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J. Exp. Mar. Biol. Ecol., 1991, Vol. 146, pp. 69-100 69 Elsevier JEMBE 01541 Relative importance of recruitment and other causes of variation in rocky intertidal community structure Bruce A. Menge Department of Zoology, Oregon State University, Corvallis, Oregon, USA (Received 11 April 1990; revision received 17 September 1990; accepted 22 October 1990) Abstract: Recruitment limitation has been advocated as a major cause of community structure on rocky shores. Earlier work was criticized for failing to assess this possible source of variation. To evaluate this suggestion in relation to factors already known to be important in such communities, I incorporated estimates of recruitment with data from prior experiments in New England and Panama and reanalyzed the results using multiple regression. Rates of increase of prey abundance in predator exclusion experiments in New England were at least an order of magnitude greater than in Panama (e.g., it took 4-6 ruth vs. 60-72 mth to reach 100% cover, respectively). Recruitment densities of sessile invertebrates and algae were variable in space and time at both sites, but were lower by at least an order of magnitude and less synchronous in Panama than in New England. The analyses indicated that, although predation, competition, recruitment, and level on the shore explained significant amounts of variation in community structure at both places, the proportionate contributions of these factors differed. In New England, recruitment explained at most 11% while predation and competition explained 50% to 78% of variation in sessile invertebrate abundance. In Panama, recruitment explained 39 % to 87 % while predation and competition explained 8 to 10% of variation in sessile invertebrate abundance. Hence, when low, recruitment density appears ,.'mportant in influencing the structure of these communities. Key words: Community regulation; Competition; New England; Panama; Predation; Recruitment; Rocky intertidal; Temperate vs. tropics INTRODUCTION Marine organisms with planktonic larvae must survive several critical stages before reaching adulthood (i.e., larva, settlement, recruitment, juvenile). A current issue in community ecology is the relative contributions to "adult" community structure of pre-juvenile factors such as larval production, settlement and recruitment density vs. post-juvenile factors such as predation, competition, and physical factors (Denley & Underwood, 1979; Underwood & Denley, 1984; Watanabe, 1984; Caffey, 1985; Conne|l, 1985; Gaines & Roughgarden, 1985; Sutherland & Ortega, 1986; Menge & Farrell, 1989; Sutherland, 1990a,b). In fact, much earlier work has been criticized for failing to incorporate variation in initial benthic stages as a factor influencing the structure of communities (Underwood & Derr!ey, 1984). However, as noted by Connell Correspondence address: B. A. Men,e,_ Department of Zoology, O,'eoo~.~..~ . . . . State .. I T,............. ~;x.... ;,,,.,,c~o..,~,ml,.~,,~ :" OR 97331, USA. 0022-0981/91/$03.50 © 1991 Elsevier Science Publishers B.V. (Biomedical Division) 70 B.A. MENGE (1985), evaluation of the importance of these factors requires study of their contribution to adult patterns of abundance and distribution relative to the contributions of other potentially important factors. Such knowledge is increasing but still rather sparse (Connell, 1985; Gaines & Roughgarden, 1985; Sutherland & Ortega, 1986; Sutherland, 1987, 1990a,b; review in Menge & Farrell, 1989). Obtaining such information is essential to the further development and testing of general models of community organization (e.g., Connell, 1975; Menge & Sutherland, 1976, 1987). Connell (1985)evaluated the relationship between adult abundance of barnacles and their settlement and recruitment densities. Settlement density was defined as the number of larvae attaching per unit area of surface while recruitment density was defined as the number of settlers surviving the initial period of time (usually days) in the adult habitat. He found that adult and recruit density were positively correlated with low recruitment but were uncorrelated with high recruitment. In one of the first studies attempting to relate densities of early and adult stages, Gaines & Roughgarden (1985)suggest that low recruitment produces low adult density, while high recruitment produces higher densities. Sutherland (1990a) and Raimondi (1990) found such relationships for species of Chthamalus in Costa Rica and the northern Gulf of California, respectively. Holm (1990) notes that, at least at low densities, settlement and recruitment can be positively correlated whether or not mortality is density dependent. In this paper, I evaluate the relative importance of recruitment density and postrecruitment factors in governing the abundance of solitary sessile invertebrates which as adults are actually or potentially dominant space occupiers in their communities. The data were obtained during two earlier studies of community organization of rocky shore communities, one each in New England and Panama. In both studies, my coworkers and I concluded that competition, predation, and physical disturbance were largely responsible for observed post-recruitment variation in the abundances of sessile organisms. The contribution of recruitment to community variation was not analyzed, however. New England communities displayed high spatial and temporal variation in structure both within and between sites (spatial scale of kilometers). Field experiments indicated that such differences were due to seasonal and between-site variation in the relative importances of predation, competition, and physical disturbance (Menge, 1976, 1978a,b, 1983; Lubchenco & Menge, 1978; Lubchenco, ! 980, 1983, 1986). Recruitment density of barnacles and mussels, the dominant space-occupying animals, appeared weakly related to variation in community structure (Menge, 1978a). Panama communities exhibited little spatial or temporal variation in structure (Menge & Lubchenco, 1981; Lubchenco et al., 1984; Menge et al., 1985, 1986a,b). Field experiments suggested that this apparent stability was due to spatially and temporally uniform predation. Competition for space and physical disturbance appeared unimportant (Menge et al., 1985, 1986a,b). However, both recruitment and adult densities of all sessile invertebrates were extremely low (Menge et al., 1986a, p. 256; pers. obs.), suggesting the possibility of recruitment limitation as well. RECRUITMENT AND COMMUNITY STRUCTURE 71 The goals of this paper are: (1)to document patterns of recruitment of the most abundant species of solitary sessile invertebrates in these habitats; (2)to expand previous analysis of experiments in New England and Panama to determine the relative impact of recruitment on community variation in comparison to the effects of postrecruitment factors; and (3)to evaluate the factors which cause low recruitment in Panama. COMMUNITY STRUCTURE IN NEW ENGLAND AND PANAMA Both systems have been described in detail elsewhere (Menge, 1976; Lubchenco & Menge, 1978; Lubchenco et al., 1984). In New England, community structure, defined as the percent cover of dominant space occupiers, varied with the tide height and exposure to waves. At most sites, barnacles (Semibalanus balanoides (L.)) dominated space in the high zone. At wave-exposed sites, mussels (Mytilus edulis L.) dominated space in mid and low zones. At more sheltered sites, fucoids dominated mid zone space (Fucus distichus L. at sites of intermediate wave exposure, Fucus vesiculosus L. at sites of lower wave exposure, and Ascophyllum nodosum (L.) Le Jolis at sites of lowest wave exposure), while the foliose red alga Chondrus crispus Stackhouse dominated low zone space. Previous studies in this system evaluated the effect of several biotic and physical factors on the abundance of the dominant sessile organisms (see Menge, 1976, for the experimental design). Predators (Nucella (= Thais) lapillus L. in the mid zone, and N. lapiilus, Asterias vulgaris Verrill, A. forbesi (Desor), Carcinus maenas (L.), Cancer borealis Stimpson, and C. irroratus Say in the low zone) were absent from the high zone, present but ineffective at controlling prey abundances at wave exposed sites, and present and effective at sites of intermediate to low wave exposure (Menge, 1976; Lubehenco & Menge, 1978). Herbivores (Littorina saxatilis Olivi in the high zone, and L. littorea L., L. obtusata L., Acmaea testudinalis Muller in the mid and low zones) were absent from the mid and low zones of sites of high to moderate wave exposure, and present and partially effective in controlling algal abundance at more sheltered sites (Lubchenco, 1978, 1980, 1982, 1983, 1986; Lubchenco & Menge, 1978). Competition for space occurred between mussels (the dominant competitor) and barnacles unless prevented by predation. Physical disturbance from waves and, to a lesser extent, cobble scour, and desiccation at higher tide levels were the main sources of mortality due to physical factors (Menge & Farrell, 1989). Other potential sources of reduced abundance of sessile animals were algal whiplash and a barrier effect of a dense algal canopy. Dominance by barnacles in the high zone was attributed to the absence of both interspecifie competition (from mussels and algae) and predators. Dominance by mussels in wave-exposed mid and low zones was attributed to their ability to outcompete barnacles and algae in the absence of effective predation. Dominance by fucoids and 72 B.A. MENGE Chondrus in more sheltered mid and low zones was attributed to their escape from competition due to elimination of mussels and barnacles by predators. Between-site variation in the macroalgal species which dominated upon escaping competition and grazing was attributed to variation in their competitive abilities with wave-exposure and level on the shore (Menge, 1976, 1983; Lubchenco & Menge, 1978, unpubl, data; Lubchenco, 1980, 1983). Although annual variation in recruitment was observed in these studies, recruitment was usually high, although seasonal, for all dominant sessile species except A. nodosum. We therefore assigned little importance to this factor. In Panama, community structure varied less in space and time than in New England (Menge & Lubchenco, 1981; Lubchenco et al., 1984; Menge et ai., 1986a). Crustose algae were the dominant space occupants in mid and low zones while most high zone space was bare rock. Although solitary sessile organisms tended to be more abundant at both wave-exposed and wave-protected sites, total abundances were always low (typically < 10% cover) at all tide heights and at all sites. Species richness was higher in Panama than in New England, and no sessile invertebrate species was an obvious dominant in unmanipulated communities (Lubchenco et al., 1984). Thus, to keep the analysis manageable but realistic, I selected community structure dominants (the dependent variables) both on the basis of their abundance in unmanipulated communities and the abundances they achieved in experiments. These included the barnacles Chthamalus fissus Darwin and Balanus inexpectatus Pilsbry, the bivalves Ostrea palmula Carpenter and Chama echinata Broderip, and foliose algae as a group. These animals made up 28% (mid zone) and 78% (low zone) ofinitial, and 84% (mid) and 89% (low) of final total solitary sessile animal cover. Algal crusts were not included in either analysis because these always decreased in abundance as sessile invertebrates and foliose algae recruited and increased; i.e., algal crusts were competitively subordinate to all other sessile organisms. Chthamalus reached peak densities in the high zone but also occurred in mid and low zones. Ostrea reached peak densities in the mid zone, and Balanus, Chama, and foliose algae became most abundant in the low zone. Studies in this community evaluated the influences of several biotic and physical factors (Menge & Lubchenco, 1981; Lubchenco etal., 1984; Menge etal., 1985, 1986a,b). The effects of four functional groups of consumers (molluscan herbivores, predaceous gastropods, large fishes, small fishes and crabs) were determined experimentally. Mortality of sessile invertebrates from competition for space was minor because space was virtually never limiting except in consumer exclusions. Desiccation/ heat, which becomes increasingly severe with increasing height on the shore, is likely to be the primary source of mortality caused by the physical environment (e.g., Garrity, 1984). The direct effects of wave exposure and cobble scour were not evaluated but, as argued earlier, seem minor (Menge & Lubchenco, 1981). Neither sessile invertebrates nor macroalgae are present in dense stands, so there are few organisms for wave force to disturb, and no cobbles occur along the steep shores upon which we worked. Moreover, neither algal whiplash nor canopy barrier are a factor in this community since macroalgae are small and extremely scarce. RECRUITMENT AND COMMUNITY STRUCTURE 73 METHODS ESTIMATES OF RECRUITMENT Methods of estimating recruitment varied with species and region. In New England, recruitment of S. balanoides was quantified in monthly photographs of 10 x 10 cm control and cage control plots in the experiments reported in Menge (1976). Barnacle recruitment in these experiments was also scored qualitatively (see next paragraph) to make indices ofbarnacle and mussel recruitment in the regression analyses comparable. Recruitment of this barnacle occurred in an annual spring pulse, usually April through June. Recruiting mussels and algae could not be accurately counted in photographs due to the usual high topographic relief of the substratum caused by the prior occupants of the rock surface (the barnacles), and counts were not taken in the field. Therefore, I scored recruitment ofM. edulis, Fucus spp., and C. crispus in photographs of experimental plots qualitatively (i.e., high or low). Independent estimates of M. edulis recruitment density were obtained by fastening 10 x 10 cm squares of shag rug in the mid and low intertidal zones at each of four sites from May to September 1974 (see Menge, 1978b). Rug squares were used in an effort to mimic the filamentous algae which are preferred settlement sites for mussels (Bayne, 1964; Paine, 1974). Mussel recruitment occurred in a long annual summer pulse from June through at least September. Independent estimates of patterns and densities of Fucus gpp. recruitment were obtained in a series of manipulations of canopy and substratum at three sites. At Chamberlain, one plot per each combination of presence or absence of canopy (F. distichus)or substratum cover (M. edulis)was established. At Grindstone Neck, one plot with and one plot without canopy (F. vesiculosus) was established. At Canoe Beach Cove, one plot per each of three combinations (Ascophyllum and Semibalanus present, Ascophyllum absent and Semibalanus present, Ascophyllum and Semibalanus absent)was established. Eight to 10 subplots were sampled in each plot to obtain densities of recently settled Fucus spp. Since the plots were unreplicated, no statistical analyses of the data were attempted. Standard errors of each mean density were calculated to indicate variability among subplots. In Panama, all sessile invertebrates were periodically counted in permanently marked quadrats. Recruitment was estimated as the average increase in density of each species per 0.25 m 2 plot per sample date (sometimes converted to no./0.01 m E for comparison to New England data). Recent recruits were generally distinct from older individuals, although high growth rates of some species (e.g., B. inexpectatus) made it difficult to distinguish individuals > 3-wk old from older individuals, i?iots were sampled nondestructively at 1-4 mth intervals unless recruitment was observed during twicemonthly inspections, whereupon we immediately monitored the plot. Despite our frequent inspections, some pulses of recruitment may have been partly missed due to high post-settlement mortality. Her~ce, estimates of recruitment in Panama are be~t viewed as conservative. 74 B.A. MENGE DATA FILES AND STATISTICAL ANALYSIS Data sets The New England set consisted of four dependent variables (percent covers of barnacles, mussels, and two types of ma~.roalgae, Fucus spp. and Chondrus; Ascophyllum did not colonize these experiments) and 11 independent variables (recruitment; presence or absence of predation, herbivores, interspecific competition, cobble disturbance, algal whiplash, canopy barrier; degree of wave exposure; level on the shore; inclination of the substratum; and year of the experiment; Appendix 1). The data set consisted of a total of 291 experimental plots divided among six study sites over a 5-yr period. Here, a plot is a one of three to six 10 x 10 cm areas included in each experimental replicate and subjected to specific treatments (e.g., presence or absence of predators, mussels, etc.; see Menge, 1976). A minimum of four replicates was set up at each tidal level (high, mid, low) at each ofthe six study sites. New experiments were begun each spring at some levels and sites. The Panama data set consisted of five dependent variables (densities of four species of sessile invertebrates and percent cover of macroalgae) and seven independent variables (recruitment; presence or absence of molluscan grazers, predaceous gastropods, large fishes and small fishes and crabs; level on the shore; and percent of substratum exposed to predation by fishes and crabs; Appendix 1). The data set consisted of a total of 64 plots divided among fou~ neighboring locations at one site (Taboguilla Island). Each plot was one of four 50 x 50 cm areas included in each experimental set and was subjected to specific treatments (different combinations of consumers; see Menge et al., 1986a). Experiments were established at three tidal levels (high, mid, and low), producing a total of 96 plots. Except for C. fissus, most sessile species were absent from the hitzh zone, however, so I analyzed the high zone data separately from the mid and low zone data. This produced the final data set of 64 plots for the community-level analysis (as opposed to the population-level analyses on Chthamalus; see below). Density of sessile invertebrates was used in the Panama data set because this was a more sensitive measure of variation in abundance of these scarce organisms. Recruitment data for three additional species in Panama (Tetraclita panamensis Pilsbry, Catophragmus pilsbryi Broch, and Brachidontes semilaevis (Menke)) are presented in the recruitment surr~nary for broader comparison; these species are not included in the above set because adult abundances were always low. The specific data used as quantitative estimates of community structure both in New England and Panama were the final abundan,:es in the field experiments. In New England, 1 used abundances observed at the end of the period of greatest growth and activity, usually in September or October, 1972 to 1976. In Panama, I used abundances obtained when the mid zone experiments were terminated in July 1979, 2.5 yr after their initiation in February i977. Quantitative estimates of recruitment were number/ 0.01 m2/settlernent season (March-June) in New England, and number/0.04 or 0.25 m2/sample date in Panama, where samples were taken every 2.4 mth on average. RECRUITMENT AND COMMUNITY STRUCTURE 75 Analysis Quantitative variables were transformed before analysis to conform with assumptions of parametric statistics. Percent covers of sessile organisms and percent of substratum exposed to fast-moving consumers were normalized with the arcsine transformation and densities were normalized with the square root transformation (Sokal & Rohlf, 1981). Mean substratum depth and its coefficient of variation were not transformed. Qualitative variables were coded as indicator variables and were also not transformed (Appendix 1). Because the final abundances of prey species (community structure, or the ~) are not necessarily independent and more than one dependent variable can respond to variation in one or mcre independent variables~ multivariate analysis techniques (canonical correlation analysis, multivariate analysis of variance= MANOVA; Dillon & Goldstein, 1984) are most appropriate for these data sets. The experimental design in Panama unavoidably involved pseudoreplication, however, thus violating the M/ANOVA assumption of independence of some treatments. In an earlier paper (Menge et al., 1986a), we evaluated this and other problems encountered in the Panama study, and concluded that despite the design flaws, multivariate statistics were still the best means available of sorting out the results. Important to this conclusion was the lack of initial differences in community structure among the different sites. Our results were interpreted conservatively, nonetheless. In the present analysis, however, we have no means of determining whether or not recruitment varied among the sites prior to initiating the experiment. Further, observation during the study indicated that recruitment of C. fissus, at least, was very patchy on a scale of meters (i.e., within a site; see below), so it is possible that recruitment density varied non-randomly among the 0.25 m 2 experimental plots. Hence, we used regression techniques which have no underlying assumptions regarding independence of sample units (Sokal & Rohl£ 1981). In the absence of a multivariate analog of stepwise multiple regression analysis, I used this essentially univariate technique to estimate the proportion of the variance in each dependent variable explained by the independent variables. The software package used was SYSTAT v. 4.0. Stepwise multiple regression estimates and ranks the contribution of two or more independent variables (the X~) to variation in each Y~independently of other Y~. For example, tide height might account for 50°,o and predation might explain 30~o ofthe variance in mussel abundance. However, since the Y~may be interdependent (e.g., mussel abundance may also depend on barnacle and algal abundance), and the X~ might interact (e.g., predation varies with tide height), estimates of the percent of variance explained by each independent variable so ebtained must be regarded cautiously. Forward stepwise selection with a probability of alpha = 0.15 to enter or remove was used to identify those variables contributing the most variance to the overall model. Subsequent multiple regression was run repeatedly until all variables with statistically 76 B.A. MENGE insignificant regression coefficients or high condition indices (an indication of multicollinearity, or correlation among the independent variables; Netet et al., 1983) had been identified. These variables were dropped from the analyses to achieve the best fit regression modeis. Indicator variables were used because I did not have quantitative data for most independent variables. To determine how this might influence the regression analyses, I also ran the New England analyses with numerical data for Semibalanus recruitment rather than indicator variables. As reported below, the effect of this was a slight change in the magnitude but not the rank order ef the R2's or significance of the variables. RESULTS PATTERNS OF RECRUITMENT New England Settlement and recruitment of S. balanoides in New England occurred in an annual spring pulse (Menge, 1978b). Significant variation in density occurred both in space and time (Fig. 1). For example, recruitment densities at three sites (Chamberlain, Grindstone Neck and Little Brewster Cove) varied with both site and level on the shore (Table IA; Semibalanus balan0ides: Recruitment Density HIGH= ~ l MID= 1500 LOW-- O4 E o O o 1000 t,= o z 500 o i 72 PPT = 74 LBPT 73 74 CH .!., ,. 74 GN LBC 74 CBC Fig. 1. Recruitment density ofSemibalanus balanoides at six study sites in New England. Sites, ranging from most to least expased to wave forces, are Pemaquid Point (PPT), Little Brewster Point (LBPT), Chamberlain (CH), Gl;i~dstone Neck (GN), Little Brewster Cove (LBC), and Canoe Beach Cove (CBC; see map in Menge, 1976). ND, no data available. Error bars in this and all subsequent figures are 1 SE of the mean. RECRUITMENT AND COMMUNITY STRUCTURE 77 TABLE I Two-way analysis ofvariance (mixed models) of recruitment density of Semibalanus balanoides by study site and tide level (A) and by year and tide level at Grindstone Neck, Maine (B). Tide level was considered a fixed effect, while site and year were considered random factors. Since both interactions were significant, the tide level mean square is tested over the interaction mean square. Source of variation Sum of squares d.f. Mean square F P A. Study site (Chamberlain, Grindstone Neck, Little Brewster Cove) by tide level (high, mid, low) Study site 2277.38 2 1138.69 13.10 < 0.001 Tide level 2737.59 2 1368.79 5.67 0.07 Interaction 965.76 4 241.44 2.78 0.03 Error 10432.14 120 86.93 B. Year (1972, 1973, 1974) by tide level (Grindstone Neck) Year 5459.08 2 .2679.54 Tide level 23'~280 2 1166.40 Interaction 4~s5.09 4 1? ! 27 Error 2674.92 57 46.93 57.10 9.62 2.58 40.001 0.03 0.047 site x level interaction was significant; p = 0.03; 4, 120 d.f.). Among these three sites, which differ in exposure to wave forces (highest at Chamberlain and least at Little Brewster Cove; Menge, 1976), average recruitment of S. balanoides in the high zone changed little with reduced wave action (Fig. 2A). In the mid and low zones, however, recruitment density decreased with reduced wave action. Recruitment density of S. balanoides also varied over time (Fig. 1). At Grindstone Neck, densities varied significantly with both year and level on the shore (Table IB; year x level interaction was significant; p = 0.047; 4, 57 d.f.). Patterns ofvariation were positively associated between levels and years in 2 yr (high at all levels in 1972, low in 1973) but not associated in 1974 (moderate density in the high zone and low density in the mid and low zones; Fig. 2B). Later, less detailed observations indicated that temporal variation in density was actually even greater than shown in Figs. 1 and 2. In 1975, recruitment density at Grindstone Neck failed completely at all tide levels in 1975 (B. Menge, unquantified obs.). In 1976, counts from two haphazardly placed photographic transects through the high zone in April showed that recruitment occurred at this level but was relatively low compared to 1972 and 1974 (mean and standard error; 367 + 80 per 100cm 2, n = 20). No samples were taken in mid and low zones in 1976, but unquantified observations suggested densities were even lower at these tide levels. As indicated by the often large error bars in Fig. 1, variation was also high on very small spatial scales (i.e., when different 100 cm 2 plots within a ~;ite are compared). Ranges of recruitment density from 1972- 74, by site, were 67-1324 (24 plots, Pemaquid Point), 126-874 (23 plots, Little Brewster Point), 0-1643 (48 plots, Chamberlain), 0-1028 (117 plots, Grindstone Neck), 0-1174 (59 plots, Little Brewster Cove), and 41-979 (20 plots, Canoe Beach Cove). Recruitment of M. edulis occurred from late spring to early autumn and varied both 78 B.A. MENGE A. INTERACTION: SITE BY LEVEL HIGH= 0 - - 0 1000 O4 ¢.) 750 MID= Q - - • LOW= A - - Z ~ 0--i~"0-- --0 0 0 v.(/1 I-- 500 C¢: C,J b.I 0 Z 250 I I I CH GN SITE LBC B. INTERACTION: YEAR BY LEVEL AT GRINDSTONE NECK HIGH= O m O MID= • - - O LOW= A - - A 1500 O O4 "5 0 0,F-. 1000 Q o (/1 I-~° Q: CJ bJ 500 d Z , ÷- 72 73 YEAR 74 -9 Fig. 2. Analyses of significant two-way interactions in Semibalanus balanoides settlement patterns. A. Interaction between study site and level on the shore. B. Interaction between year and level on the shore at Grindstone Neck. over time (month-to-month) and space (between 100 cm 2 plots, tide levels, and study sites; Fig. 3). Since natural substrata are more variable than the artificial settlement surfaces sampled in Fig. 3, spatial and temporal variation in mussel settlement are undoubtedly more variable than suggested by these data. Yet, mussel recruitment in New England was also predictable. As noted by earlier workers (Paine, 1974b; Seed, 1976), mussels settle preferentially on filamentous algae and byssal threads of adult mussels. In New England, recruits could be consistently located in these microhabitats. Additionally, ,.~bservations indicated that rugose rock surfaces and particularly substrata occupied by S. balanoides are also favored settle,ment surfaces (Menge, 1976). In ~11 experiments ,t all sites and levels, mussels invariably recruited to barnacle-covered RECRUITMENT AND COMMUNITY STRUCTURE 1 MID LOW 4000 3500 79 ! ! ! I I | II B. GRINDSTONE NECK A. CHAMBERLAIN 3000 2500 2000 OI E ~d 1500 O O 5OO 6¢- 0 M J J A $ I I I I I MEAN M J J A S ! w w ! i I MEAN I Or) Z IJJ C3 I-Z) w' O bJ ne 4000 3500 I C. LITTLE BREWSTER COVE ! ! D. CANOE BEACH COVE 3000 2500 2000 1500 1000 500 M d d A $ g~Mq M d J A S MEId~ Fig. 3. Recruitment density e,fMytilus edulis from May to September 1974 in the mid and low zones at four sites in New England. Data are counts ofthe number ofmussds < ! mm shell length in 10 x 10 cm squares of shag rug (see Menge, 1978b): each bar represents a single rug. Rugs ~Tere replace~.' r:~onth]~' e,~cept for four plates in July (two each at Chamberlain and Grindstone Neck), and four in A=gus~ ~ail four plates at Little Brewster Cove which were collected after 2 mth. In these cases, the 2-mth totals were divided by two to obtain the I-ruth estimate. As a consequence of this, and small sample sizes (two rugs per site and level, except Cano~ Beach Cove low, which had one plate), no statistical analysis was attempted. Overall means for each site-level combination are shown to facilitate comparison. Missing bars reflect dates for which no data are available. surfaces, either eliminating them by competitive exclusion or being eliminated themselves by predators. T h e mid z o n e at three sites ( C h a m b e r l a i n , G r i n d s t o n e N e c k , a n d C a n o e Beach C o v e ) w a s domi.nated by a fucoid canopy, but each by a different species (F. disdchus, F. vesiculosus, and ,4. nodosum, respectively: Menge, 1976; L u b c h e n c o , 1980). M o s t recruits at C h a m b e r l a i n grew into F. distichus, while m o s t recruits at both G r i n d s t o n e N e c k a n d C a n o e Beach C o v e grew into F. vesiculosus. ,4. notlosum recruits were always 80 B.A. MENGE scarce, even at sites such as Canoe Beach Cove at which they dominated and reproduced. At Chamberlain, recruitment of Fucus spp. was high only where both the canopy was absent and mussel cover was low (Fig. 4). Fucus recruit density under such conditions was also much higher here than at either of the other fucoid-dominated sites (compare Fig. 4 to Figs. 5 and 6). At Grindstone Neck, mussels were always scarce in the fucoid zone, and recruitment of F. vesiculosus was low regardless of the presence or absence of a canopy (Fig. 5). Other factors were not tested, although grazing by L. littorea is probably a key factor in keeping Fucus recruitment low at this site (Lubchenco, 1983). Both barnacles and mussels are normally scarce in the fucoid zone even in patches lacking a canopy, and hence seem unlikely as inhibitory agents. A, + F u c u s +Mytilus 100 B. + F u c u s - M y t i l u s , ': 300 ........ Me 90 80 ~'-- - - - - ~ 250 Fd 70 60 50 150 04 qg 30 100 10 OE LIJ > 0 t) 0 Frec ..... 9 -10 J./F 76 Z C. - F u c u s tu t) n~u CL tO0 "" 0 ci 20 /~P 76 +Mytilus , so ~ ae 0 Frec J/F 7e ~ 7e , i 90 •~ BO v, 300 Frec Fd ~: Z bJ C~ D, - F u c u s -Mytilus i c~ z n, ¢j 2so LIJ n- 70 200 60 b. 50 / 40 i ~oo / 20 5O / 10 0 ~5o / 30 -10 / 7J/F 76 -I T ° Me Frec AP 76 I I J/F 76 AP 76 o Fig. 4. Recruitment density of Fucus distichus in the mid zone at Chamberlain, Maine. Fucus recruits are plants ~ 3 cm long. Original plots ranged in area from 100 to 2500 cm2 in area. All values were adjusted to 100 cm2 before averaging. +0 present: - , absent (removed in October 1975). Number of subplots per treatment was eight. RECF:UITMENT AND COMMUNITY STRUCTURE + Fucus 81 Fucus - 100 Fv 90 Frec 60 n, 2 Frec 70 fq E O 60 O 5O ~: 4o :D ¢w fO la.I n- 30 iX. 20 10 Me o Sb' -10 O . Me Sb ~ 1/76 4/76 Fv ! I 1/76 4/76 c~ Z -1 Fig. 5. Recruitment density ofF. vesiculosus in the mid zone at Grindstone Neck, Maine. Fucus was removed from the - Fucus plot in October ~975; means are averages of eight subplots. See caption to Fig. 4 for further details• At Canoe Beach Cove, F. vesiculosus ro,-r,,itmo,,~ was ~_l~_,t~,,,, ~,, ,,ao o~,,,~o,,h, inhibited by the Ascophyllum canopy (Fig. 6). Grazing evidently also helped to suppress recruitment at this site. In the abser~.ce of the canopy but presence ofbarnacles, Littorina recruit density increased tremendously at first, but declined steadily thereafter. Adult Littorina density remained normal in this treatment, but increased dramatically in. tt~e absence of both canopy and barnacles. Fucus recruitment was highest in the absence of canopy and presence of barnacles at the time of lowest overall Littorina abundance (Fig. 6). These data, and the studies of Lubchenco (1983), suggest that Fucus recruitment will be highest at such sites when the canopy is absent, and grazers ~e scarce and/or inhibited from grazing by a rugose substratum such as that provided by barnacles. Thus, many factors, including a dense canopy, high cover of preemptive space occupiers, and grazers may suppress recruitment of these fucoids. Recruitment density of F. distichus is evidently potentially much higher than that of F. vesiculosus. Although no data are presented here, recruitment density of Asco,~hyllum was even !ess that that of F. vesiculosus. Recruitment density of these fucoids clearly varies over a broad range, and variation is due at least to species identity and local biological conditions. Further investigation of recruitment in these species would be rewarding. Panama In Panama, average recruitment densities per sample date varied with level on the shore, species, space and time. In the mid and low zones, for example, recruitment of all of the eight most abundant solitary sessile animals except the mussel B. semilaevis 82 B.A. MENGE -Asco +Semibolanus +Asco +Semibalanus 100 90 60 u 60 u 40 t i ! , 8 m 6~'E 5 q0 y~-- Sb Me,An 3= = U lU 2 n, Frec 2O 10 0 -10 350 N 'T' 7 ~ ~o • " I Ii "~"~" I Frec I • " ~ 5o o. -Asco -Semibalonus Frec SbuL~,.ira..m..i..~ "Me, T . T . 9 12 3 6 10 I I a s w 1.o i . 9 300 [. \ .., 12 3 6 10 ! I T I w -~ v i 1 I v I l 9 12 3 6 10 0 1F -1 25OO <3ram 250 E 0 2000 N m tsoo E u~ 20O ¢N i/1 m d 15o 1000,~ 100 z 5O0 50 <3ram 0 -- g 12 74 v 3 v 5 75 v 10 I 12 ' 9 74 i 3 I 6 75 I 10 i i i a 9 12 3 6 74 , 0 ~0 75 Fig. 6. Recruitment of F. vesiculosus in the mid ~one at Canoe Beach Cove, Nahant, Massachusetts. Ascophyllumwas removed in March 1974; Semibalanus was repeatedly removed during spring and summer, 1974 and 1975. Densities ofL. littorea "adults" (i.e., animals > 3 mm in shell length) and "recruits" ( < 3 mm shell length) are also shown• Means are averages from 10 ( + A s c o + Semibalanus), 8 ( - A s c o + Semibalanus), and 9 ( - Asco - Semibalanus) subplots. and the barnacle T. panamensis varied with tide level (Fig. 7). Chthamalus and Ostrea recruit more densely higher on the shore, while Balanus, Catophragmus, and Chama rec~'uit more densely lower on the shore. Among mid zone species, mean recruitment varies by more than two orders of magnitude, from 0.4 (Tetraclita) to 116/2500 cm 2 (Chthamalus). Average recruitment of Chthamalus, which occurs at all tide levels, varies over space and time. Although statistical analysis is compromised by pseudoreplication, examination of the data makes certain trends apparent. First, on a spatial scale of tens of meters, Chthamalus recruitment density clearly increases with increasing level on the shore (Figs. 7 and 8). Second, on a spatial scale of I m 2 and less, recruitment is patchy (e.g., Fig. 9). At any time, one plot might receive no recruits, while another only a few meters away will receive several hundred. Third, Chthamalus recruitment varies with the presence or absence of both grazing molluscs (H in Fig. 8) and predaceous gastropods (P in Fig. 8). Recruitment is usually highest in the high zone, but at this level is greatest in the presence of grazing molluscs and the absence of predaceous gastropods. In the mid zone, recruitment is highest in the absence of grazing molluscs, while in the low R E C R U I T M E N T AND C O M M U N I T Y S TRUCTURE cN '5 MID= LOW= 100.0 tD o o u3 t'N n,, bJ n 83 10.0 03 n,, t~ 1.0 I,.=J n,, d z 0.1 o-'o I,O .... o E" w t~ a "3 t s.. =r _~.:1 ~ Q o" ¢.. Fig. 7. Recruitment density (note logarithmic scale) of several sessile invertebrate species in the mid and low zones at Taboguilla Island, Bay of Panama (see map in Menge et al., 1986). Chthamalus, Tetraclita, Balanus, and Catophragmus are barnacles; Ostrea and Chama are oysters; Brachidontes is a mussel. HIGH= ~ MID= RX'R~ LOW= 1000.0 500.0 II L) oo t'N n," L=..I 100.0 50.0 a.. 10.0 ~-- 5.0 tr) ,. at" O "n,"' 1.0 d 0.5 Z 0.1 i +H+P +H-P 1 -H+P ! -t4-P Fig. 8. Recruitment density (note logarithmic scale) of Chthamah~sfissus in the high, mid, and low zones in the presence ( + ) or absence ( - ) of herbivorous molluscs (H) and/or predaceous gastropods (P) at Taboguilla Island, Bay of Panama. zone, recruitment is highest in the absence of predaceous whelks (Fig. 8). The difference in the postulated effect of grazers (neutral or positive in the high zone, negative in the mid and low zones) probably reflects a difference in species composition and method of grazing. High zone grazers are primarily littorine and neritid snails, while mid zone 84 B.A. M E N G E 60 HIGH= I MID= ~ (N=S0) (N-96) 40 o. ,, , 0 I'-- I =,H 51- I01- 151- 2 0 1 - 2 5 1 - 3 0 1 - I =,_ 50 I00 . 150 200 .... 250 , nn,. 300 nn,. , 401+ 400 RECRUITMENT DENSITY/PLOT Fig. 9. Frequency distribution ofrecruitment densities of C.fissus per 0.25 m2 plot in the high and mid zone at Taboguiila Island, Bay of Panama, N = total number nf plots sampled. The high zone has a significantly higher frequency of plots with high recruitment densities than does the mid zone ( i f = 20.7, 5 d.f., p = 0.001). grazers are mostly limpets and chitons (Lubchenco et al., 1984). Limpets apparently have greater ability to excavate hard substrata, and to dislodge barnacles, than do snails (e.g., Menge, 1976; Steneck & Watling, 1982). Fourth, temporal variation in Chthamalus recruitment is also great with at least some plots receiving recruits on each sample date in the high zone and on 10 of 13 sample dates in the mid zone (Fig. 10). No consistent annual pattern is apparent either, since dates lacking recruitment in one year had recruitment in other years (e.g., compare July 1977 to July 1978 (high zone) or November 1977 to November 1978 (mid zone)). Similar Density C h t h e m o l u s fissus: R e c r u i t m e n t 350 HIGH= O 3OO ('~ 250 oo 200 o U3 ~4 O MID= • -- . • f I Of , 1 .~V. • ~ / 10 r~ 5 J 10050 • -•" I~ M J 1977 Fig. 10. Recruitment density of S N J -. ! o ~ o gl -~g~o ~vM.j.~.a.j.,M,-M=~ 1978 1979 Chthamalusfissus in the high and mid zones from March at Taboguilla Island, Bay of Panama. 1977 to July 1979 RECRUITMENT AND COMMUNITY STRUCTURE 85 variability in space and time was seen with several of the other solitary sessile species in Panama (B. Menge, unpubl, data). Although recruitment varies over space and time in both New England and Panama, a comparison of recruitment patterns between these regions reveals two major differences. First, recruitment in New England occurred more or less regularly with the o n s e t of the warm, long-daylight season. Virtually all barnacle recruitment occurred during 2 mth in spring, mussels recruited during summer, and Fucus spp. recruited in spring and autumn. In Panama, recruitment occurred irregularly (e.g., Fig. 10; see Discussion). Second, recruitment density is higher in New England than in Panama. For example, over all sample periods, the highest per plot recruitment density of Chthamalusfissus, the species in Panama which recruited most densely, was 24 per 100 cm 2. This was at least an order of magnitude lower than average densities of S. balanoides at all levels and sites but two in New England (Fig. 1). In summary, in both Panama and New England, recruitment of the dominant sessile organisms is variable in space and time. Average recruitment density is much higher and more synchronous in New England than in Panama, however. The question to be considered next is, what are the relative contributions of recruitment patterns and other factors in determining patterns of adult community structure? CONTRIBUTION OF RECRUITMENT AND OTHER FACTORS TO COMMUNITY STRUCTURE The contribution of recruitment densit, to final adult density was evaluated by regressing density of recruits on density of adults (Fig. 11). In New England, the prediction of Semibalanus ada!t density by recruit density was significant (p ,~ 0.001) but weak, with recruitment explaining only 36°o of the variance in adult abundance (Table I1). The plot in Fig. 11 shows that most data points lie below a 45 ° line through the origin, suggesting that adult densities are most often determined by post-recruitment processes. No similar data were available for either mussels or fucoids, although the facoid data presented above also suggest a weak relationship between recruit density and abundance of adult plants. In fact, in two of three cases, recruitment and adult abundance of the fucoids seemed negatively related. In Panama, recruit density is a much better predictor of adult density for five of six species (Fig. 11, Table II). Excluding C.fissus, recruitment explains from 55 to 97°~o of the variation in adult density of solitary sessile bivalves and barnacles (66.1°h if all species but Chthamalus are lumped). In contrast, Chthamalus recruit density expLfins only 29% of the variation in final adult density. The apparent large effect of Chthamalus on the overall regression including this small barnacle (only 24.3~'o of the variance explained if Chthamalus is included) is due to the much greater densities this species normally reaches. These high densities are largely restricted to the high zone, however, and in terms of both biomass and percent cover, this species is a minor component of mid and low zone assemblages. I thus s u r e s t that the ecologically significant con- 86 B.A. MENGE . 2000 , I I I I A. NEW ENGLAND 1500 1000 0 0 0 500 o° E I-,, C) d 6 -500 -500 I I I I I 0 500 1000 1500 2000 1 2500 3000 c V Z ILl 10 L ' ! I B. PANAMA l-.J o Q <C ° V V D -2 -0.5 I i I 0.0 0.5 1.0 .l 1.5 2.0 RECRUIT DENSITY (no./O.01 rn 2) Fig. i I. Relation between density of recruits a-.d density of adults of solitary sessile animals on rock.'/shores in (A) New England and (B) Panama. A. Semibahmus balanoides: data are from six study sites over time periods ranging from i to 4 yr. Recruit numbers are counts taken in May or J,me ofeach year from cleared 10 x 10 cm plots in experiments studying community development. Adult numbers are counts from the same plots in autumn, after settlement, growth, and predator activity had ceased. See text and Table 11 for further details, i~. Data for five species (see text and box in figure). Recrui: numbers are average number of recruits/plot/sample date (14-21 dates over 3 yr). Adult numbers are number/plot at the end of the experiment. Plots at both New England and Panama vary in the presence or absence of consumers and abundance of competitors and thus in influence of post-recruitment mortality. clusion from these data is that recruitment density explains a high proportion of the variation in adult densities in the actually or potentially dominant sessile solitary invertebrates in Panama. This contrasts strongly to the situation in New England, where recruitment density of Set,,libalanus explains a low proportion of variation in adult density. RECRUI'i MENT AND COMMUNITY STRUCTURE 87 TABLE 11 Linear regressions (y = a + bx) between densities of recruits (abseissal~ and adults (ordinate) in New England and Panama. p ,~ 0.001 for all regressions. The sum of adults and recruits of all five or six species in each plot was used in "overall" regressions. In Fig. ! 1B, the data for each species are plotted separately to retain the species-specific relationships. i Region New England Panama Species Semibalanus balanoides Balanus inexpectatus Chthamalusfissus Tetradila panamensis Catophragmus pilsbo'i Chama echinata Ostrea paimula Overall: (without C.fissus) (with C.fissus) o b F - 20.8 - 0.84 20.1 - 1.34 - 0.18 - 3.64 - 1.55 0.36 4.01 0.93 6.38 4.05 7.16 3.74 115.o 100.0 26.8 78.8 2092. ! 380.8 78.8 0.358 0.611 0.290 0.553 0.971 0.858 0.552 205 64 64 64 64 64 64 - 0.40 2.16 4.83 0.91 123.8 21.3 0.661 0.243 64 64 r2 I'/ T~BL~ I!1 Stepwisc multiple regression analysis of New England experiments to determine the percent of variance in the abundance ofeach dominant sessile organism explained by recruitment, predation, herbivores, competition for space, other biotic factors, and the physical environment. ( + ) and ( - ) indicate that the independent variable had a positive or negative effect on the dependent variable, respectively. A dot i n d i c a e s that no statistically significant association excists between the relevant variables. Ind. var., indicator variable; no.. number m 2. Dependent variables Independent variables M vtilus edulis Semibalam~s balanoides Ind. var. Recruitment S. balanoides M. edulis Fucus spp . . Predation Herbivores Littorina spp . . Tectur~ i¢studinalis (Muller) Competition Physical environment Tide height Waves Other biological factors A N O V A summary Source Regression Residual F p Multiple R 2 11.2 ( + ) . . 25.7 ( - ) No. . • . 2.6 ( + ) • • 2.4 ( - ) • . 19.2 ( - ) . . . . . d.f. MS 3 14.948 223 0.079 i 89.9 ~ 0.00 i 0.715 • . 34.7 ( - ) 25.6 ( - ) d.f. MS 3 15.663 287 0.098 159.4 ~ 0.001 0.621 18.0 ( - ) . . . • . . . Fucus spp. 78.0 ( + ) . 1.0 ( + ) • . . d.f. MS 2 28.945 288 0.053 542.6 < 0.001 0.789 d.f. MS 2 0.018 288 0.002 7.59 < 0.001 0.043 88 B.A. MENGE What is the relative importance of recruitment in relation to other variables in explaining a statistically significant amount of community variation ? In New England, stepwise regression analysis on each community dominant suggests that predation and competition each accounted for about 269/0 of the variance in Semibalanus abundance, while recruitment of this species accounted for about 11.2% of the variance (Table III). Use of recruit number for Semibalanus rather than indicator variables changes the proportions of variance explained somewhat but retains their relative ranking, lending confidence in the use of indicator variables. Competition accounted for most of the variance (789/o) in Mytilus. Although other factors contributed statistically significant amounts of variance in these species and in Fucus, all these percentages were ecologically trivial (2.6% or less; Table Ill). Thus, this analysis suggests that predation and competition were the major factors and recruitment was a minor factor influencing abundance of the dominant sessile animals in the New England community. In Panama, stepwise multiple regression explained a rather high proportion of the variance for each community variable, ranging from about 45 % for Chthamalus to 87 for Chama (Table IV). Partitioning these variances among the statistically significant TABLE IV Stepwise multiple regression analysis of Panama experiments to determine the percent of variance in the abundance of each dominant sessile organism explained by recruitment and consumers. ( + ) or ( - ) indicates that the variables were positively or negatively correlated, respectively. A dot indicates that no statistically significant association exists between the relevant variables. Independent variables Dependent variables Chthamalus fi~sus Recruitment C. fissus B. inexpectatus O. palmula C. echinata Consumers Molluscan herbivores Predaceous gastropods Large fishes Small fishes and crabs Physical environment Tidal height Substratum complexity ANOVA summary Source Regression Residual F p Multiple R 2 Balanus inexpectatus 38.7 ( + ) ¢.. 9.1(-) 61.2(+ ) • MS 875.04 32.22 27.16 ,~0.001 0.454 d.f. 3 60 MS 170.50 2.26 75.47 .~0.001 0.780 . 86.8 ( + ) 2.! ( - ) . 8.2(-) . • d.f. 2 61 Chama echinata Foliose algae t 68.1 ( + ) 1.9(-) 8.4 ( - ) Ostrea palmula d.f. MS 3 79.02 60 1.58 50.15 0.00 ~ 0.70! . d.f. MS 1 299.86 62 0.74 405.96 < 0.001 0.865 . 2.0 ( + ) 11.9 ( - ) 50.3 (-) d.f. MS 3 1.638 60 0.032 38.62 ~ 0.001 0.642 RECRUITMENT AND COMMUNITY STRUCTURE 89 predictor variables suggests that recruitment explains the greatest amount of variance in the abundance of each animal species (39 to 87~o, depending on st~ecies), while level on the shore explains the greatest proportion of variance in cover of !i?01iosealgae (50~o ; Table IV, foliose algae are most abundant in the low zone). Consumers are generally the second most important factors by this analysis, explaining a total of about 8 to 12~o of the variance. DISCUSSION This analysis suggests that recruitment accounts for a higher proportion of community variation in Panama than in New England. In addition, the recruitment data show that, despite considerable local variation, sessile dominants in New England tend to have high recruitment while sessile dominants in Panama tend to have low recruitment. Two major questions need resolution. First, what are the causes and consequences of variation in recruitment? Second, does the statistical analysis necessarily mean that recruitment is the most important factor determining community structure in Panama? Although the two questions are tightly interwoven, I will address each in turn. COMPARISON BETWEEN THE TWO SYSTEMS" CONSEQUENCES AND CAUSES OF RECRUITMENT VARIATION What are the community consequences of such differences in recruitment density? Examination of rates of change in prey abundance during community development suggests that, compared to New England, colonization rates of sessile animals in Panama were low (Menge et al., 1986a). At Taboguilla Island, mean total cover of barnacles plus bivalves in low zone treatments from which all consumers had been excluded (total exclusions) was 15, 45, and 55 ~o after 12, 24, and 36 mth, respectively. In New England, total cover of barnacles plus bivalves in mid and low zone total exclusions was 90-100~o after 2 to 6 mth. Assuming that 100~ cover would be achieved in Panama after 5 to 6 yr (60-72 mth) of predator exclusion, the rate of prey increase in New England is at least an order of magnitude higher than that at Taboguilla Island. Similarly rapid animal colonization rates were observed in, or can be inferred from studies in Washington state (Dayton, 1971), Scotland (Connell, 1961a,b), New Zealand (Paine, 1971), and Australia (Un6erwood et al., 1983). What is the cause of such great differences in rates of increase in abundance of sessile species ? Potential causes for organisms with planktonic larvae (such as barnacles and bivalves) include: (1) low production of larvae per unit area of shore by adults, (2) low rates of survival oflarvae in the plankton to the settlement stages, (3) low rates of return to the shore due to unfavorable currents, (4)low rates of survival from settlement to recruitme~lt stages (i.e., low recruitment), (5)slow growth rates after recruitment, (6) high mo~tality of juveniles, (7) high mortality of adults, or (8) some combination of 1-7. 90 B.A. MENGE The available information suggests that differences in rates of increase in prey abundance may be due to all factors but slow growth rates. Although individual growth of sessile prey was not quantified, recent recruits of B. inexpectatus, C. echinata, and Ostrea iridescens were observed to grow rapidly. Basal diameters or shell lengths of 2 cm were commonly reached within a month after metamorphosed individuals were first observed (B. Menge, pers. obs.). Larval production is likely to be low in Panama since the abundance of adult barnacles and bivalves is extremely low compared to temperate sites. For example, maximum barnacle densities in New England in late summer at sites of intermediate wave exposure were 1850 per m 2, or 119 x greater than at TaboguiUa Island in the late dry season with intermediate wave exposure (11.9 and 15.6 per m 2 in the low and mid zones, respectively; Lubchenco et al., 1984; Lubchenco & Menge, 1978). Similarly, bivalve densities in New England were 654 x greater than those observed at Taboguilla (86,900/m 2 vs. 132.8/m2). Second, although there is no available information on mortality in the plankton, in Panama such loss rates are probably at least similar to, and more likely greater than those in New England and other temperate sites. Planktivores, both fishes and invertebrates, are abundant and present year round in Panama, (B. Menge, pers. obs.), while in New England, planktivores are abundant mostly in summer, but vary from scarce (most of the time) to very abundant (rarely). The role of larval transport offshore has not been determined at either site. In Panama, dry season (December to May) tradewinds blow offshore (Glynn, 1972) and probably transport larvae away from shore for about 6 mth of the year. In the wet season, wind speed is low and direction is variable (Glynn, 1972) and removal of larvae by wind-generated currents may be less. Sutherland (1990b) has pointed out, however, that the North Equatorial Current begins in this region of the East Pacific, and that this large-scale physical oceanographic process may continually drain larvae of benthic organisms from this region to the western Pacific. In New England, winds blow both on- and offshore throughout the year, suggesting that losses due to larval transport away from shore may not be a chronic problem. Third, as documented above, recruitment success in Panama was much less than that observed in New England. The highest density of B. inexpectatus recruits recorded in Panama was 400/m 2 while the highest density of S. balanoides recruits observed in New England was 123,200/m 2 (Menge, 1978b), a 308-fold difference. Similar differences exist for the other dominant species in each region. What causes such great differences in survival to recruitment? Two biotic factors differing between New England and Panama which could influence survival between settlement and recruitment are predation and biotic disturbance. In New England, limpets, chitons, urchins and all other grazers except Littorina spp. ( -- potential sources of biological disturbance) are scarce or absent, predator diversity is relatively low, and consumer activity is highly seasonal. In Panama, grazers are abundant, predator diversity is high, predators include groups RECRUITMENT AND COMMUNITY STRUCTURE 91 uncommon in New England (e.g., omnivorous fishes), and consumer activity is aseasonal and incessant (Menge et al., 1986a,b). Does this different consumer regime have an influence on recruitment? Although pseudoreplication precludes use of inferential statistics in attributing causation to different treatments, parametric statistics may be used to determine whether or not plots differ (S. Overton, pers. comm.). For instance, in the mid zone, Chthamalus recruitment is lower in the presence than the absence of molluscan grazers, and in the low zone is low regardless of consumer presence or absence (Fig. 8; ANOVA; significant interaction between molluscan grazers and level on the shore, F = 23.2, 1,40 d.f., p < 0.0005, significance level adjusted with Bonferroni correction in this and all other cases). Similarly, Balanus recruitment is low in the presence of predaceous gastropods (ANOVA; F = 5.3, 1,40 d.f., p < 0.05), and is higher in the low zone than in the mid (F = 70, 1,40 d.f., p < 0.0001). Ostrea edulis recruitment is lower in the presence than in the absence of predators in the mid zone, and like Chthamalus, is low regardless of the presence or absence of predators in the low zone (predaceous gastropod x level on shore interaction is significant; F = 10.7, 1,40 d.f., p < 0.05). Finally, Chama recruitment is highest in the low zone (F = 75, 1,40 d.f., p < 0.0001), and is generally low in the presence of consumers (significant interactions between molluscan grazers, predaceous gastropods, and crabs, and between fishes and level on shore). Thus, in the zones in which abundances of these four sessile species is greatest, recruitment is lower when predation and biotic disturbance are high. Low settlement, or even failure to settle (Chthamalus, Ostrea) may keep densities low in zones of low abu,adance of these animals. Finally, survival of both metamorphosed individuals and adult prey was always low unless predators were excluded (Menge et al., 1986a,b). The intensity of this predation pressure is suggested by reduction in abundance of sessile prey after removal of the cages which had protected them from piscine and invertebrate predators. Four-year accumulations of large prey (mostly Chama) were reduced by about 43~o within a month of exposure to predators. Further decrease in cover of these prey was slower, probably because the remaining prey were large (7-8 cm diameter) or nearly flush with the rock surface and thus hard to crush. Small prey (mostly Chthamalus) exposed to predators were completely eliminated within 1 to 4 days (Menge et al., 1986b). In summary, low rates of prey colonization in Panama seem due to several factors, including low recruitment rates, low survival of juvenile and adult prey, low production of larvae, and possibly low numbers of competent larvae for settlement. Predation, biological disturbance, lack of settlement, and possibly seasonal transport of larvae away from shore appear to be the major mechanisms influencing these rates. MULTIFACTORIAL REGbLATION OF COMMUNITY STRUCTURE: RELATIVE IMPORTANCE OF RECRUITMENT In apparent support of previous suggestions (Connell, 1985; Gaines & Roughgarden, 1985; Holm, 1990; Raimondi, 1990; Sutherland, 1990a,b), the analysis indicates that 92 B.A. MENGE when recruitment density is low it is a major cause of adult density. However, this conclusion needs qualification in at least four ways. 1. Multiple causation First, recruitment is not the only factor, or even necessarily the primary factor structuring these communities. For example, in New England, experiments analyzing the role of a single factor such as recruitment would produce significant results (Table III). This might lead one to conclude that recruitment was the most important factor in organizing the study system. This conclusion would obviously be equivocal in the absence of tests of the role of other factors. Despite great differences in food web complexity and structure between New En$md and Panama, in both regions variation in community structure depended on three or more factors, including predation, competition, recruitment, and level on the shore. However, the analyses suggest that the rank order of these factors differs between regions. Biotic interactions explained the most variance in New England, while recruitment and level on the shore explained the most variance in Panama. That other factors also influence community structure in each region underscores the importance of emphasizing (and analyzing) simultaneous, multiple causation in the regulation of community structure. 2. Indirect VS. direct causation Second, even the moderately complex conclusion that several factors structure the community may be only a partly satisfactory interpretation of community dynamics. Even complex experimental designs cannot always include all possible causal factors. Moreover, the potential role(s) of indirect factors is difficult to both anticipate and evaluate (e.g., Bender et al., 1984; Abrams, 1987; Yodzis, 1988). For example, as indicated above, low recruitment in Panama may depend both directly and indirectly on predation operating at several stages in community development. By holding adult prcv to low abundances, predators could indirectly reduce larval production. Cohorts of larvae in Panama may thus be small in comparison to those in New England. Once larvae are in the plankton, their abundance may be depleted further by planktivores (e.g., Gaines & Roughgarden, 1987; Olson & McPherson, 1987) as well as offshore currents. The observed presence of an abundant, multispecific assemblage of planktivores suggests that mortality from planktivory could be high at Taboguilla Island. Moreover, offshore movement of surface waters during the dry season, December to April, may also deplete larval abundance. Thus, low numbers of larvae available for settlement may contribute to low recruitment densities. Predation after settlement may also have influenced prey abundance. At Taboguilla, recruitment was generally low where abundance of grazing invertebrates (limpets, chitons, snails, and sea urchins), predaceous gastropods, fishes, and crabs was high and vice versa (see above). Others have observed that mobile consumers such as limpets can reduce abundance of new recruits (e.g., Connell, 196la; Dayton, 1971; Farrell, 1988). Finally, few juvenile sessile invertebrates survived to adulthood, probably due largely to predation by fishes, crabs, and gastropods (Menge et al., 1986a,b). The few RECRUITMENT AND COMMUNITY STRUCTURE 93 surviving adults were generally restricted to holes and crevices in the rock surface, although predation (by gastropods) was relatively high even in these havens (Menge & Lubchenco, 1981; Menge et al., 1983, 1985). Hence, low recruitment may have resulted from predation at several stages during community development at Taboguiiia island and to transport of larvae offshore by currents. For instance, different sets of predators could have reduced availability of settlers both indirectly (by reduction of the abundance of both benthic juveniles and reproducing adults) and directly (by reduction of abundance of planktonic larvae). Survival of settlers to recruits was likely reduced by grazers, presumably via "biological disturbance" from incidental consumption and bulldozing. 3. Pattern, duration and strength of an effect The statistical contribution of recruitment density to community variation in New England and Panama may be enhanced by differences in the patterns and duration of recruitment, predation, and competition. That is, regression analysis can overemphasize the importance of factors which fluctuate greatly vs. those which fluctuate less, even though their ecological importances may differ little (N. Gotelli, in press, pers. comm.). As numerical abundance of sessile organisms varies through time, increases can be due only to recruitment, while decreases can be due to many factors, including predation,, competition, physical and biological disturbances, physiological stress, etc. Changes in abundance may be due to any of several types of variation. These include changes in periodicity (many or few changes per time period), duration (i.e., a change occurs over brief or long time periods), and amplitude (i.e., the magnitude of the change). For species in which recruitment occurs in brief irregular pulses, increases resemble a rapid positive "spike" in the prey abundance signal. Prey decrease, on the other hand, oh.~.n resembles a more gradual negative "decay" in prey abundance. In New England, recruitment of barnacles and mussels occurred in single large pulses per year (Fig. 12a,b). Predators in the interaction web consisted of one to six invertebrate species. High recruitment caused large increases in prey (relative to Panama), mad differences between predator treatments became distinct rapidly (Fig. 12A,B). In the absence of preda:;~on, competition led to a highly predictable result; elimination of barnacles by mussels (Menge, 1976; Lubchenco & Menge, 1978; Fig. 12A,B, - P + C treatments). Most variation in New England was thus due to predation and competition. In Panama, recruitment occurred in numerous small, asynchronous pulses per year (e.g., Figs. 10, 12C,D). Predators in the interaction web consisted of ~ 15 vertebrate and invertebrate species which differed more widely in relatively mobility, sensory acuity, and feeding mode than did predators in New England~ The effects of predation were nonlinear. That is, substantial increases in prey abundance were not observed until all, or nearly all consumers were deleted (Menge et al., 1985, 1986a,b). Thus, densities generally remained steadily low in most treatments due to predation (e.g., Fig. 12C, D + P treatments). Recruitment pulses were small, frequent and led to short-lived 94 B.A. MENGE GN LB Cove 100 600 B. [o +P+cl MY~iIlu= I o -P+c I cr~o--o A. T 500 400 :~ 300 o. o O4 w o c) \ o/ 50 2o0 6 z Ioo ill I J'-- . i , . , l l . . - ' 11w 11w ~ .lw .il..I., 11w I i ~ - . ' - ,.ll, 11w q l r 4 4 5 6 7 8 9 1011 5 7 5 6 7 8 9 1011 1974 1973 200 f C. Balanusinexpectatus 150 lB.. ° -p ÷P I 100 O4 U') d h.I O3 Z3 Z 50 o S L recr~.o, rV/ od~u,ts I L 12 .2 .. 3 5 / • 7 9 = ." . ' o - O 11 I 3 4 '.. 10 11 2 . 4 7 100 D. Chamoechinata 75 50 D ~ recruits .° 'o\ o ". " ". od, It. n .. 25 I I I I I I I I I I 2 3 5 8 9 11 1 3 4 7 1977 I I 10 11 1978 I I I 1 4 7 1979 Fig. 12. Examples of experimental results in New England (A,B) and Panama (C,D). In all panels, number of recruits is indicated by dotted lines ( .... ), number or cover of adults by solid ( ) and dashed lines ( .... ). Note that the ordinal scales for numerical abundances differ between the New England and Panama panels, and that the abscissas are not continuous (months indicated by number with January = 1). Results from single plots are shown to more clearly indicate the relation between plot-specific changes in recruit and adult abundances. A and B. Recruitment density of Semibalanus balanoides (right ordinate) and percent cover (left ordinate) of Semibalanus (~) and Mytiius ed,~lis ( - - ) in three treatments of single replicates at each of two sites. Little Brewster Cove (LB Cove) and Grindstone Neck (GN). Treatments (see box in B) are predation and competition present ( + P + C), predation absent competition present ( - P + C), and predation and competition absent ( - P - C). Predation was by the whelk Nucella lapillus and was manipulated with exclosures. Competition was between the barnacles and mussels and was manipulated by removing mussels. C and D. Number of adult and recruited Balanus inexpectatus (C) and Chama echinata (D) in the presence ( + P) and absence ( - P) of four groups of predators. Predation was manipulated by removal (mobile invertebrates) and exciosures (fishes and crabs). increases in prey abundance in all treatments but those with the lowest predation where high prey densities gradually developed (Menge et al., 1986A,B; Fig. 12C,D, - P treatments). Thus, in both systems, consumers and/or competition regulated the "frequency" of RECRUITMENT AND COMMUNITY STRUCTURE 95 the community response (i.e., level of prey abundance). Recruitment regulated the magnitude, periodicity, and synchrony of spikes in prey density. In New England, large recruitment events contributed to variation in abundance of sessile organisms, but most variation was subsequently produced by differing intensities of predation and competition in space and time (e.g., Fig. 12A,B). In Panama, uniformly high predation (low zone) or predation and physical stress (high and mid zones) held prey abundances steadily low most of the time, so that most community variation was due to changes associated with recruitment (e.g., Fig. 12C,D). Hence, statistical analysis of community variance emphasized recruitment in Panama and biotic factors in New England, even though predation was important in both systems. In other words, the pattern of an effect may be important as well as its strength. 4. Unit of abundance The unit of measurement of abundance can qualify the interpretations of a statistical analysis. In this paper, I have emphasized changes in numerical abundance of prey species, largely because recruitment is not readily quantified or is almost meaningless in other terms, such as percent cover. Yet, prey abundance is ultimately strongly dependent on individual growth and differences in size reached as adults after recruitment, especially when recruitment density is low. For example, Chthamalu~, which contributes most of the variation in recruitment in Panama (e.g., Table II), is the smallest of the prey species as an adult. Numerically, on TaboguiUa it contributes 53°/~, 47~, and 16~o of the total number of solitary sessile invertebrates at high, mid, and low levels of the shore. With respect to surface area covered, however, this small barnacle contributes less to total abundance; 33, 23, and 1~ ofthe total cover of solitary sessile invertebrates at high, mid, and low levels. Thus, in this case, numbers overemphasize species biomass. Conversely, in species of large adult body size, numbers would underemphasize species biomass. Hence, at least at the community level, statistical analysis could lead to over- or underemphasis of the actual ecological influence of a species because size/biomass and numerical density of most populations are inversely related. Hence, the statistically based conclusion that recruitment density explains the greatest proportion of community variance at Taboguilla needs qualification. Recruitment density is presumably a complex function ofmany factors, and understanding of community dynamics depends on efforts to identify, and ideally, to quantify these factors. Here, I implicate predation by a variety of agents as a major factor both directly and indirectly underlying low recruitment density. Other factors, such as offshore transport of larvae or severe desiccation are also possible in this and other benthic habitats. Still others are known (e.g., depletion of larval populations moving inshore over broad fiat benches; Gaines et al., 1985) or may be postulated (e.g., larval starvation). The ecologically robust conclusions of this analysis are that: predation and competition (in New England) or predation and factors associated with level on the shore (in Panama) directly regulate adult prey abundances, and that recruitment density directly regulates the rate of change in the adult prey community (in both systems). Further 96 B.A. MENGE insight into regulation of benthic communities will depend on efforts which simultaneously combine an understanding of the factors which regulate settlement and recruitment density with an understanding of processes and mechanisms which regulate juvenile and adult prey abundances. ACKNOWLEDGEM ENTS Field assistance was provided by many; L. Ashkenas, R. Emlet, S. Gaines, S. Garrity, C. Hibbard, M. Lubchenco, P. Lubchenco, J. Lucas, P. McKie, D. Spero, S. Strauss, B. Walker were the principal helpers. Reviews and other assistance, advice, or discussion was offered by T. Farrell, M. Hixon, A. Olson, W. Rice, B. Tissot, P. van Tamelen and an anonymous reviewer. J. Sutherland, in particular, contributed invaluable input on both the intellectual content and analysis. J. Lubchenco was a Co-PI on the Panama research, and has contributed in countless ways. The research was supported by NSF Grants, including Nos. OCE76-22251, OCE78-17899, OCE8019020, and OCE-8415609. 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APPENDIX I Structure of the New England and Panama data sets and examples of the basic statistical models used in each analysis. Percent cover normalized with the arcsine transformation, density normalized with the square root transformation. Region Dependent variables Estimator 1. Semibalanus balanoides % cover (at end of ¢xperiment) % cover A. New England 2. Mytilus edulis 3. Fucus spp. 4. Chondrus crispus Independent variables Recruitment: 1. S. balanoides 2. M. edulis % cover 3. Fucus spp. ~o cover 4. PREDATION Herbivores: 5. Littorina spp. Estimator Range of values No./100 cm 2 or high low 0-1,643 +1 - 1 high low high low present absent +1 - 1 +1 - 1 +1 - 1 present absent +1 - 1 RECRUITMENT AND COMMUNITY STRUCTURE 99 APPENDIX I (continued) Region Dependent variables Estimator Independent variables 6. Acmaea testudinalis 7. INTERSPECIFIC COMPETITION Physical factors: 8. Waves 9. Height on shore 10. Inclination of shore 11. Cobble scour Other biotic factors: 12. Algal whiplash 13. Canopy barrier Estimator Range of values present absent present absent + + - 1 1 1 1 severe to calm high, mid, low 1 to 5 1, 2, 3 horizontal or sloping vertical present absent + + - 1 1 1 1 present absent present absent + + - 1 1 1 1 Model: S. balanoides, M. edulis, Fucus spp., C. crispus = constant + a(S. balanoides recruitment) + b(M. edulis recruitment) + c(Fucus recruitment) + d(predation) + e(Littorina spp.) + f(A. testudinalis) + g(interspecific competition) + h(waves) + i(height on shore) + j(inclination of shore) + k(cobble scour) + l(algal whiplash) + m(canopy barrier) B. P a n a m a 1. Chthamalus fissus 2. Balanus inexpectatus 3. Ostrea palmula 4. Chama echinata 5. Foliose algae density at end of experiment density at end of experiment density at end of experiment density at end of experiment % cover Recruitment: 1. C.fissus Ave. no./2500 per sample cm 2 0-604 2. B. inexpectatus Ave. no./2500 cm 2 per sample 0-11 3. O. palmula Ave. no./2500 cm 2 per sample 0-11 4. C. echinata Ave. no./2500 cm 2 per sample 0-26 present absent present absent present absent present absent + + + + - mid low + 1 - ! Consumers: 8. Herbivores Molluscs 9. Predaceous Gastropods 10. Large fishes 11. Small fishes & crabs Physical environment: 12. Level of shore 1 1 1 1 1 1 1 1 1O0 B.A. MENGE APPENDIX I (continued) Region Dependent variables Estimator Independent variables Estimator Range of values 13. Ave. depth of substratum 14. Coefficient of variation of 13 15. % of substratum exposed to fast-moving consumers no. cm below surface plane (SD/~) • 100 42-116 Percent 55-95 1.4-7.7 Model: C.fissus, B. inexpectatus, O. palmula, C. echinata, foliose algae = constant + a(C.fissus recruitment) + b(B. inexpectatus recruitment) + c(O. palmula recruitment) + d(C. echinata recruitment) + e(herbivorous molluscs) + f(predaceous gastropods) + g(large fishes) + h(small fishes and crabs) + i(level of shore) + j(% of exposed substratum)