energy and nutrients through food chains, and an ultimate goal is to understand patterns and efficiencies of transfer between levels. Both approaches have limitations on the insight they provide into community dynamics (Paine, 1980; Dayton, 1984). Most importantly, experimental, communitylevel (as used here, co~unity = all macroscopic species at all trophic levels in a particular habitat) studies which focus on the roles of individual species rapidly become impractical as diversity increases. Only in very simple communities can we realistically investigate nearly all of the important species and thereby understand the regulation of the entire community (e.g., Menge, 1976; Lubchenco & Menge, 1978; Lubchenco, 1980, 1983, 1986). Since most communities are complex, community studies usually evaluate the regulation of subunits (e.g., guilds, subwebs; Root, 1973) rather than entire communities, and our present underst~d~g of community regulation is based mosdy on studies which have ignored portions of the community. In contrast, studies concerning the function of ecosystems sacrifice the insight obtained from species manipulations for the knowledge of rates of transfer among components of the entire community, and thus complexity and diversity are typically not major foci of study. Instead, species are lumped into trophic groupings with “functional groups” (Cummins, 1974) typically representing the finest level of subdivision investigated. If complex co~~ities are so d~cult to study, why not simply ignore them in favor of rigorous study of entire simple co~~ities? First, communities sufficiently simple ORGANIZATIONOFATROPICALINTERTIDALCOMMUNI~ 221 to permit manipulations with no lumping seem to be the exception rather than the rule. Second, at least some aspects of the organization of complex communities are likely to be different from those of simple communities. For example, dete~ination of the regulation of complex communities is necessary if we are to determine how the relative importance of different structuring agents (e.g., predation, competition, mutualism, physical factors) varies in time and space (Connell, 1975; 1983; Menge & Sutherland, 1976; Schoener, 1983; Lubchenco, 1986; Sih etal., 1985). This paper presents an effort to investigate the regulation of an entire complex community in a tropical rocky intertidal habitat. The study drew on features used in both approaches to study community dynamics described above. In so doing, we chose to grapple with problems of experimental design, complexity, analysis, and interpretation heretofore avoided in most investigations of community regulation. Our goals are: (1) to report (in the main body of the text) the results of experiments done to detente the organization of an entire complex community, which is, to our knowledge, the first of its kind, and (2) to evaluate (mostly in the Appendix) the pros and cons of the hybrid approach employed in this investigation. Some aspects of the study were less than ideal: (1)the experimental design included pseudoreplication (sensu Hurlburt, 1984), (2) the effects of individual consumer species were blurred by the necessity of lumping species into trop~c~ly and/or pra~ati~~ly defined groups, and (3) the separate effects of one of the consumer groups (small fishes & crabs) could not be experimentally isolated (i.e., the design was non-orthogonal). However, these or similar flaws appear unavoidable in such studies since biological constraints and pragmatic limitations in funding, techniques, and personnel can greatly compromise the ideal experimental design and execution. Despite these problems, the experimental results provide ecologically important insights into the regulation of a complex community. STUDY SITE The main study site has been described elsewhere (Menge & Lu~hen~o, 1981; Lubchenko et al., 1984). Briefly, studies were established along a 0.5 km stretch of the southern shore of Taboguilla Island in the Gulf of Panama (15 km south of the Pacific entrance of the Panama Canal; Fig. 1). Supplementary information on community structure was taken at several other sites including Chitre Island, Tortola Island, and Punta Bruja (Gulf of Panama), and Uva Island (Gulf of Chiriqui; Fig. 1). Tidal amplitude is 7.2 m (Z 21 feet), and five zones occur on this basaltic rocky shore ( = splash, high, mid, low, very low; Lubchenco et al., 1984). The emphasis here is on the low zone, which ranges from + 0.6 to + 2.4 m above MLW. The climate is tropical (about 9”N latitude), with dry (mid December-April) and wet (May-mid December) seasons. 228 BRUCE A. MENGE ETRL. 4 CARIBBEAN PACIFIC \ PERLAS ARCHIPELAGO 5km , I Fig. 1. MapshowingPanamastudy~eas:TA = TaboguiIla,themain study site; supplement~obse~ations were made at CH = Chitre Island (Perlas Archipelago), TT = Tortoia Island, and PB = Punta Bruja (all in the Bay of Panama), and U = Uva Island (Gulf of Chiriqui); other islands labelled include: TB = Taboga, UR = Urava, NA = Naos, PE = Perrico, CU = Culebra, and FL = Flamenco; PC = Panama Canal. COMMUNITY STRUCTURE Our previous reports about this community focused on the total impact of predation on prey abundance (e.g., Menge & Lubchenco, 198 1; Lubchenco et al., 1984; Menge etal., 1986) or quantitative variation in predator abundance (Menge et al., 1986). Prelimin~y qu~itative differences among consumers were considered in some cases, but onfy with respect to how they affected the differential use of refuges by prey species (e.g., Menge & Lubchenco, 1981; Menge et al., 1983). The present paper focuses on qualitative differences among the consumers in this community. The structure of this system has been described in detail (Lubchenco et al., 1984). Briefly, the shores of Ta~guilla Island (and all nei~bo~ng islands; authors’ pers. obs.) appear barren, particularly in comparison to temperate rocky shores. Closer inspection reveals that most space in the low (and mid) zone(s) is covered by algal crusts. Sessile ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 229 invertebrates and foliose algae, so abundant in most temperate regions, are scarce, although most of the higher taxa of invertebrates typical of many temperate shores (e.g., barnacles, bivalves, gastropods, &tons, crabs, sea urchins, seastars) do occur in the Bay of Panama. Fishes are abundant and forage extensively both on mobile and sessile organisms throughout the rocky intertidal. Taxonomic authorities and references used to identify the species in this community are given in Lubchenco et al. (1984). COMPOSITION OF CONSUMER GROUPS Compared to well-known temperate rocky intertidal communities, the food web at Taboguilla is particularly complex (Lubchenco et al., 1984; Menge et al., 1986). Animal (but not plant) species richness is high (> 100 species occur in the low zone) and trophic links are numerous, occurring both between trophic levels and among species of similar trophic status. Consumers include both slow-moving and fast-moving, benthic invertebrates and fishes (Menge & Lubchenco, 198 l), with up to 29 species of four types of predator, up to five species of two types of omnivore, and up to 13 species of six types of herbivore (“type” = major taxonomic groups like crabs, neogastropods, sea urchins, fishes, etc. ; see Lubchenco et al., 1984). Since simultaneous determination of the separate and combined effects of each species of consumer in this food web was deemed impossible, we focused on groups of consumers which were species foraging in similar ways (“functional groups”) (Cummins, 1974). Functional groups are distinct from “guilds” (= groups of species using similar resources; Root, 1967) in that the focus is on the method of foraging, not on the resource. To evaluate the single and combined effects of four groups of consumers on the abundance of sessile organisms, we removed one, most combinations of two and three, and all four groups of consumers. We were particularly interested in identifying “strong” interactions; i.e., those in which the consumer had a large effect on the abundance of prey species (see Paine, 1980). Although interactions among the species within the consumer groups could conceivably cancel out the predatory effects of some species, observations of behavioral interactions, data on diets, and evidence from experiments (Menge et al., 1986) suggest that such effects are probably minor. The four consumer groups were: (1) slow-moving invertebrate predators (hereafter termed “slow-moving predators” or, in some cases, abbreviated as “P”), which consisted primarily of predaceous gastropods; (2) slow-moving invertebrate herbivores (“slow-moving herbivores”, or “H”), primarily limpets; (3) small fishes and crabs (“SF,,‘), blennies and fast-moving herbivorous crabs; and (4) large fishes (“LF”), large-bodied, fast-moving fishes. Although each group includes several species (Table I; see Lubchenco et al., 1984 for a complete species list by group), much of the effect of each group on sessile organisms is probably attributable to one or a few abundant species. This is because most consumer species in each group are either rare or restricted to microhabitats. Herewith is a brief description of the dominant species in each of the four functional groups. BRUCE 230 A. MENGE TABLE ETAL. I Consumer species in each group: species occurring only in the low zone are included; abundance expressed as absolute number per m2 and percent of total at minimum and maximum densities recorded (as reported in Lubchenco er al., 1984); numerical dominants, defined as comprising > 20% of the total abundance of the group are indicated with an asterisk. 7’ Total Consumer group Composition Slow-moving predators (P) Slow-moving herbivores (H) Small fishes, crabs (SFC) Large fishes (LF) Unmanipulated Acanthina brevidentata* (gastropod) Thais melones* (gastropod) others (12 gastropods, 1 seastar) Fissureha virescens* (keyhole limpet) F. longzzssa (keyhole limpet) Siphonaria maura* (pulmonate limpet) S. palmata* (pulmonate limpet) Acanthochitona hirudiniformis (chiton) Ceratozona angusla (chiton) Chiton stokesi (chiton) Echinometra vanbrunti (sea urchin) others (2 limpets, 1 pulmonate limpet, 2 chitons) Ophiobliennius sreindachner? (blenny) Grapsus grapsus* (crab) Herbivores Eupomacentrus acapulcoensti (damselfish)* Kyphosus elegans (chub) Scarus perrico (parrotfish) others (4 species, including 3 damselfishes, 1 surgeonfish) Predators (P) Bodianus diplotaenia (wrasse)* Holacanthus passer (angelfish)* others (6 species, including 2 puffers, 1 porcupine fish, 2 triggertishes, 1 angelfish Pachygrapsus transversus (grapsid crab) Abundance” (range of X no./m2) Minimum Maximum 0.7 to 6.5 19.4 43.3 2.4 to 6.4 66.7 42.7 0.5 13.9 14.0 73.8 25.0 5.6 to 2.1 12.4 to 24.0 0.8 to 5.4 4.8 0 to 31.0 0 32.4 0 to 23.5 0 24.5 0 to 1.5 4.8 1.6 0 to 2.5 0 2.6 0.8 to 2.0 4.8 2.1 1.1 to 3.2 6.6 3.3 0.9 to 2.7 5.4 2.8 0.011 to 0.024 47.8 15.6 0.012 to 0.13 52.3 84.4 0.29 87.9 71.9 0.013 to 0.046 3.9 9.4 0.006 to 0.035 1.8 7.2 0.008 to 0.026 2.4 5.3 to 0.35 0.008 to 0.017 0.003 to 0.005 0.002 to 0.008 2.4 (61.5% of P) 0.9 (23.1% of P) 0.6 (15.4% of P) 3.5 (56.7% of P) 1.0 (16.7% to P) 1.6 (26.7% of P) 2.3 to 4.8 a P and H values = ranges of four neighboring sites; small fishes, LF = ranges of three neighboring sites; Grapsus = range of six neighboring sites; from Lubchenco et al., 1984 and authors’ unpublished data. ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 231 The effect of the slow-mo~ng predators (Z 15 species) was probably due mostly to Thais melones and Acunth~n~brevidentfftu, whelks with broad, overlapp~g diets (Menge et al., 1986; Table I). The effect of slow-moving herbivores appeared largely due to four of 13 species (the limpets Fissurella virescens, F. longifissa, Siphonaria maura, and S. pafmata; Table I). Neither sea urchins (Echinometra vanbrunti) nor chitons were very abundant in this zone (Table I). We assume that their effect was small or restricted to holes (Menge et al., 1983). The effect of small fishes and crabs was probably attributable primarily to the bienny Uphiob~ennius steindachneri and the crab Grapsus grapsus (Table I)_ Grffpsus use their claws to scrape org~isms off the rock (Hawkins & Hartnoll, 19X3), and to crush small solitary invertebrates. They cannot crop algae as close to the rock as can the slow-moving grazers. Juveniles of larger species of fish, although generally scarce, were also included in this group. The effect of large fishes appeared due mostly to five species, including the territorial damselfish Eupomacentrus acapulcoensis, the chub Kyphosus elegans, the parrottish Scarus perrico, the wrasse ~odiffnus diplotaenia, and the porcupinefish Diodon hystrix. All reside in the shallow subtidal zone and frequently forage in intertidal zone at high tide. All use jaws to scrape food from the substratum or crush hard-shelled prey, have broad diets, and, although they are often categorized as either herbivores or carnivores, are actually omnivorous consumers of both mobile and sessile benthic animals and algae (Wellington, 1982; Menge et al., 1986). Although numerically scarce, Diodon is potentially important due to its large size, large foraging ambit, and voracious consumption of limpets, predaceous gastropods, barnacles, bivalves, and crabs (Table I in Menge eta/., 1986; see also Palmer, 1979; Bertness eta/., 1981; Bertness, 1981; Garrity & Levings, 1981; 1983; Gaines, 1983; 1985). Field observations indicate that the damselfish defend loosely-defined subtidal territories against all other fishes, but will move well into the low zone to forage at high tide. The wrasses defend dens under boulders or in deep crevices in the shallow subtidal. Aggression and chasing seemed infrequent or absent when both these species and the other fishes foraged intertidally. Our observations suggest that the different fish species are quite similar to one another in their effects. We thus assume that the large fishes constitute generalized, non-specific sources of mortality for most sessile prey. This assumption is strengthened by field observations of foraging, in which both individuals and schools of fishes would graze intensely in “lawnmower” fashion over rock surfaces, and by the uniform spread of the characteristic graze marks left by the fishes on the substratum. Although there is some species-specific predation, the above-described generalized foraging is our justification for lumping these fishes together into a single functional group. One fish and one crab species were too small to be excluded in our fish and crab exclosures. Both the herbivorous crab ~achygrupsus trffnsvers~ and the wrasse Thalassoma lucasanum could pass through the 1 cm2 holes in the mesh of the cages, and thus had access to both caged and uncaged plots. Since we never observed any differential use of any treatment by either of these species, we assumed that their effects were similar across treatments. Later experiments focusing on the specific effect of 232 BRUCE A. MENGE ETAL. P~c~~g~~~~s indicated that this crab reduces the abundance of filamentous algae and colonial invertebrates such as bryozoans, ascidians, and hydrozoans in holes and crevices (Menge et af., 1983). METHODS CONSUMER MANIPULATIONS Expe~ments were initiated between January and March 1977 (Menge & Lubchenco, 1981; see Table II for the experimental design and the Appendix for an evaluation of constraints and potential artifacts), and run through August 1980. The experiments were maintained at least once, and usually twice per month. Conservatively, z 11,410 person-hours were devoted to establishing and maintaining this particular study. Briefly, biologically and physically similar semi-isolated sections of shoreline 200 to 600 m2 in area were selected as sites for manipulation of slow-moving consumers. Although random placement of the plots along the shore would have produced *‘true” replicates, removal of the mobile invertebrates from some plots would have influenced their abundances in neighboring, non-removal plots, thereby introducing a diflicultto-control experimental artifact. Given the constraints in funding and personnel, this biological constraint thus imposed a pseudoreplicated design (Hurlbert, 1984) on the study. We removed slow-moving predators from Site 1, both slow-moving predators and herbivores from Site 2, slow-moving herbivores from Site 4, and left all slowmoving consumers on Site 6. At each site, large fishes, which forage with their bodies pe~endicul~ to the substratum (e.g., Menge & Lubchenco, 198 1; Choat & Kingett, 1982), were excluded with stainless-steel mesh roofs ( = two-sided cages). The 5 to 10 cm gap between the roof and rock was too narrow for large fishes to enter and forage on the protected surface. Both large fishes and small fishes and crabs were excluded with cages (= complete, or four-sided cages), each of which covered 0.25 m2. Blennies and crabs forage with their bodies parallel to the surface, and field observations indicated that both fed without hindr~ce under roofs but were too large to penetrate the 1 cm* mesh openings of the cages. Comparison of these exclosures with marked, uncaged plots at each site permitted us to evaluate the effects of the fast-moving consumers. Abundance (percent cover and/or density) of sessile organisms in experimental plots were monitored from March, 1977 to February, 1980 using standard methods (Menge & Lubchenco, 1981; Lubchenco et al., 1984) at intervals ranging from 1 to 6 months (see the Appendix for further details). STATISTICAL ANALYSIS Analyses employed primarily multivariate analysis of variance (MANOVA), because more than one variate (i.e., sessile species) changed in response to the consumer manipulations (Morrison, 1976; Morin, 1983). Statistical interactions between con- ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 233 sumer groups were evaluated using a MANOVA model developed by F. Ramsey. We generated linear regressions for changes in abundance of each sessile species (Y) vs. time in months since initiation (X) for each plot. Before analysis, percent covers were transformed with the arcsin transformation, and densities were transformed with the TABLE II Design of consumer deletion experiments: number presence and absence of plots of each treatment of the consumer groups. Treatments Slow-moving consumer manipulations (removals) Fast-moving consumer manipulations (exclusions) No manipulation (Site 6) No exclusion Code in parentheses; Presence or absence Slowmoving predators (=P) Slowmoving herbivores (=H) of consumer Large fishes (=LF) +H+P+LF+SFC + + + +H+P-LF+SFC + + _. +H+P-LF-SFC + + _ +H-P+LF+SFC - + + +H-P-LF+SFC _ + - +H-P-LF-SFC _ + _ -H+P+LF+SFC + -t -H+P-LF+SFC + _ -H+P-LF-SFC + -H-P+LF+SFC _ + -H-P-LF+SFC _ + -H-P-LF-SFC - (4) 2-sided cage (2) Complete cage (2) Predator removal (Site 1) No exclusion (4) 2-sided cage (2) Complete cage (2) Herbivore removal (Site 4) No exclusion (4) 2-sided cage (2) Complete cage (2) Predator and herbivore removal (Site 2) No exclusion (4) 2-sided cage (2) Complete (2) cage + and - , group Small fishes large crabs (= SFC) 234 BRUCEAMENGEETAL. square root transformation (Sokal & Rohlf, 198 1). Although we consider the experimental plots to be as spatially independent as allowed by biological constraints (see above and Appendix), results are not independent with respect to time within plots. To achieve greater independence of treatments and replicates, we removed time as a variable by using regression coefftcients as indices of short-term and long-term change, and by deriving an index of seasonal change for each species in each plot (Appendix). The Y-intercept was presumed to index short-term change because it should reflect whether or not rapid changes in abundance occurred in experimental vs. control plots after initiation of the experiments. Similarly, the slope of the regression was presumed to index long-term change because it should reflect more gradual divergences in abundance over the 3-yr period of the experiment. Finally, we derived an index of seasonal change (see Appendix) because fluctuations in abundance of sessile organisms from the dry (December-April) to the wet (May-November) seasons (Glynn & Stewart, 1973) were possible (although virtually no biologically significant seasonal changes in abundance occurred in unmanipulated plots; Lubchenco et al., 1984). The MANOVAs were performed on each of these indices of change in abundance of sessile organisms. RESULTS NATURAL CHANGES IN SPACE OCCUPATION Patterns of space use in unmanipulated plots in the low zone changed little, either seasonally or annually (Lubchenco et al., 1984) although cover of Ralfia sp. gradually increased (Fig. 2; Spearman rank correlation coefftcient = 0.65, P < 0.01). From 1977 to 1980, 80 to 95% of primary space was covered by algal crusts and the remaining 5 to 20% of the space was covered by dead coralline crust, the rock oyster Chama echinata, shells of dead bivalves and barnacles, and traces of other sessile organisms (Fig. 2). Ranked from most to least abundant, algal crusts included the brown Ralfsia sp., the blue-green Schizothrix calcicofa, a multi-specific (but difftcult-to-identify-in-thefield) pavement of coralline crusts, and the red Hildenbrandia sp. (Fig. 2; Lubchenco et al., 1984). Foliose algae were virtually absent throughout this period, except for trace amounts located in holes and crevices (Menge & Lubchenco, 198 1; Menge et al., 1983 ; 1985). The most noticeable seasonal fluctuations at Taboguilla were small changes in cover of coralline crust (increased during wet season and decreased during dry season) and ephemeral green algae (increased during dry season and decreased during wet season ; Lubchenco et al., 1984). Together, these data and qualitative observations at Taboguilla beginning in January, 1973 and ending in February, 1983 suggest a pattern of near constancy in community structure. Large seasonal and annual fluctuations like those usually observed in temperate regions were not observed at Taboguilla. Observations at other sites in the Bay of Panama suggest that near constancy in community structure is widespread in this region. ORGANIZATION 2 3 7 5 9. 7 RALF. a f.2 m 4 10112 1978 1977 a 235 OF A TROPICAL INTERTIDAL COMMUNITY 7 1979 cc m HILD. 1 19 XXI 80 C* Fig. 2. Changes in patterns of abundance of quantitatively dominant occupants of primary space in control plots (n = 4) in the low zone from February, 1977 to January, 1980: RALF. = Ru&u, SC = Schizothrix calcicolu, cc = coralline crusts, HILD. = Hildenbrandiu (all algal crusts), and Ce = Chama echinata; the remainder of the primary space was covered by other sessile organisms, bare space, dead shells of solitary invertebrates, and dead coralline crust. EXPERIMENTALLY-INDUCED CHANGES IN COMMUNITY STRUCTURE Community development in the absence of all consumers In strong contrast to the persistent patterns of space utilization in unmanipulated plots, community structure changed dramatically in the absence of consumers (Fig. 3). Immediately after consumer deletion, foliose algae increased in abundance from 0 to nearly 80% cover. This bloom was dominated by ephemeral greens for 3-4 months (Fig. 3C). At this time (July 1977), cover of Jania spp. and, to a lesser extent, Gelidium pusilIum and the hydrozoan Abie~nu~a sp. increased, and cover of crustose algae decreased sharply (Fig. 3B, C). The decline of crustose algae (including the near elimination of Schizothrix calcicola and Hildenbrandia sp.) was due primarily to overgrowth by Japria spp., whose holdfasts form a dense turf. Jania was the lone turf dominant until November 1977, when successive blooms of Giffordia mitchelliae (peak in November) and the filamentous greens (peak in March 1978) were temporarily co-dominant foliose algae (Fig. 3C). Barnacles (primarily Balunus inexpectatus) appeared in late 1977, and co-dominated primary space with crustose algae (still dwindling in cover), Jania holdfasts, and, in late 1978, the rock oyster Chama echi~ata (Fig. 3B). Thereafter, Chama increased, while Jania holdfasts, crustose algae, and eventually, barnacles, decreased in abundance until January 1980, when the experiment was terminated. Note that despite a decrease in the primary space its holdfasts covered, BRUCE A. MENGE ETAL. 236 the upright portions of Junia remained abundant (compare Fig. 3B and C). This was due to the increase in cover of sessile animals, which decreased the availability of primary space, forcing Jade to shift to an epibiotic mode on the shells of barnacles and oysters. Thus, in the absence of all consumers, crustose algae are slowly eliminated by a sequence of increasingly effective competitors for space (Fig. 3). A. CRUSTS Ralfsia Schizo. COR. CR. Hilden. me, C. SECONDARY SPACE FII.. 1977 1978 1979 GRNS. = Gelidium = GifforUia = Jsnia = Abiet. = 1980 Fig. 3. Changes in abundance of qu~titatively dominant sessile animals and plants in the low zone after deletion of all consumers, February, 1977 to January, 1980: Schizo. = Schizothrix, COR. CR. = coralline crust, Hilden. = Hildenbrundiu, CRUSTS = summed cover of the four algal crusts shown in A., Jania HF = primary space covered by the holdfasts of J&a, DD SES = primary space covered by shells of dead barnacles and bivalves, FKL. GRNS. = filamentous greens, Abiet. = Abietinariu; other names are self-evident. ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 231 Comparison among treatments Although initial patterns of space occupancy among the 12 treatments were similar (Appendix), final patterns varied (Fig. 4). For example, algal crusts still covered > 70% of the surface in all treatments from which single groups of consumers had been deleted, and covered 2 50% of the surface in all but three of the rest of the treatments (Fig. 4). -++--++--+:::+-+LF + + + SFC+++++++--+=IN rem. 0 I I -++--- _+_+_- 122223334 Fig. 4. Comparison of final community structure (mean percent cover of prey species or groups) in each experimental treatment (experimental code and number of consumer groups removed is listed below each bar): codes in boxes are listed in the same order in which they are given on the histograms; abbreviations are as given in previous figures, except: PERENN. = perennial foliose algae, EPHEM. = ephemeral foliose algae, dd. sol. ses. = remains of dead solitary sessile animals (e.g., barnacle tests, oyster shells), dd. crusts = bleached and presumably dead encrusting coralline algae, other biv. = other bivalves, or. sponge = unidentified encrusting orange sponge, Schizo. = Schizorhrix culcicola. Chama, which covered l-2% of the surface in unmanipulated plots, covered 2 10% of the surface in four treatments, most strikingly in total deletions, where it covered z 50% of the surface. Solitary sessile animals such as barnacles and bivalves tended to cover more space in nearly all consumer deletion treatments, as did total abundance of species of ephemeral and perennial algae and arborescent colonial animals (Fig. 4). The magnitude of changes in abundance during the 3 yr of the experiment were equally variable, with even the controls demonstrating modest changes in cover of algal crust species (Fig. 5). Although the largest changes occurred in treatments from which either slow-moving herbivores, large fishes, or both were deleted, in general, the greater the number of consumer groups that were deleted, the greater the total change in abundance of sessile species (Fig. 5). BRUCE A. MENGE ETAL. 238 n 1 R INCREASE II= 20 A = Ablebinaicr P =PERENNIALS E =EPHEMERALS IO 08 qOTHER BIVALVES 0; ‘ORA’RA: SPONGE E C = Cbama echinata M 5 hfesospom H = Mdenbrandia CC = COR. CRUST S = Schizatkix R = Raifsia = DECREASE 0 H+-++--++--+p++-+-+-+-+-Sk; N Rem $2 0 = I I T ; _t + 1 T I- - 122223334 Fig. 5. Mean total change in percent cover of secondary and primary space-occupying sessile species or groups from initiation to conclusion of experiment: codes in box are listed in the same order in which they are given on the histograms; height of each bar represents the total sum of increases (shaded bars) and decreases (open bars) in abundance; each bar is broken down into the magnitude of change for each prey species or group contributing to the total; percent covers are untransformed for ease in interpretation; no statistics were performed on these data; secondary cover species are separated from primary cover species by a space; treatment code and number of consumer groups removed is listed below each bar. Separate effects of consumer groups What was the contribution of each of the groups of consumers in this assemblage to the overall effects shown above? The MANOVA analysis suggests that each consumer group had statistically and ecologically significant effects on community structure (Table III). However, as indicated by the rates and sequences of changes in prey abundance, these effects varied both seasonally and between years (Table III; Figs. 6, 7). Moreover, some consumer groups had effects on some prey which were independent of other groups (see below), while some had effects which depended on the presence or absence of other consumer groups (as suggested by the significant statistical interaction terms; see next section). Independent effects of removing slow-moving herbivores included: (1) over the short term (0 to 6 months), ephemeral and perennial foliose algae (the green Ulva-Enteromorpha, and the reds Gelidium and Jania) increased, and two of the crusts (the red Hildenbrandia sp. and the blue-green Schizothrix calcicola) decreased in abundance (Fig. 6A1, II, III, row 1). (2) Seasonal fluctuations were induced in covers of perennial foliose algae Gelidium and Jania), Ulva-Enteromorpha, and the algal crust Ralfsia sp. (Fig. 6B1, II, III, row 1). (3) Over the long term (6 months to 3 yr), cover of algal crusts as a prey group decreased and cover of solitary sessile animals as a prey group increased (Fig. 71). The only prey species effected over the long term, however, were Hildenbrandia ORGANIZATION A. SHORT-TERM B. SEASONAL I. OF A TROPICAL GROUPS: lo & 2O INTERTIDAL II. 239 COMMUNITY SPECIES: lo III. SPECIES: 2’ CHANGE CONSUMER GROUP = PRESENT = ABSENT N= I I = ABSENT* Fig. 6. Impacts of consumers on: A, short-term (top two rows of panels), and B, seasonal changes (bottom two rows of panels) of: I, prey groups (occupants of both primary = 1’) and secondary = 2” space); II, prey species occupying 1’ space; III, prey species occupying 2” space; left and right bars of each pair are values with consumers present and absent, respectively; significant effects of deletion of a consumer group independent of the composition of the remaining consumer assemblage are indicated by solid right bars; significant effects of a consumer group which are dependent on other consumer groups (i.e., interactions) are shown in Figs. 8 (short-term effects) and 10 (seasonal effects); - H = slow-moving herbivores removed; - P = slow-moving predators removed; - LF = large fishes excluded; - SFC = small fishes and crabs excluded; CRUSTS = algal crusts; SOL SES = solitary sessile invertebrates; ENC COL = encrusting colonial invertebrates; ARB COL = arborescent colonial animals; other code names are as given earlier or are self-evident. NS = not significant; * P < 0.05 or less. sp., which decreased less, and Ulva-Enteromorpha, which increased greatly in the absence of slow-moving herbivores (Fig. 711, III). The only independent direct effects of removing slow-moving predators were a short-term increase in the barnacle Balunus sp. (Fig. 6AI1, row 2) and a long-term increase in all solitary sessile animals (Fig. 71, row 2). Other independent effects of slow-moving predators (i.e., those on plants) were indirect, however, because the predaceous gastropods were strictly carnivorous. These will be considered in a later section. 240 BRUCE A. MENGE ETAL. Jania UlvalEnteromorpha Giffordia Gelidium Jania filamentous greens Abietinaria Species all (secondary) Balanus Chama crustose corallines Ra&a Schizothrix Hildenbrandia Groups all Encrusting Colonial Animals Perennials Species all (primary) *** ** ** ** *** loo*** ** ** ** ** ** ** ** 41*** *** *** 45.1*** ** NS 126 I,26 1,26 1,26 I ,26 1,22 1,22 1,22 6,21 1,26 I,22 I,22 1,22 1.21 I,27 6,17 I,24 6,22 I,24 1,24 I,24 1,24 ** f ** 5,20 22/I*** (none) (none) (none) (none) P x H x SFC P x SFC (none) P x LF P x H x SFC (none) (none) (none) P x H, P x LF, P x SFC, H x SFC, PxHxLF (none) (none) (none) (none) (none) P x LF, P x SFC, H x LF (none) (none) P x LF, P x SFC, H x LF, P x LF P x LF :Ytal) H P LF (Total) (Total) kotal) (Total) LF H (Total) H, LF, SFC P H P, H, SFC il Raw data were percent covers taken on 18 sample dates from February or March 1977 to January 1980. Statistics were performed on transformed data (arcsin transformation; Sokal & Rohlf, 1981). “(none)” under Interactions indicates that no interactions between consumer groups were significant for that particular prey category. “(Total)” under Main effects means that the significant effect indicated in the Total effect column was due to the entire assemblage of consumers, not to a single group of consumers. No consumer groups are listed in the Main effects column when they are included in an interaction because the interaction supercedes the main effect in such cases. F-values are listed only for the overall MANOVA for each analysis; significance levels and degrees of freedom (d.f.) are listed for all prey categories. *** P i 0.001; ** P < 0.01; * P < 0.05; NS = P > 0.05. C. Induction of seasonal fluctuation (R = seasonal index) Ulva/Enteromorpha G@ordia ~lamcntous greens Abietinaria Species all (secondary) BRUCE A. MENGE ETAL. 242 The independent effects of removing the remaining two consumer groups were fewer. Small fishes and crabs had a single short-term effect (ephemeral algae increased in their absence; Fig. 6A1, row 2), no effect on seasonal fluctuations, and two long-term effects (ephemerals decreased and the rock oyster Chama increased in their absence; Fig. 71, I. GROUPS: 0.5 i”& 2’ II. SPECIES1 III.SPECIES: 2’ 0.4 0.2 0 0 0 -0.5 0.4 -0.1 0.5 0.2 0 Y 0 0.2 ^r 0 ;: 0.5 0 -0.5 Fig. 7. Impacts of consumers on long-term changes of: I, prey groups (occupants of both 1’ and 2” space); II, prey species occupying 1” space; III, prey species occupying 2” space; significant long-term effects of a consumer group which are dependent on other consumer groups are shown in Fig. 9; see caption for Fig. 6 for further explanation of codes. II, row 4). Large fishes had no short-term effects, induced a single seasonal fluctuation (encrusting colonial animals, a relatively scarce prey group, fluctuated more in their absence), and had direct long-term effects on solitary sessile animals as a prey group, although Balanus sp. was the only species affected independently (Fig. 71, II, row 3). Non-additive effects of consumer groups Statistical interactions among consumer groups are indicated when a significant amount of the variation in the abundance of a prey is explained by combinations of two or more consumer groups (i.e., the effect of one group depends on whether or not one ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 243 or more groups is present; Table III). In such cases, the statistical interaction supersedes any significant effect of a single consumer group, unless, of course, the single group was not included in the interaction. To interpret the statistical interactions, celI means of each of the significant interactions (out of five possible two-way interactions and two possible three-way interactions) were plotted (Figs. 8-10). Statistical interactions are indicated whenever the lines joining cell means are not parallel. A brief scan of Figs. 8, 9, and 10 indicate that there were many statistical interactions among the consumer groups in this experiment. A. P x ;:jy=j H 6. PxHxSFC ;;Ei + H + - + - H Fig. 8. Short-term changes of sessile prey: analysis of significant statistical interactions between consumer groups. A, interaction between slow-moving predators (P) and herbivores (H) with respect to their short-term (i.e., shortly after the experiment began) effects on tilamentous green algae (FIL. GRNS.). B, interaction between slow-moving predators (P), herbivores (H), and small fishes and crabs (SFC) with respect to their effects on the brown tilamentous alga, Giffordiu mitchelliae. In this figure, and Figs. 9 and 10, dots indicate the average Y-intercept when the consumer group is present ( + ), triangles indicate the average Y-intercept when the consumer group is absent ( - ). Interpreting the biological significance of statistical interactions is difficult even in simple systems, so it is not surprising that interpretations are not always self-evident in this complex food web. Among the factors that need to be considered are the following. (1) There were four consumer groups, each dominated by one to four species (Table I). (2) Trophic links occur among the consumer groups, as well as between consumers and sessile organisms (Menge et al., 1986). (3) The sessile organisms interact (e.g., compete for space, provide substratum or microhabitats for each other, etc.; see above). (4) The richness of sessile organisms is high (Lubchenco et al., 1984; Menge et al., 1985), which by itself increases the chances for complexity in species interactions. For these reasons and to save space, we do not attempt a detailed examination of the ecological meaning of all significant statistical interactions. Instead, we discuss two examples for the sake of illustration, and then use these analyses to emphasize three results. (1) All consumer groups have major effects on the abundance of one or more prey, either independently or through interactions. (2) Some changes in prey abundance noted earlier (decline in crust cover due to increases in solitary invertebrates and foliose algae) were at least partly due to rather complex, and still unexplored, interactions among many mobile and sessile species. (3) Many significant statistical interactions are due to indirect effects of consumers on prey they do not directly eat (see below). 244 BRUCE A. MENGE ETAL. An example of an interaction of uncertain meaning is shown in Fig. 8A. Here, removal of slow-moving herbivores (for brevity, we here use H) led to a short-term decrease in the cover of tilamentous algae when slow-moving predators (P) were present (dots and solid line in Fig. 8A) but to an increase when P were absent (right triangle in Fig. 8A). Although P eat H, P shun algae, and their effect in this case must be either spurious or indirect. A spurious effect could be due to patchy blooms of filamentous greens A. l- - PREY ClUBTO : -+ LF GROUPS 1l OI. f A”. SLB f COL pi’-+ - 0.5-- LCHLY LF ---f--l-- - + ,L”l!” LF- +LF_ :+ O/ - : p -: : t f- +- +SFC +- +- fLF +- +- +- + +- +- - t +- - H Fig. 9. Long-term changes of sessile prey: analysis of significant statistical interactions between consumer groups. A, interaction between slow-moving predators (P), herbivores (H), and large fishes (LF) with respect to five prey groups; CRUSTS = crustose algae; SOL SES = solitary sessile invertebrates; ARB COL = arborescent colonial animals; EPHEM = ephemeral fohose algae; and PEREN = perennial foliose algae. B, interactions between P and H, P and LF, P and SFC, H and LF, and P, H, and SFC with respect to various prey species (except SOL SES in the first panel of B: see the caption to Fig. 6 for further explanation and codes of prey groups or species. ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 245 unrelated to experimental manipulations. However, the blooms we observed occurred uniformly along the shore and were usually barely perceptible, while dense growths occurred in consumer exclusion experiments. Alternatively, P may indirectly affect filamentous green algae through predation both on H, which graze filamentous algae, PxHxSFC B. PxSFC A. 9 JaniaSFc + + 5 11*. - J C. nr.- A>+ ..A- I +- +- H SFC lo- i :’ p - + ; HxSFC l e “1 cc I* en SFC D. PxHxLF Fig. 10. Seasonal changes of sessile prey: analysis of significant statistical interactions between consumer groups. A, interaction between slow-moving predators (P) and small fishes and crabs (SFC) with respect to seasonal changes in crustose coralline algae (COR CR). B, interaction between slow-moving predators, slow-moving herbivores (H), and small fishes and crabs with respect to seasonal changes in the foliose coralline alga,Junia. C, interaction between slow-moving herbivores and small fishes and crabs with respect to seasonal changes in Rufjikz (RA), Schimhrix (SC), ~ffde~bra~djff (HI), coralline crusts (CC), Balanus (BA), and Chuma (CH). D, interaction between slow-moving predators, herbivores, and large fishes (LF) with respect to the same prey categories as in C. See caption and text for Fig. 8 for further explanation. 246 BRUCE A. MENGE ETA. or on sessile solitary invertebrates (e.g., barnacles, bivalves). The latter might shelter small grazers, foster settlement of lilamentous greens (and other foliose algae), and filter algal spores from the water. We do not presently know enough of the natural history of this system to discern among these possibilities. A more readily interpreted example is shown in Fig. 9A. Here, the effect of P on cover of solitary sessile animals depends on the presence or absence of H and large fishes (LF). Removal of P (or H) alone produces no long-term change. Removals of LF only, H and P, H and LF, or P and LF all produce long-term increases in cover of solitary sessile animals of similar magnitude, while removal of all three consumer groups produces the largest effect. All these effects are most likely direct; P and LF consume metamorphosed juveniles and adults of most sessile animals, while H probably bulldoze recently settled juveniles. Thus, each consumer group has effects on sessile prey, some independent of, and some dependent on, the composition of the remainder of the consumer assemblage. Many interdependent effects are quite complicated. For example, in Fig. lOD, seasonal changes in abundance of six sessile species depend on interactions between P, H, and LF. Further, as considered below, many statistically significant effects of consumers on prey are indirect; i.e., they are due not to trophic interactions, but to interactions between a prey and a non-prey organism (e.g., Bender et al., 1984; Holt, 1984). Direct vs. indirect eflects The above analyses, coupled with knowledge of the links in the food web of this system (Menge et al., 1986), permitted us to distinguish between direct and indirect interactions (Table IV}. Most direct effects were obvious and were noted above. Indirect effects, although clearly indicated by significance in the statistical analyses, are more difficult to characterize without detailed field investigation. Thus, some of the interpretations offered in Table IV are tentative, although all are based on our field observations and most have been documented in other, usually simpler systems. Our interpretations fall into four categories. (1) Indirect beneficial effects of consumers on one species or group of sessile organisms by removing competitively superior sessile organisms. For example, H, P, and LF benefit the bier-resist~t algal crusts by removing solitary sessile animals or foliose algae or both gable IV). (2) Indirect negative effects on foliose algae by removing sessile prey, whose irregularlyshaped shells serve as partial havens from consumers which graze most effectively on smoother surfaces. For example, LF consume barnacles and oysters, whose shells evidently provided somewhat better conditions for perennial foliose algae than did smoother rock surfaces. The probable advantage of settling on irregular surfaces like shells of invertebrates is that limpets and chitons, which crop foliose algae completely from smoother substratum, are less eficient on more rugose surfaces. In contrast, the beaks or jaws of fishes and the claws of crabs are less effective than are the radulae of the molluscs and neither fishes nor crabs can graze foliose algae closely, regardless of ORGANIZATION substratum rugosity. OF A TROPICAL (This interpretation algal turfs only on smooth surfaces accessible to fishes and crabs.). is suggested lacking TABLE Direct and indirect INTERTIDAL by the occurrence shelter for grazing effects of consumer H groups community I Jnmanipulated Ralfsia sp. Schizothrix calcicola encrusting corallines Hildenbrandia sp. Manipulated community Chama echinata Balanus inexpectatus Abietinaria sp. Jania spp. Gelidium pusillum Giffordia mitchelliae tilamentous greens Ulva-Enteromorpha molluscs _ f Dir. but still group LF SFC + + + + I I 1; I _-,+ + -_ + _ -1 _ _ + _ Ind. of on sessile organisms. P Dir. of patches IV Consumer Dominant space occupiers 247 COMMUNITY Ind. Dir. Ind. Dir. Ind. + + + f : _ -1 _ _ _ _ _ _ _ + + +_ + = positive effect; - = negative effect; . = no effect on abundance; brackets indicate that the consumer effect is exerted on a prey group rather than on individual species, e.g., consumers affect algal crusts positively, not individual crust species; H = slow-moving herbivores; P = slow-moving predators; LF = large fishes; SFC = small fishes and crabs; Dir. = the effect of the consumer group is direct; Ind. = the effect of the consumer group is indirect. (3) By eliminating foliose algae, H inhibit recruitment of Chama, which seems to recruit best to algal turfs (possibly because of the cooler, moister microhabitat found among the holdfasts of the turf; authors’ personal observations). In contrast, grazing of foliose algae by H may enhance recruitment of Balanus, since these barnacles virtually never recruited to turfs (and our observations in New England indicate that Semibalanus balanoides is inhibited by thick mats of the filamentous greens, Ulothrix and Urospora). However, this latter effect is likely to be balanced by the direct bulldozing/grazing of barnacle recruits by slow-moving grazers. Finally, (4) H, which are directly preyed upon by P, may thus indirectly enhance abundance of larger barnacles and oysters by diverting some of the predation pressure exerted by P on these prey. This possibility is suggested by the PxH and PxHxSFC interactions for solitary sessile prey, Balanus and Chama in Fig. 9B, where Balanus, for instance, declined in the absence of H and presence of P but increased in the absence of both consumers. 248 BRUCE A. MENGE ITAL. We emphasize that our interpretations have not been subjected to field testing and must remain hypothetical at present. However, the main point, that numerous indirect interactions occur in this community, is robust. We further note that, with the exception of the indirect effect of consumers in maintaining the dominance of the algal crusts (by consuming foliose algae and solitary sessile invertebrates), most effects occurred under experimental conditions. Thus, interaction complexity among sessile organisms in this system may not normally be as high since most sessile species are rare and interspecific encounters among these organisms are infrequent. High interaction complexity may thus be a transient phenomenon, occurring under unusual conditions of temporarily reduced consumer pressure. An implication of the statistical interactions which revealed these indirect effects is that interactions among consumers can have important effects on prey community structure. One important theoretical question concerning the structure of consumer assemblages is the extent to which generalized consumers have equivalent effects on their prey. Previously, we have suggested (Menge & Lubchenco, 1981) that there is no true “keystone” predator (sensu Paine, 1969) at Taboguilla. That is, no consumer has effects which are overwhelmingly greater than those of any other consumer. We consider this possibility below. Impact of consumer groups: individual vs. combined effects To evaluate how interactions between consumer groups influence their individual vs. combined impact on major prey groups, we performed the following analysis. To determine the effect of a consumer group (slow-moving herbivores, for example) on abundance of sessile prey, we subtracted the mean cover of each prey group in controls from that in slow-moving herbivore deletions on each sample date. This was done for each single-consumer group deletion (with the exception of small fishes and crabs) and for the total consumer deletion treatments for each of five prey groups (crustose algae, solitary sessile animals, holdfasts of foliose algae, upright portions of foliose algae, and arborescent colonial animals). The effect of small fishes and crabs was estimated as the average prey cover in large fish plus small fish and crab deletions minus that in large fish deletions. Statistical analysis of the resulting trends over the 1977-80 time period was done using analysis of covariance. This analysis permitted us to consider three questions. First, do the consumers remaining after deletion of a given consumer group compensate for the absence of the deleted group? Second, is the sum of the effect of the consumers in the single-consumer group deletions equivalent to their effect in the total consumer deletions? Third, is there a keystone consumer (or consumer group), and if so, which is it? The evidence suggests that the consumers remaining after the deletion of a singleconsumer group can partially compensate for the absence of that group. Since cover of sessile organisms is inversely related to cover of crustose algae (Figs. 2-3, we used cover of crustose algae after consumer deletion as an inverse index of increase in total ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 249 cover of sessile organisms (Fig. 11). The results indicate that deletions of single consumer groups (i.e., H, P, LF, or SFC) eventually lead to slight decreases in cover of crustose algae, while deletion of the entire consumer assemblage (TOTAL deletion) CRUSTOSE -*cm! 1 , , , , , s 8 7 . 1 1977 I I 1978 ALGAE , , 1 7 , 10 , 11 , I , I 1979 , 7 9 1980 Fig. 11. Effects of consumers on space availability as estimated by the difference between abundance of crustose algae in consumer-deletion treatments and controls: TOTAL is the total effect of consumers as estimated by the difference between y0 cover of crusts in total consumer deletions and controls (i.e., TOTALS - CO); SUMMED is the total effect of consumers as estimated by the sum of the effects of each of the four consumer groups as separately estimated by the differences between y0 cover of crusts in deletions of single consumer groups and controls (i.e., [H - CO] + [P - CO] + [LF - CO] + [SFC LF] = SUMMED effect); - H, - P, - LF, and - SFC indicate the separate effects of the single consumer groups which when added yield the SUMMED line in the figure; the negative initial values (i.e., for 2/77) indicate that the average abundance of crusts in treatments was initially slightly less than that in controls; we regard this as within the range of normal variation. leads to near-elimination of crustose algae (Fig. 11). If the consumers did not compensate, then the sum of the separate effects (i.e., SUMMED in Fig. 11) of each group would equal the effect of the TOTAL deletion, which is not the case (Fig. 11, Table V). Since there were three major groups of prey which increased in response to consumer deletions (solitary sessile animals, both upright and holdfast portions of foliose algae, and arborescent colonial animals), the answer to questions 2 (are the effects of the consumer groups additive or not) and 3 (is there a keystone consumer group) are best approached by considering each prey group separately. As indicated in Fig. 12, the answer to these questions depends on which group is considered. The separate effects of each consumer group on solitary sessile animals was small (Fig. 12A, dashed lines), and the sum of these separate effects was significantly less than their effect in TOTAL deletion plots (Table V). In contrast, the sum of the separate effects of consumers on foliose algae and upright colonial animals did not always differ from the effects in total exclusions (Table V). Further, the effect of a single group on these prey (H on foliose algae, SFC on colonial BRUCE A. MENGE ETAL. 250 animals) is similar to the effect in total exclusions, suggesting a keystone effect (Fig. 12B, C, D). Specifically, the effect of H on the holdfasts of the foliose algae was not si~i~c~tly different from either the sum of the separate effects (ANCOVA; P = 0.32), TABLE V Analysis of covariance comparing regressions on effects of the four consumer groups estimated either by summing the effects of removing single groups in four separate treatments or by the effect observed in removals of all groups simultaneously in a single treatment. Ho Sessile group Algal crusts Solitary sessile animals Algal holdfasts Foliose algae Arborescent colonial animals Treatment Sum of single group effects Total effect Sum of single group effects Total effect Sum of single group effects Total effect Sum of single group effects Total effect Sum of single group effects Total effect Y-intercept Slope _ 24.9 - 0.205 - 16.8 Slopes equal Slopes =0 Regressions equal - 1.997 0.459 *** *** *** - 13.3 19.6 2.13 - 0.437 *** *** *** 31.6 60. I - 0.768 0.512 NS * NS 66.1 -0.622 0.381 * NS * 0.497 NS ** * - 1.90 10.5 2.68 Degrees of freedom (slopes equal) = 1, 22; d.f. (slopes = 0, regressions equal) = 1,23. *** P < 0.001; **P< 0.01; *P< 0.05; NS = P> 0.05. or the effect in total exclusions (ANCOVA; P = 0.07). Similarly, the analysis suggests that small fishes and crabs have a keystone effect on arborescent colonial animals (Fig. 12D, Table V; ANCOVA; SFC regression is not different from that for either the sum of the separate effects [P = 0.068] or the total effect [P = 0.8431). The keystone effect of slow-moving herbivores on the thalli of foliose algae was similar, although it appears a bit more complex in Fig. 12C. The long-term effect of H on the thalli of the foliose algae was not different from the effect in total exclusions (ANCOVA; P = O.OSS), ~thou~ a difference (P = 0.011) between the slopes of the regressions for the H and total effects suggests an initial. difference in these effects (Fig. 12C). The effect of H on thalli was parallel to (ANCOVA; slopes are equal, P = 0.943), but less than the sum of the separate effects (ANCOVA; P < 0.0004), suggesting that other consumers (mostly SFC) also had an effect on the abundance of foliose thalli. Unexpectedly, the total effect was also less than the sum of the separate effects (Fig. 12C, Table V; regressions were not equal, P = 0.02). This implies that the effect ORGANIZATION 251 OF A TROPICAL INTERTIDAL COMMUNITY of a given consumer group was less when excluded with all other consumers than when removed separately, or, in other words, that the effect of some consumers was enhanced by the presence of others. However, this apparent “super-keystone” effect seems best explained as an artifact of an indirect effect. Since solitary sessile animals covered more A.SOL 50 SES B. ALGAL ANIMALS HOLDFASTS I 0 0 3 5 7 9 13 4 C. FOLIOSE 7102 4 7 1 ALGAE 3 5 7 9 D.ARB 1‘3 4 CO1 7102 4 7 ANIMALS 10 5 4---T , , 3 7 9 5 TOTAL= * I 13 I 74 I__... 7102’4 SUMMED= 7 l 1 -H=m 3 : 5 7 -p=+ 9 13 -LF=& 4 7102 : 4 -SF& 7 A Fig. 12. Effects ofconsumers on each of four major prey groups: A = solitary sessile animals; B = holdfasts of foliose algae; C = secondary cover of foliose algae; D = secondary cover of arborescent colonial animals; symbols coded as in Fig. 11; the third lines in B and C ( - H line) and D ( - SFC line) are connected by solid lines because ANCOVA analysis indicates that they are statistically indistinguishable from the TOTAL and/or SUMMED LINES; see text for details of the analysis. space in total exclusions than in single group exclusions (e.g., Fig. 12A), less space was available for the turfy foliose algae in total exclusions. That is, the lower cover of foliose algae in total exclusions is probably due to competition for space with solitary sessile invertebrates, and not to some dependence of some consumers on the presence of others. Thus, this analysis suggests that the roles played by the consumer groups depends, in part, on the prey group considered. Since the impact of consumers on solitary sessile 252 BRUCE A, MENGE ETAL. prey is revealed only when all are s~ult~~usly removed (Fig. 12A), their effects on these prey are ~terdependent but not additive. In contrast, the elects of consumers on foliose algae (mostly the perennials Gelidium and Janiu) and arborescent colonial animals (mostly the hydrozoan Abielinatiu sp., but also bryozoans) are attributable mostly to slow-moving herbivores and small fishes and crabs, respectively (Fig. 12B-D). DISCUSSION COMMUNITY DYNAMICS AT TABOGUILLA Normally, space in this low zone habitat is dominated by four algal crusts (Lubchenco et al., 1984; Fig. 2). This pattern is both temporally and spatially persistent: quantitative transects taken at 11 sites in the Bay of Panama and the Gulf of Chiriqui were dominated by algal crusts (Menge & Lubchenco, 198 1; Lubchenco et a/., 1984; Menge, unpubl. data), and obse~ations and transects at Tanaka from 1973 to 1983 indicate that these patterns vary little over time (Lubchenco eta& 1984; B. Menge and J. Lubchenco, pers. obs.). What are the causes of this apparent stability? Physically, this habitat is harsh, especially during the dry season when waters are often calm and one of the two daily low tides occurs in midday (Garrity, 1984; Lubchenco et al., 1984). In the high and mid zones, evidence indicates that heat and desiccation stresses reduce the activity and cause mo~~ity of mobile ~ve~ebrates (Garrity, 1984), and may suppress abundances of sessile organisms (Lubchenco et al., 1984; B. Menge 62J, Lubchenco, unpubl. data). With one exception, our results suggest that physical stress has a small effect in the low zone. This exception is a slight seasonal fluctuation in the upper limit of crustose coralline algae (Lubchenco et al., 1984), with a lower upper limit occurring during the dry season. We have not investigated longer-term, sublethal effects of heat or desiccation stress on community structure. Consumers and interspecific competi~on are two major dete~n~ts of community structure in the low zone at Taboguilla. Prey groups most strongly affected were foliose algae, arborescent colonial invertebrates, solitary sessile invertebrates, and, indirectly, crustose algae. Each of these groups is discussed in turn. First, the total abundance of foliose algae was suppressed by both grazing invertebrates and browsing fishes and crabs (see summary in Table IV). Thus, reduction in the abundance of grazing molluscs produced a rapid increase in foliose algae, with ephemerals, and then perennials exhibiting large increases in abundance (e.g., Fig. 3). Although slow-moving herbivores evidently had the largest short-term effect, large and small fishes and crabs also reduced the abundance of foliose algae, especially over the long term. While grazing molluscs also had longer-term effects, these were dependent on interactions with other consumer groups (e.g., Figs. 6-10). Important qualitative differences may exist in the ways in which these groups ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 253 influenced the abundance of foliose algae. On a local spatial scale, grazing molluscs appeared to crop foliose algae closer to the surface than did fishes and crabs. However, at larger spatial scales, molluscan grazers appeared relatively inefficient in controlling macroalgal abundance, presumably due to their sluggish nature and limited perception of their surrounding environment, and local patches of algae may occasionally be found which have evidently escaped the molluscan grazers. In comparison, fishes and crabs have a small short-term effect on fohose algae, probably because they cannot crop it close to the surface; hence, short algal turfs may be essentially unavailable to these consumers. However, fishes and crabs are large, fast-moving, have wide perceptual fields, and powerful feeding apparatuses, all of which presumably combine to allow rapid detection of, movement to, and consumption of concentrations of food. Since the effects of fishes and crabs are greatest on prey taking longer to become large or abundant (several months for foliose algae, a year or more for sessile invertebrates; Figs. 7,9), the effects of grazing molluscs and of fishes and crabs on foliose ma~ro~gae at Taboguilla appear complementary. Indeed, were it not for substratum heterogeneity, which provides temporary microrefuges from these consumers (Menge & Lubchenco, 1981; Menge et al., 1983), foliose algae would probably virtually disappear from these shores. Second, the abundance of arborescent, colonial invertebrates, primarily the hydrozoan Abietinaria sp., was controlled by large fishes. This hydrozoan is normally ubiquitous, but scarce, on Taboguilla. Deletion of large fishes led to significant increases in its abundance (Fig. 9B, Table IV), although it never truly dominated any pIot. This species achieved its greatest prominence as an epibiont on shells of sessile invertebrates (e.g., Fig. 3). It did not appear to be an effective competitor for space. Finally, the abundance of solitary sessile invertebrates was strongly affected by all consumer groups, usually in combinations of two or more groups. Recruitment of the dominant sessile invertebrates, Balunus and Chama, was evidently inhibited by slowmoving grazers, and possibly, small fishes and crabs, and larger individuals were preyed upon by large fishes and predaceous snails. Like the foliose algae, these animal prey are normally scarce on the shore, and Chama, but, interestingly, not Balanus, are usually found primarily in holes and crevices in the rock. Expl~ations for the occurrence of palace on surfaces exposed to fishes and crabs are (1) that they grow exceptionally fast, and/or (2) that fast-moving consumers do not consider barnacles to be attractive prey. Regarding the first hypothesis, we have observed growth of recently settled individuals (i.e., within the past week) to “adult” size within a month or less. Thus, B&anus may simply escape fishes over the short-term by rapid growth, but eventually be discovered and consumed. Regarding the second hypothesis, adult barnacles seem to live only a matter of weeks before they are preyed upon by predaceous snails, primarily Thaiv melones. However, their empty shells may persist on the shore for months, thus, dead barnacles typically outnumber living ones. Hence, unless an empty barnacle shell displays outward signs of attractiveness as food (e.g., it is used as a shelter by a small crab or other potential prey, or is festooned with 254 BRUCE A. MENGE ETAL. foliose algae), crabs and fishes may ignore all barnacles, because in their experience, most are empty and not worth the effort. As we have discussed elsewhere (Menge & Lubchenco, 1981; Menge et al., 1985), bivalves (Chama, and possibly the mussel Modiolus cupax and the giant oyster Ostrea iridescens) are likely to eventually replace the crustose algal monopoly with a bivalve monopoly. This is, however, a result observed in experiments, and it is pertinent to ask whether or not bivalve-dominated shores wouid ever occur naturally in this region. We have observed sites in the Bay of Panama (ChamC Island) and the Gulf of Chiriqui (Uva Island) where abundances of C~~~~ are higher than that seen anywhere on Ta~guilla or neighboring islands. In all instances, high cover of Chama is correlated with either alterations in the consumer assemblage, or factors that might alter the effectiveness of consumers, or both. Thus, oysters are often more abundant on shores with severe water turbulence; that is, sites where all consumers are likely to be less efficient foragers (e.g., Menge, 1978a, b, 1983). Also, oysters are generally more abundant at sites with persistent high turbidity, which may influence fish presence and foraging ability. Despite these examples, however, we never observed a non-expedite rocky surface in the > 80% covered) by bivalves. Combined Gulf of Panama which was truly dominated ( = with the intensive maintenance required by our consumer deletions and the diversity of the consumers in this community, our observations suggest that such sites will generally be rare in this region. Thus, we suggest that the spatial and temporal uniformity of dominance of low intertidal rock surfaces by algal crusts at Taboguilla is due to uniformly intense predation by a complex assemblage of consumers. Each consumer group has a distinct effect on sessile prey: molluscan grazers virtually eliminate foliose macroalgae and alter the relative abundances of algal crusts; all consumers interact to keep solitary sessile invertebrates scarce; and fishes and crabs browse both foliose macroalgae temporarily escaping the control of molluscan grazers, and arborescent colonial invertebrates, COMPLEX FOOD WEBS AND COMMUNITY THEORY The results presented above bear on several general issues, including the concept of “keystone” species (Paine, 1969), diffuse competition (MacArthur, 1972), indirect vs. direct effects (e.g., Bender et al. 1984), and whether or not observed spatial variation in community structure indicates differences in underlying causes. Our experiments suggest that a keystone species is lacking in this community. Although slow-moving herbivores have by far the largest effect on foliose algae, and small fishes and crabs have the greatest effect on abundance of arborescent colonial animals, neither of these prey groups includes a strong competitor for space. The dominant competitor for space is evidently Chama echinata, and our analyses suggest that this species is collectively controlled by all consumer groups. Although no single-group removal had a dramatic effect on the dominant space-occupants, one might ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 255 argue that large fishes, especially Bodiunus and Diodon, play a keystone role by virtue of their ability to control prey that escape from the intermediate trophic level consumers. Our experiments and observations are not consistent with this interpretation, primarily because prey do not appear to escape from the abundant, trophically intermediate predaceous gastropods. Thais melones, for example, can successfully attack and consume the largest sessile invertebrate prey we have seen on the shore (Menge et al., 1986). Also, in the absence of large fishes and presence of other consumers, sessile invertebrates did not become abundant even after more than 3 yr. Hence, at present, the evidence suggests that a keystone species is lacking in this community. The absence of a keystone species may be related to a number of characteristics of the community, including the high species richness at upper trophic levels and the fact that the intertidal zone represents only part of the foraging range of the top predators. Virtually all fishes at Taboguilla reside in the subtidal region, and although many forage intertidally at high tide, feeding time in that region is limited by both the tidal cycle and wave action during high tide. Furthermore, since few of the intertidal prey species occur in the subtidal region, the fishes forage on different prey in the intertidal and subtidal zones (authors’ unpublished data). Hence, fishes may not form the strong predator-prey relationships with potentially dominant intertidal prey that are known for communities with keystone species (e.g., Paine, 1974, 1980; Menge, 1982a). The existence of diffuse competition (sensu MacArthur, 1972) among many of the predaceous species and among many of the herbivorous species in the community is suggested by both the exceptionally broad diets of most species and the compensatory changes expressed when single consumer species were removed. This idea warrants testing in this community. Recent theoretical work has emphasized the potential importance of indirect (vs. direct) species interactions (Holt, 1977, 1984; Abrams, 1984; Bender et al., 1984). As summarized above (Table IV), statistically significant indirect effects are common in this complex community. For example, the occasional formation of patches of foliose algal turf on smooth rock surfaces seems dependent on localized reduction of limpet densities by fishes. Further, the dominance of space by algal crusts depends on the normal elimination of foliose algae by grazers. Since indirect effects are also important in certain simple communities (e.g., Lubchenco & Menge, 1978; Brown et al., 1986; Dungan, unpubl. data), it seems unlikely that indirect effects are uniquely associated with tropical or complex communities. Although we can predict that indirect interactions will be prominent features of the dynamics of all communities, we know too little about the details of interactions between most types of organisms to predict with much accuracy where and when they will occur, let alone their importance in community regulation. The existence of indirect interactions serves to underline the complexities of the dynamics of communities, and emphasizes the necessity of knowing natural history. Despite much progress, we still know far too little about even reasonably well-studied systems. The structural differences between this and temperate communities (i.e., dominance BRUCE A. MENGE ETAL. 256 by encrusting algae vs. dominance by sessile invertebrates and large erect algae) are not due to uniquely different causal agents, but to variation in the intensities of the same set of causal agents. The structure of this community depends on the same processes stressed in studies of other communities, but the rates and intensities of these processes appear different at Taboguilla. Specifically, predation appears more intense, competition for space and physical disturbance seem much less intense, and recruitment rates are much lower than in most cool temperate rocky intertidal communities. As we argue elsewhere (Menge & Lubchenco, 198 l), some of these differences probably derive from such physical features as (e.g.) relatively high and little seasonal variation in water temperatures. Presumably, this permits consistently high metabolic rates, and yeararound activity of consumers (see Discussion in Gaines & Lubchenco, 1982). Combined with other factors, near-continuous consumption of prey would seem to be a key factor in producing high levels of predation. Despite the many environmental and biological differences between Taboguilla and certain temperate communities, experimental deletions of consumers usually lead to dominance by sessile invertebrates, particularly bivalves (e.g., Paine, 1966, 1971, 1974, 1984; Dayton, 1971; Menge, 1976; Lubchenco & Menge, 1978; Peterson, 1979; Menge & Lubchenco, 1981; Fairweather et al., 1984; this paper). This not only supports our argument that such communities are organized by different intensities of similar forces, it is also consistent with models of community organization which stress such commonality in the dynamic processes which underlie divergent patterns of community structure (e.g., Paine, 1966; Connell, 1975; Menge & Sutherland, 1976), counter arguments notwithstanding (Underwood & Denley, 1984). Although these models are not without flaws, they have also enjoyed modest success in predicting community organization in a variety of other habitats (e.g., Morin, 1983; McNaughton, 1983; Peckarsky, 1983). As empirical knowledge increases, revision of these partially successful theories should lead to continued progress in the development of a conceptual framework in community ecology. TRADE-OFFS IN ALTERNATIVE EXPERIMENTAL APPROACHES TO COMMUNITY REGULATION There are a number of different experimental approaches that could be used to understand the dynamics of communities. They include: (1) a simultaneous investigation of the role of all major species in the system, (2) sequential studies of different ecological processes (competition, predation, etc.) focussed at the species level, (3) a simultaneous evaluation of different. ecological processes focussed at the level of functional groups, or (4) some combination of these three approaches. Each approach has advantages and disadvantages. Although approach (1) has been successfully employed in some communities (Lubchenco & Menge, 1978) and provides the most comprehensive analysis of community dynamics, it is impossible to use in complex communities. There are simply too many species, for example, in the low zone at Taboguilla for separate, simultaneous manipulations of even only the major ones. ORGANIZATIONOFATROPICALINTERTIDALCOMMUNITY 251 Approach (2) has long been the dominant method of choice in co~unity ecology, and it offers the advantage of detail and precision in ident~~g the ecological processes at play in the community. However, as recently noted by numerous authors (e.g., Connell, 1983; Schoener, 1983; Bender et al., 1984; Lubchenco, 1986), this approach does not reveal the relative impact of each process and is subject to problems in interpretation due to the necessity of having to ignore some species. Although sequential or separate investigations may show that physical disturbance, competition, and grazing all have significant effects in a community, determination of the relative importance of each is impossible unless all factors were studied in the same experiment. Moreover, if some species are ignored, as they must be using this approach, especially in complex communities, then experiments may produce misleading results (Bender et al., 1984). For example, a species may be regarded as having no effect, when in fact its effect was masked by an unmanipulated species. In approach (3), the complexity demanded by simultaneous investigation of different processes is offset by the simplification provided by a focus on functional groups. This was the appro~mate method used in the experiment reported in the present paper, and it offers the advantages of revealing the overall organization of the community, suggesting the relative importance of different processes and reducing problems of interpretation caused by ignoring species. However, it lacks precision, loses detail, and introduces problems of interpretation caused by lumping. For example, in the present paper, consumer species were lumped together (reducing precision in manipulations and blurring detail in the responses to manipulations). Nonetheless, this type of experiment produces a broad understanding of the org~ization of a community, and has the potential to provide sufficient detail to identify fruitful avenues of further research. Approach (4), a combination of (2) and (3), offers probably the best solution for a complex community. We found that using approach (3) first, then approach (2) provided a very reasonable way to attack the dynamics of a complex system. The initial studies described above and in Menge & Lubchenco (1981), Menge et aZ. (1985, 1986) and Lubchenco et al. (1984) all used approach (3), using manipulations of functional groups to elucidate the overall dynamics of the system. These studies laid the ~oundwork for and have been supplemented by more in-depth investigations of individual species, i.e., approach (2) (Garrity & Levings, 198 1, 1983 ; Levings & Garrity, 1983 ; Menge et al., 1983, in prep.; Garrity, 1984; Gaines, 1983). Together these studies have shown a feasible way of approaching the dynamics of a complex community. ACKNOWLEDGEMENTS We thank S. Gaines, C. Marsh, W. Rice, J. Sutherland, T. Turner, A. Brown, T. Farrell, A. Olson, L. West, R. Paine, and an anonymous reviewer. Many others assisted in this study, most notably R. Emlet, S. Garrity, P. Lubchenco, J. Lucas, A. Matson, L. Olds, S. Sargent, S. Strauss, J. Valenter, and P. Williams. The Republic of Panama BRUCE 258 A. MENGE ETAL. and the Smithsonian Tropical Research Institute provided field access, laboratory space, and logistical support. I. Rubinoff, V. Vergel, J. Budria, E. Lombardo, R. Ely, and L. van Valkenhoef of STRI were particularly helpful. The research was supported by NSF grants OCE76-22251, OCE78-17899, and OCE80-19020 to B. Menge and J. Lubchenco, and writing was supported by NSF grant OCE-8415609 to B. Menge. APPENDIX EVALUATION OF EXPERIMENTAL DESIGN, EXECUTION, AND ANALYSIS OF RESULTS “Proper statistical methods should be used, but the biologically defined objective should dominate use the statistics, rather than the reverse” (Green, 1979, p. 6). Below, we describe the design, execution, and analysis of this study, and evaluate problems therein Hurlbert, 1984). In particular, we consider the independence of the experimental plots, since assumption of independence of errors is the only one in most statistical methods for which violation is serious and impossible to cure after the data have been collected” (Green, 1979). EXPERIMENTAL and (e.g., “the both DESIGN Premanipulation similarity among sites Some physical and biological variation among sites (and plots) is inevitable in experimental field studies; the best we can manage is to make initial conditions in sites or plots as similar as possible (see, e.g., Connell, 1974). Since our experiences in temperate rocky intertidal habitats accustomed us to observing large between-site differences in community structure, we were initially struck by the apparent homogeneity of the community on the shores of Taboguilla (and neighboring islands). More detailed sampling supported these impressions (Fig. A-l). Physically, all sites are: southerly in aspect, intermediate in wave exposure, and probably virtually identical in desiccation stress, water and air temperature, insolation, and water turbidity (Lubchenco et al., 1984). All have a heterogeneous basaltic substratum, but Sites 4 and 6 are somewhat less heterogeneous than Sites I and 2 (Fig. A-l). Tidal levels of the plots were similar, but varied somewhat (Table A-l) due to our use of “biological indicators”, rather than absolute tidal height when selecting plot locations. That is, because the vertical locations of the biota vary with local variation in wave exposure, aspect, etc. (Lewis, 1964), local patterns of species composition and abundance are better indicators of a particular “realized” tidal height than is absolute height with respect to a fixed reference point (e.g., mean low water). When the entire range of physical conditions in rocky intertidal habitats in this region is considered, most differences between sites are ecologically insignificant. Biologically, the sites also differed little prior to the experiment (Fig. A-IB,C). Premanipulation percent cover ofthe eight most abundant occupants ofprimary space did not differ among sites (MANOVA, F = I .80; P = 0.13; 8, 2 I d.f.). However, premanipulation cover of the eight most abundant occupants of secondary space did differ (MANOVA, F = 2.66; P = 0.03; 8,2l d.f.), as indicated by a significant slow-moving predator ( = P) x slow-movingherbivore ( = H)statistical interaction. Graphic analysis indicates that this effect is due largely to relatively high abundance of several foliose species on Site 4 (average percent cover ranges from 0. I to 8.4%) compared to the normal 0 to I % ; Fig. A-2). However, no single prey species differs significantly in abundance among sites (multiple comparisons tests = one-way ANOVA with critical value adjusted with theBonferronimethod;P > 0.05inallcases).Therefore,thesignificantMANOVAmustbeduetothecollective effect ofhigher abundances offoliose algaeon Site 4. The premanipulation difference attributable to P-removal sites (as indicated by the P x H interaction) is evidently due to slightly higher abundance of the hydrozoan Abietinaria sp. at site 1 than at the other sites (Fig. A-2). Since ecological systems commonly exhibit considerable variation, and differences in percent cover < 10% are difficult to detect without extensive replication, we consider the premanipulation difference in foliose algae among sites to be ecologically trivial. However, correlated with this slightly higher abundance ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 259 of algae is the absence from site 4 of the herbivorous crab Grupsus grupsus (Fig. A-l). Although this crab is one of the most abundant members of the small fish and crab ( = SFC) consumer group, and its absence from Site 4 could have led to a partial escape by the algae from consumers, other members of this consumer group (e.g., Ophioblennius, juvenile fishes, the crab Ertphides hispida) were present at this site. We conclude that the study sites were initially similar in physical and biological characteristics, and that the exceptions were unimportant relative to the questions being asked in this study. A. PHYSICAL ENVIRONMENT EXP. 8. SESSILE 5 b 40 BAR Biv ORGANISMS COL. SES. m. COR. CR. FOL. ALG. T C. MOBILE ORGANISMS Fig. A-i. Physical environmental characteristics (A) and community structure (B and C) prior to experiment initiation at Taboguilla (summarized from Lubchenco et nl., 1984): bars are 95% confidence ranges at each site; mean and 95 % confidence intervals are given For temperature, windspeed, and visibility; data were not transformed before plotting to make interpretation easier; abscissa labels: 1,2,4, and 6 refer to experimental site numbers; D and W refer to dry and wet seasons. A, MSD = mean substratum depth, an index of substratum heterogeneity; EXP. TO FMC = percent of the substratum exposed to fast-moving consumers (large fishes and large crabs), ASPECT = arrows indicate direction each site faces; TEMP. = temperature (open symbols = air, and solid symbols = surface sea-water temperatures); WIND = windspeed (open symbols); VISIB. = lateral visibility in the water column (solid symbols). B, BAR. = barnacles; BIV. = bivalves; COL. SES. = encrusting colonial animals (all sessile animals); Hild = Hi~denbrandia spp., COR. CR. = encrusting coralline algae; Rulf: = Ra(fsiu sp., S.C. = Schizothrix calcicola (all encrusting algae); FOL. ALG. = foliose algae. C, SMC = slow-moving consumers; F.v. = ~~s~re~~a virescens (keyhole limpet); LIMP. = tota limpets; CHIT. = chitons; E.v. = Ech~ometra vanbrunti (sea urchin); P.t. = Pachygrapsus tramversus (crab); PRED. GAST. = total predatory gastropods; FMC = fast-moving consumers; G.g. = Grupsus grapsus (crab); E.a. = Eupomacentrus acapulcoensis (damself&h); K.e. = Kyphosus eiegans (chub); 3.d. = Bodianus diplotaenia (wrasse). of treatment plots) +H-P-LF+SFC Predator removal and large fish exclusion tH+P-I.F-SFC tH+P-LFtSFC +H+PtLFtSFC Code +H-PtLFtSFC (roof) A-I (+6.3) (+ 6.5) (t6.1) (+ 5.3) t 0.8 (t 2.6) +0.6 (t2.1) + 0.7 ( + 2.4) + 0.7 ( t 2.2) + 0.7 ( t 2.2) + 0.7 ( + 2.4) 1 2 tl.6 (+5.2) + 1.8 (t 5.8) t 1.8 (t 5.8) ND +I.6 (+5.2) t 1.6 (t 5.3) t 1.8 (t 5.8) ND t 1.9 + 2.0 +I.9 + 1.6 m (ft) Tide height 1 2 3 4 2 3 4 I 1 2 3 4 1 2 3 4 Plot NO. 5.9 5.9 3.9 3.5 4.8 c 0.6 2.8 5.5 4.1 3.7 4.0 + 0.6 2.9 3.2 3.4 4.3 3.5 f. 0.3 7.0 7.7 5.1 4.7 6.1 k 0.7 7.8 8.4 8.1 k 0.3 M&III Substratum 60.4 15.2 92.5 72.0 75.0 * 6.6 63.5 52.6 47.4 67.6 57.8 r 4.7 96.9 66.2 54.6 62.5 70.0 + 9.3 77.1 78.8 106.4 63.1 81.4 * 9.1 59.9 62.9 61.4 k 1.5 Coefficient of variation heterogeneity” 4.4 3.5 2.8 2.1 * k i i 8.0 4.2 5.6 5.6 & 2.8 + 3.6 * 7.3 + 7.6 & 3.2 + 2.4 + 4.6 f 6.4 + t k f. 1 SE 5-5 1 17-84 5-96 12-61 17-94 12-91 O-38 19-65 17-87 17-95 20-59 10-39 26-64 17-79 18-75 18-54 11-44 12-33 Range Herbivores 25.5 + 3.6 43.2 + 5.4 50.9 33.6 46.4 47.2 12.5 38.8 39.3 36.2 39.5 23.8 45.9 37.4 45.8 36.1 24.7 21.2 i* depending date. on the treatment, 16 16 16 16 16 16 16 16 IO 10 16 15 9 10 15 15 16 14 N i f f f ( 1.6 ( 1.8 (2.2 ( 1.1 0.6 0.4 0.5 0.3 1.6 1.4 1.3 5.6 1.2 1.2 1.9 2.2 (5.1 + 1.4 ( 5 3 * 0.9 k i_ i_ t * * k i ? 0.9 & 0.4 + 0.8 + 0.7 7.4 9.8 5.4 19.7 4.1 7.7 6.2 6.7 4.9 1.9 4.1 4.1 O-20 o-12 O-8 O-6 o-7 o-4 O-18 3-24 o-14 o-47 o-17 o-14 O-18 O-23 o-12 O-6 o-11 O-8 Range Predators No. slow-moving consumers present or entering (in parentheses) each plotb plot tide heights (datum = MLW) and substratum heterogeneities and number of slow-consumers: latter data are mean number present per sample date or mean number removed per sample Predator removal (marked plots) (cage) Large, small fish and crab exclusion (roofI Large fish exclusion (marked Control Treatment Summary TABLE 16) 16) 16) 16) 16) 16) 16 16 10 10 16 15 9 10 15 15 16 14 the -H-P-LF-SFC Herbivore and predator removal, large, small fish exclusion (cage) il See Lubchenco et al. (1984) for methods N = number of dates monitored. -H-P-LFtSFC -H-P-LF+SFC -H+P-LF-SFC -HtP-LF+SFC -H+P+LFtSFC +H-P-LF-SFC Herbivore and predator removal, large fish exclusion (roof) (roof) Herbivore removal and large, small fish and crab exclusion (cage) Herbivore and predator removal (marked plots) Herbivore removal and large fish exclusion (cage) Herbivore removal (marked plots) Predator removal and large, small fish and crab exclusion used + 1.1 t 1.1 I I + + t + in obtaining 2 3 4 I 2 3 4 + + + + 0.8 0.8 0.8 0.8 1.1 1.0 0.7 0.8 data. 2.7) 2.6) 2.6) 2.6) 3.5) 3.4) 2.3) 2.6) these t + t + t + + t t 1.0(+ 3.3) t 1.0 (+ 3.2) + 0.7 t 2.3) + 0.9 + 2.8) I 2 3 4 +1.1 (+3.6) +l.O (t3.2) + 3.5) t 3.6) + 3.7) + 3.4) 13.1) + 2.9) 2.5) + 2.5) t 1 2 2 2 3 4 + + t + 1.1 1.0 0.9 0.9 t 0.8 t 0.8 I 1 2 ’ Data (and 66.6 44.8 66.0 55.7 58.3 f 5.1 115.6 87.2 101.4 k 14.2 68.4 57.6 63.0 k 5.4 71.3 62.1 69.4 72.6 68.8 + 2.3 66.6 76.0 93.1 58.2 73.5 * 7.5 58.9 64.0 120.5 66.4 77.4 * 14.4 66.5 65.2 65.8 f 0.6 are mean 4.1 4.5 2.5 6.8 4.5 * 0.9 4.3 2.2 3.2 + 1.0 2.2 3.5 2.8 f 0.7 8.0 4.6 6.0 4.9 5.9 + 0.8 6.3 7.1 3.8 3.8 5.2 f 0.8 5.3 5.4 4.2 3.4 4.6 ?r 0.5 4.6 3.5 4.0 + 0.5 * + k + 1.4 0.3 1.5 2.3 + * + k + t f k 1.4 1.2 1.6 4.5 4.0 5.0 2.7 4.4 & 3.6 + 3.0 + 7.0 f 2.8 1 SE) and range (7.8 (5.4 (8.9 (29.8 (22.3 (22.6 (17.9 (28.7 (21.1 (20.9 (12.7 (14.4 (5.5 + 1.7 (16.6 + 2.5 (8.1 f 1.9 (5.7 f 1.1 (5.5 (1.2 (6.9 (13.9 39.9 + 4.5 37.3 + 5.3 of number O-25 o-19 2-17 IO-61 O-62 3-80 4-27 4-47 o-47 o-35 1-31 2-35 O-24 5-33 l-22 1-14 O-20 o-4 O-16 3-31 lo-65 lo-78 per plot. 18) 18) 10) 10) 18) 18) 10) 10) 18) 18) 18) 16) 13) 14) 13) 13) 13) 13) 14) 14) 16 16 i * f -f 0.4 0.7 0.4 0.4 + * * * + + ? * f + f + 1.8 0.9 1.4 2.1 0.9 1.4 0.5 0.9 0.3 0.4 0.5 0.3 18) 18) 10) 10) 18) 18) 10) 10) 18) 18) 18) 16) 13 14 13 13 13 13 14 14 16) 16) available; O-29 o-12 O-13 o-2 1 o-17 O-18 O-16 O-8 O-6 O-6 O-6 o-5 o-5 1 1-33 2-9 l-31 o-5 o-7 o-5 o-5 o-43 o-13 ND = no data (7.2 (4.0 (3.7 (7.7 (2.7 (5.2 (1.8 (2.6 (1.7 (1.1 (1.8 (0.6 15.2 + 3.9 10.6 + 2.6 5.6 + 0.7 10.6 f 2.6 0.8 2.2 1.1 3.1 (6.2 & 2.6 (4.4 f 1.0 BRUCE A. MENGE ETAL. 262 ‘0 F. GRN. Ab. -I G. m. &!-I!5 O-P *tP !s Y % ‘“7 G.p. +H Jani0 -H rH F. RED 4 +H -H TURF +H -H Fig. A-2. Analysis of significant statistical interactions between study sites in MANOVA done on abundance of secondary space occupants prior to initiation of experiments: percent covers were transformed before analysis; ordinate labels were back-transformed to percent cover for easier interpretation; means indicate untransformed percent cover ofeach species or group on Site 4 = - H + P; Site 1 = + H-P, Site 2 = -H-P, Site 6 = + H + P; Ab. = Abietinariu sp. (arborescent hydrozoan); F. GRN. = green filamentous algae; U.-E. = Vlva-Enteromorpha (green foliose algae); G.m. = Gt@rdiu mitchelliae (brown foliose alga); G.p. = Gelidium pusilium; Jania = Jania spp.: F. RED = red filamentous algae; TURF = multispecific turf of foiiose algae. Constraints Our experimental design departed from the ideal dictated by purely statistical considerations. Most departures were imposed by biological characteristics of the system and were thus, we believe, largely unavoidable, at least in the context of studying the dynamics of the entire community. Because benthic consumers were highly mobile and more frequent monitoring (e.g. every 2-5 days) was simply unfeasible (tidal patterns prevented low tide monitoring for periods of 1 to 3 wk per month), we selected sites with natural barriers (e.g., surge channels, deep crevices, boulder fields) to hinder lateral reinvasion by slow-mo~ng consumers. P were removed from Site 1, H were removed from Site 4, both P and H were removed from Site 2, and neither was removed from Site 6. Thus, with respect to slow-moving consumer manipulations, the plots were not intermingled among sites but were clustered within sites. Because P, H, and PH removals were each done at one site only, the plots were “pseudoreplicates” (Hurlbert, 1984). To conform to Hurlbert’s “replicates”, each plot of each treatment should have been assigned randomly along the shore, and manipulations should have been focused on plots, not sites. However, manipulations at the level ofthe plot may also have artifacts associated with them, such as dilution of consumers from nearby controls (Underwood, 1981; Hawkins & Hartnoll, 1983). In fact, this is the biological constraint that prevented us from scattering plots and treatments at random along the shore. Mark-recapture studies confirmed that P in particular have large, wandering ambits. Had a P-removal plot been randomly located near a P-present plot, our removals would have eventually had a strong effect on non-removals, introducing uncontrolled variation into the design. Alternatively, replication of the slow-moving consumer treatments could have been done by locating four additional semi-isolated sites and performing another set of P, H, PH, and control manipulations. Unfortunately, funding, and thus personnel, limitations eliminated this option. As it was, monitoring the design we used kept two to four investigators more than fully occupied, and over 11000 person-hours were devoted to its maintenance. ORGANIZATION OF A TROPICAL INTERTIDAL COMMUNITY 263 However, after the initial removals ofslow-moving consumers from the sites, we concentrated subsequent removals on, and around, each plot. Hence, non-adjacent plots were at least partially independent with respect to slow-mov~g consumer m~ipulations. Roofs and cages excluding fast-moving cons~ers (SFC, LF) were placed over locations which were amenable to attachment within each tidal zone; otherwise, these plots were intermingled within sites. The experimental design was not orthogonal (Table II), because we could not separate the effects of SFC from those of LF. Although roofs exclude LF alone, and cages exclude both groups, we could not devise a practical method of excluding SFC alone. However, subsequent statistical comparisons between treatments differing only in the presence or absence of SFC gave an indication of the effects of this group (see Table III). As in many experimental field studies, relatively few plots per treatment (two to four) were used. Plot number per treatment was a compromise between the statistical ideal of high replication and several constraints ofthe system. For example, preliminary studies (in 1974) indicated that photographic monitoring of the plots would not permit accurate taxonomic identification. We therefore resorted to monitoring plots directly in tbe field, which was time-consuming, and often hazardous (due to wave surge). Limitations of personnel availability, and the broad scope of the study also set limits on plot number. Despite these problems, we have confidence in our results because similar results were obtained in a separate experiment of similar design (in prep.). Execution of the experiment Experiment initiation included (1) identifying surfaces similar in heterogeneity and species composition, and, for exclosures, amenable to attachment of roofs or cages; (2) quantification ofpre-manipulation species abundances in each plot; and (3) removal or exclusion of consumers. We did not denude the substratum prior to initiation of the experiments because open rock surfaces were virtually devoid of sessile organisms except for algal crusts (Menge 62 Lubchenco, 1981; Lubchenco et al., 1984; Fig. A-l). The effect of aIga1 crusts on colonization was minor as indicated in a later set of experiments of similar design (authors’ unpubl. data). Abundances were quantified using flexible vinyl quadrats (0.25 m’) with 100 randomly-plotted dots to estimate percent cover of sessile organisms, and aluminum frame quadrats to estimate numbers of mobile and solitary sessile invertebrates. Two categories of space use were distinguished. Primary space occupants used space on the rock surface, while secondary space occupants were attached to, but grew away from the surface to a height of 1-3 cm (see Connell, 1970; Dayton, 1971; Menge, 1976; Lubchenco & Menge, 1978; Menge & Lubchenco, 1981 for further details). Large, rather than more manageable smaller (e.g., 0.1 x 0.1 or 0.2 x 0.2 m) plots were used both because substratum heterogeneity was high, and abundance of sessile organisms was low (e.g., Lubchenco et al., 1984) relative to intertidal habitats we had studied elsewhere. We judged that 0.25 m2 plots were the best compromise between the opposing constraints of including a large surface area, and the practical problems of attaching, monito~ng, and majnt~ning large exclosures. Statistical ana(ysis Hurlbert (1984) recommends against using inferential statistics when the assumption of independence of sample plots is not met, although this recommendation can be disputed (S. Overton, pers. comm.). Even when “replication” is “pseudo”, parametric statistics may be used to determine whether or not plots differ, but cannot be used to infer that the difference is due to the treatments. Moreover, temporal synchrony of replicates may not be required either, and repetition of an experiment at some other point in time can serve as a replicate (S. Overton, pers. comm.). The goals of this study were basically (I) to determine whether or not different consumer groups had an effect on the structure of the sessile organisms, (2) if so, how large the effect was, and (3) whether or not the consumers exerted their effects independently of each other. Although inspection and comparison of the results ofeach treatment without using statistics should provide a partial answer to both of these goals, detection of interactions (in the statistical sense) among consumer groups using this method would be exceedingly difftcult. We thus used a combination ofinferential statistics (see below) as a means ofdetecting such interactions, and graphical examination of results as a guide in interpreting our data. However, we stress that our conclusions, particularly regarding the interactions among groups, must be regarded as tentative. BRUCE 264 A. MENGE ETAL As noted in the text, changes in abundances of prey species (i.e. the dependent variables, Yi) constitute a multivariate response to variation in consumer deletions (i.e. the independent variables, Xi ). To eliminate time as a variable (sampling was done at intervals from 1977-80), a regression equation for percent cover ofeach species (i.e., the response variables,y[abm]) at time t(in months from the beginning oftbe experiment) was calculated using multiple linear regression: where y (abm) x t = the percent cover of a species m at time t in plot n (n = 2 or 4) of treatment b (n = 12); BO = the Y-intercept of the response vector; B 1 = the slope of the response vector; E x t = the error term, and the remaining terms are used to generate the seasonal (R) and phase (PHI) statistics. Thus, BO and 5 1 are taken directly from the regression equations, while R, the index of seasonal variation in the response vectors, = ,/(B2’ + B3*), and PHI, the index of change in phase in the response vector, = tan-l (B3/82). Each index quantifies a distinct aspect of changes in abundance of sessile prey over time. Differences among: (1) Y-intercepts (= BO) reflect the magnitude of short-term (0 to 6 months) changes in prey abundance, (2) slopes (Bl) reflect long-term (6 months to 3.5 yr) changes in prey abundance, (3) seasonal indices (R) indicate the magnitude of differences in seasonal pattern of abundance, and (4) phase indices A. Y-INTERCEPT SLOPE 1 ANALYSIS & B. C. SEASONAL PHASE m 0 ANALYSIS ANALYSIS -CONSUMERS -CONSUMERS i TIME i il (YR) CONTROL= -A- Fig. A-3. Illustration of hypothetical sessile organism response vectors. A, Y-intercept and slope analysis: for species 1, the Y-intercept (EO) would be different from controls but the slope (Bl) would not; for species 2, both Y-intercept and slope would be different; for species 3, the Y-intercept would not be different, but the slope would. B, seasonal analysis: the prey species exhibits more strongly seasonal patterns in experiments (the line labelled - CONSUMERS) than in controls. C, phase analysis: peak abundance of seasonally variable species shifts (e.g., from wet to dry season) in the absence of consumers. (PHI) indicate differences in the phase of seasonal pattern. Examples of the types of prey response are diagrammed in Fig. A-3. The general form of the MANOVA model is: Yi = pi + CtiXi+ /?kXk + dmXm f E vectors , where Yi = the coefficients (BO, B 1, R, or PHI) estimated from the prey response vectors for each plot of each treatment for each of species i (i = 1 to n); pi = constant, estimated by the overall mean of the Yi; Xj = the effects of the consumer groups j (j = 1 to 4) with coefficients uj; Xk = the two-way interactions k (k = 1 to 5) among the consumer groups with coefficients fik; Xm = the three-way interactions m (m = 1 to 2) among the consumer groups with coefficients 6m; and E = the error term. ORGANIZATION For example, the MANOVA Table III) was: OF A TROPICAL equation INTERTIDAL for the BO (Y-intercept) analysis COMMUNITY on primary space occupants 265 (see Ra,Hi,Sc,CC,Ba,Ch = CONSTANT + P + H + LF + SFC + P x H + P x LF +PxSFC+HxLF+HxSFC+PxHxLF+PxHxSFC, where Ra, Hi, etc. signify the coefftcients (i.e., BO values) for each of the most abundant occupants of primary space (Ralfsia, Hildenbrandia, Schizothrix, Coralline Crust, Balanus, and Chama), and P, H, LF, etc. indicate the effects of the consumer groups (main effects, two-way, and three-way interactions). The CONSTANT and the coefficients of the consumer groups (i.e., the x and /I values, etc.) are estimated by the MANOVA analysis. The effects of each consumer group (i.e., the Xi values) and the interactions (i.e., the Xk, and Xm values) are incorporated using indicator variables. That is, + I in the data file indicates that a group was present, and - 1 indicates that it was absent. Thus, only four MANOVAs (one each for Y-intercepts, slopes, seasonal patterns, and phase patterns), rather than 14 (one for each abundant prey species) were necessary. Further, we were able to evaluate five two-way interactions and two three-way interactions, as well as the “main” effects (the four consumer groups; see above). Interpretations of results Interpretations of the effects of each consumer group were based on both inspection of the average coefficients (i.e., the BO, Bl, and R values; no phase changes were significant) and the MANOVA analyses. Prey were included in an analysis only if they were common (i.e., occupied z 5% cover) in all plots in all treatments. If MANOVAs on Y-intercept, slope, or both indicated differences between plots, both the changes over time and the coefficients (i.e., BO, B I, R, and PHI) were graphed and inspected to determine what changes had occurred (Fig. A-3). Differences in Y-intercepts indicated that the sessile organism had increased or decreased in abundance almost immediately after the experiment began. Differences in slope indicated that the sessile organism had increased or decreased in abundance more gradually. Differences in the index of seasonal change indicated that the sessile organism had exhibited seasonal fluctuations. Finally, combinations of difference in more than one index were also possible. If the analyses indicated that differences were due to statistical interactions (i.e. the effect depended on the combination of consumers removed), subsequent analyses were performed to identify which prey groups or species had changed. Unless the statistical interactions were significant at P < 0.01 or less, and could be detected by graphic analysis (Figs. 6-9) they were ignored. POSSlBlF AR-,ItAC,'SDUE TO EXPERIMENTAL DEVICES Potential artifacts of exclosures included direct effects of the devices and indirect effects through alteration of either the physical environment or the biology of the organisms. Direct effects included either detrimental or beneficial influences ofthe device on survival. A potential detrimental effect, use ofgalvanized mesh exclosures for the first 6 months (after which we switched to stainless steel mesh), was done to reduce the expense of cage loss while attachment techniques were perfected. Fortunately, no detrimental effects of the galvanized cages were observed in low zone experiments. Potential indirect effects ofexclosures included: enhanced survival ofcolonists due to reduced desiccation or ultraviolet radiation, positive or negative effects due to sediment accumulation or alteration ofwater flow, and heightened effects of small consumers due to use of exclosures as shelter from large consumers (e.g., Menge, 1976; Menge & Lubchenco, 1981; Dayton & Oliver, 1980; Hulberg & Oliver, 1980; Virnstein, 1978; Peterson, 1979; Edwards et al., 1982). However, relatively few artifacts of artificial devices have been detected in studies on rocky shores, particularly in lower zones (e.g., Connell, 1961a,b, 1970; Dayton, 1971; Menge, 1976; Lubchenco & Menge, 1978; Menge & Lubchenco, 1981; Menge, 1982b). Because ofthe complexity ofthe food web and the rapid recruitment of mobile invertebrates in this system (see below), the usual controls for exclosure effects were ineffective, or were themselves exclosures for some consumers (see above). We thus used alternative methods of evaluating potential experimental artifacts. Merhods To determine the effects of shade by the mesh in the low zone, we performed shade (SHADE; described in Menge & Lubchenco, 1981). water evaporation (EVAPORATION), and exclosure removal/replacement 266 BRUCE A. MENGE ETAL. (DESICCATION) experiments. In the SHADEexperiments, colonization of prey under roofs of differing mesh size was examined. In the EVAPORATION experiments, we compared water loss rates with and without shade from the mesh. Preweighed, water-filled vials were placed under roofs and in nearby marked plots (10 vials per treatment) at Site 2 from 1030to 133531 January, 1979, a typically clear, cloud-free, hot day. The vials were capped at the conclusion of the experiment and taken to the laboratory where they were reweighed. In the DESICCATION experiments, we used plots that were due for termination after being roofed or caged for 4 yr. We removed cages and roofs for the entire day-time low tide for 5 days during a series of spring tides. Cages and roofs were replaced just before they were covered by the advancing tide each day. We compared changes in abundance of sessile organisms in the presence or absence of shade from the mesh during low tide. Four exclosures in which prey abundances were still high were selected as experimentals and four as controls (two roofs and two cages each) at Site 2 z 1 yr after intensive monitoring of the experiments had stopped. Prey covers were estimated 21 January (premanipulation) and 25 January (postmanipulation), 1981. Sunny, hot, and dry conditions prevailed during this period, and desiccation and heat stress were presumably high. Experimental artifacts from roofs and cages appear minimal. First, over an 11 month period in the SHADE experiments, changes in abundance of sessile organisms under mesh roofs with 2.5 x 2.5 cm (“large”) openings were not different from those with 1 x 1 cm (“standard”) openings (for details, see Menge & Lubchenco, 1981). Second, in the EVAPORATIONstudy, the rate of water evaporation under mesh (X f. 1 SE = 3.6 + 0.2%; initial volume = 26 ml/vial) was lower than the rate away from mesh (5.1 rl: l.O%), but the difference was not significant (one-way ANOVA; F= 1.97, 1,18 d.f., P > 0.1). Third, in the DESICCATIONstudy, cover of the major space-occupying groups (coralline crust, fleshy crust, foliose corallines, fohose fleshy algae, hydroids, sponges, barnacles, oysters, mussels, dead coralline crust, and dead foliose coralline algae) did not differ in control and experimental plots (one-way ANOVAs with Bonferroni correction, P > 0.05). 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