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MakingMountainsoutofBarnacles:TheDynamicsofAcornBarnacleHummocking

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Making Mountains out of Barnacles: The Dynamics of Acorn BarnacleHummocking

Mark D. Bertness; Steven D. Gaines; Su Ming Yeh

Ecology, Vol. 79, No. 4. (Jun., 1998), pp. 1382-1394.

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Ecolng~ ,79(4). 1998. pp. 1382-1394 G 1998 by the Ecological Society of >\merica

MAKING MOUNTAINS OUT OF BARNACLES: THE DYNAMICS OF ACORN BARNACLE HUMMOCKING

MARK D. BERTNESS,' STEVEN D. GATNES,? A N D SU MING YEH'

!Departntent of Ecology and Evolufionaiv Biology, Brown University, Providertce, RIzode l.slrrlzd 02912 USA ZDel~artinenrof Ecology, E~~nlu t inn , and Marine Biology, L'rliversity of California,

Santa Barbara, C'A 93106 USA

Abstract. Like plants, sessile invertebrates are often morphologically modified at high densities. At high densities acorn barnacles commonly form hummocks of tall, densely packed individuals. We examined hummock development and its consequences on filter feeding and growth in the northern acorn barnacle, Sernibalunus bala~zoides.

Hummocking occurs in response to high recruitment densities and growth rates, which intensify competition for primary substrate space. In the field, hummocks are more common at low than at high tidal heights, paralleling within-site variation in recruitment and growth rates. Hummocks also are most pronounced at high-flow sites with high barnacle recruitment and growth rates.

In laboratory flume studies, particle capture rates were higher for individuals near the peak of hummocks than for solitary individuals, and were lower for individuals in the troughs between hummocks than for solitary individuals. Hummocked individuals are el- evated above the surface and are thus likely exposed to higher particle fluxes than either individuals between hummocks or solitary individuals. These patterns in particle capture closely match patterns of barnacle shell growth in the field. The shells of hummocked individuals were larger than those of solitary individuals, which were larger than the shells of individuals in troughs between hummocks. The tissue growth of solitary individuals, however, is lower than that of crowded individuals, apparently since, without neighbors, solitary barnacles must allocate more resources to structural support than do crowded individuals. Thus, crowding may benefit individual barnacles by reducing their skeletal support costs.

Most studies of crowding in sessile, space-limited invertebrates have focused on neg- ative, competitive effects. Our results, along with previous work showing that crowding may benefit northern acorn barnacles by buffering them from heat and desiccation stress and increasing reproductive output, illustrate that crowding also can positively affect sessile organisms. We suggest that interactions among most sessile, space-limited invertebrates are best viewed as a balance between negative and positive effects.

Key words: acorn barnac1e.s; density-dependent nzorfality; ,filter feeding; group benefits; Semi-balanus balanoides.

INTRODUCTION Competition for space among recruits has been par-

plant and sessile invertebrate assemblages space ticularly well studied in plants. At high seedling den-

is frequently limited, and with adults can sities mortality and morphological modifications pri-

preclude successful juvenile establishment. a result, marily associated with access to light lead to ~ r e d i c t -

recruitment is commonly restricted to sites where able self-thinning patterns (Ellison 1989). Morpholog-

adults have been killed (for plant examples see platt ically, crowded seedlings become tall and etiolated in

1975, Brokaw 1985, Bertness and Ellison 1987; for to low light levels (Sinnott I9601 and air

sessile invertebrate examples see Dayton 1971, sousa movement (Niklas 1992) within seedling canopies. he

1979). Recruitment to disturbance-generated free space plants are often structurally (Holbrook

can be high as a result of seed shadows (Ellison 1987), et al. 1991) and On neighbors for support

seed and seedling banks (Leck et al. 1989), and prop- and Bertness 1996),

agule dispersal patterns (Gaines and Roughgarden Interactions among sessile invertebrates on hard sub-

1985). In both plants and sessile invertebrates this often strate are but have not been as studied.

leads to density-dependent mortality and the morpho- Competition for space among sessile invertebrates.

logical modification of the individuals that ultimately however, is not driven by access to light, but by access coinprise adult assemblages. to their food in the water column. Density-dependent

mortality among crowded sessile filter feeders is anal-

Manuscript received 9 September 1996; revised 21 May ogous to self-thinning in plants (Wethey 19834 b, El-1997: accepted 21 June 1997. lison and Harvell 1989), and crowding has been shown

1382

GROUP BENEFITS IN BARNACLES

FIG. 1. Illustration of l o a tidal h e ~ g h t hummocklng in Setr~ibalarzits balarzo~dev Indl\ldual barnacle d~ameters In the photograph range from 0 5 to 0 75 mm

to lead to morphologically-modified mussels (Bertness and Grosholz 1985), barnacles (Barnes and Powell 1950, Wethey 1984b, Bertness 1989), and tunicates (Paine and Suchanek 1983).

When crowded, many sessile invertebrates (e.g., mussels, barnacles, and tunicates) form hummocks, or elevated areas within dense assemblages. In mussel beds, where individuals often are not directly attached to the substrate but to one another, hummocks occur when lateral crowding causes buckling and localized vertical growth of the bed (Harger and Ladenberger 1971). In these barnacles and tunicates in which in- dividuals are attached directly to the substrate, hum- mocks are localized areas of morphologically-modified individuals (Fig. 1). Like etiolated plants, individuals in these hummocks, unable to expand laterally, grow vertically, resulting in tall, cylindrical individuals. In acorn barnacles, hummocks commonly occur in strik- ingly regular spatial patterns in dense stands of recruits. In contrast to solitary barnacles, which are usually 2- 4 times wider than they are tall, the barnacles that comprise hummocks are often 4-10 times taller than they are wide and can protrude 1.0-2.5 cm above sur- rounding individuals. Like the tall, etiolated seedlings that develop at high seedling densities, the individual

barnacles that compose hummocks should be unstable without their neighbors.

Barnacle hummocks have long drawn the interest of biologists. Barnes and Powell (1950) and Wu (1980) have described barnacle hummocking; Wethey (1984b) and Bertness (1989), respectively, have examined the reproduction and survivorship of hummocked individ- uals; and Pullen and LaBarbera (1991) have examined feeding behavior within hummocks. No previous work, however, has examined the proximate causes of hum- mocks or the consequences of hummocks to individual barnacle feeding success.

Invertebrate filter feeding, in general, has also at- tracted considerable recent empirical (e.g., Merz 1984, Okamura 1984, 1992, Sebens 1984, Patterson 1984, Cancino and Hughes 1987, Best 1988, Eckman et al. 1989) and theoretical (Rubenstein and Koehl 1977, LaBarbera 1984, Shimeta and Jumars 1991) attention. The effects of crowding on filter feeding, however, have received less study. Field (Bertness and Grosholz 1985) and laboratory (Okamura 1984) studies have suggested that crowding in mussel beds can limit individual growth and particle capture rates (also see Peterson and Black 1987). Other studies, however, have suggested that hydrodynamic aspects of filter-feeding groups and

13111 MARK D. BERTNESS ET AL. Ecolog). Vol 79, Xo 1

morphology can positively influence feeding success (Carey 1983, Merz 1984, Lidgard 1985, FrCchette et al. 1989, Johnson 1990).

In this paper we explore the causes and consequences of hummocking in the northern acorn barnacle, Semi-balanus balanoides. Specifically, we: (1) quantify hum- mock development in field populations, (2) examine the proximate cause of the position of hummocks with- in dense stands of barnacle recruits, and (3) explore the consequences of hummocks on individual barnacle feeding and growth.

Hunzmock ~Zevelopnzent irz Jield populations

Our field studies were designed to quantify hummock development in S. balanoides recruits and to examine the effect of barnacle growth rates on hummocking. To do this, we followed benthic barnacles at low and high tidal heights, since barnacle growth rates decrease with increasing tidal height (Bertness 1989), and at three sites in Narragansett Bay, Rhode Island, where barnacle growth rates were known to differ substantially (Bert- ness et al. 1991, Sanford et al. 1994). The sites, Seal Cove. Mt. Hope, and Portsmouth, arc located within 5 km of each other, but are exposed to different flow regimes, which influence barnacle growth. Near-bed flow rate, which mediates the delivery of food to bar- nacles and thus affects barnacle growth, is lowest at Scal Cove, intermediate at Mt. Hope, and highest at Portsmouth (Sanford et al. 1994). Barnacle predators (predaceous snails and starfish) are very rare at these estuarine sites and account for little barnacle mortality (Bertness 1989, Sanford et al. 1994, uizyublislzed data).

At each site we: (1) followed permanent photograph- ic quadrats, and (2) quantified barnacle size and mor- phology during hummock developmcnt. In December 1992 before the annual S. balaizoides settlement (Bert- ness et al. 1991), we established ten 100-cm2 (10 X 10 cm) quadrats at high (+1.0-+ 1.3 m) and low (+0.3- i 0 . 5 m) tidal heights at each sitc. Quadrats were clcared of all sessile organisms in early January before S. balanoides recruitment began. The quadrats were photographed in the middle of April, after S. balanoides recruitment ceased. Subsequently, the quadrats were photographed every other week for the next 3 mo (ex- cept in mid-May due to poor weather conditions). From thcse photographs we counted recruits and quantified hummocking intensity by scoring 50 random locations/ photograph for the presence of hummocks.

To quantify the size and morphology of barnacles during hummock development, we measured barnacles from each site in mid-June 1992. At high (+ 1.0 t 1.3 m) and low (+0.3 i 0.5 m) tidal heights at each site wc haphazardly collected eight cobbles (20-30 cm in maximum dimension) with hummocking barnacle re- cruits. At Mt. Hope and Seal Cove we also collected eight cobbles with solitary recruits not in physical con-

tact with neighbors. At Portsmouth recruit densities were nearly twice as high as at the other sites, and solitary recruits were not available. We randomly se- lected 16 individuals on the peaks of hummocks, 16 individuals in the troughs between distinct hummocks, and 16 solitary individuals (n = 2 individuals pcr cat- egory per cobble). For each individual we measured maximum basal diameter and height with calipers un- der a dissection microscope ( l o x ) and then separated soft tissue from the shell. Shell and tissue from each individual were dried and weighed. Shell and tissue mass will be used as an indicator of barnacle growth, since all juveniles sampled were from the same settlc- ment cohort and were initially all of similar size.

Examining the effect of individual barnacle growth rates on hummocking from the photographic quadrats described above was difficult, since recruit densities and growth rates covaricd within and among sites (San- ford et al. 1994). To separate the effects of variable growth rates from variable recruit densities (Barnes and Powell 1950, Bertness 1989), we transplanted sub-strates with similar high and low recruit densities to high and low growth rate (i.e., flow) locations. In De- cember 1992 before S. balaizoi~Zes settlement, we placed 40 granite settlement surfaces (20 X 30 X 4 cm) at a high tidal height at the Portsmouth study sitc. In April. following settlement, recruits on half of the substrates were thinned to low (<1 individual/cm2) densities, and then all substrates were photographed. Half of the high and low recruit density substrates were then transplanted to a low tidal height (+0 .5 m) at Portsmouth (a high growth rate site, n = 10 high den- sity and n = 10 low density), and the remaining sub- strates were transplanted to the same low tidal height at Seal Cove (a low growth rate site, 11 = 10 high density and n = 10 low density). These substrates were photographed every other week (from April to July) and recruit densities and hummocking were quantified from the photographs as described above.

U'hat are tlze proxzmate causes qf hunznlocks?

To eliminate the role of substrate relief in nucleating hummocks (Barnes and Powell 1950). we examined the proximate causes of hummocks on ceramic settlcment tiles. We hypothesizcd that the exact locations where hummocks occurred could be triggered by either initial size differences among recruits and/or areas of high initial recruit densities. The settlement tiles were 15 X

15 X 1 cm, individually numbered, and had textured, uniform surfaces. We attached the tiles with marine epoxy at a low tidal level (0.5 m) at Seal Cove in January 1992 before S. balanoides settlement. All tiles (12 = 90 total) were photographed every other week from the first week of February through August. Sixty of these tiles were used for the flow tank feeding ex- periments outlined below. From the photographic re- cords of the rcmaining 30 tiles we chose 15 tiles with complete photographic records that showed distinct

1385 June 1998 GROUP BENEFITS IN BARNACLES

hummocks in August without hummock breakage. Us- ing the August photographs projected at actual size, we outlined the distribution of hummocks on cach tile. We then overlaid the hummock outlines for each tile on mid-March photographs and quantified initial recruit densities and sizes in what ultimately became hum- mock and nonhummock locations. For each tile, we used calipers to measure the size of 16 random recruits and counted recruit densities (in 1-cm2 sampling quad- rats) monthly in areas near the peaks of future hum- mocks and in areas between future hummocks.

Laboratory studies oJ bnrizncle ,feeding

To supplement our field studies of hummock for- mation, we measured barnacle feeding rates in a small recirculating f u m e (1 4.5 cm wide X 18 cm deep work- ing area; see Vogel 1981 for design). Small recircu- lating flow tanks like the one used in this study can have flow artifacts that are generated by interactions with the wall. This type of tank is too narrow for gen- erating uniform flow. To characterize the pattern of flow within the tank, we illuminated a narrow slit section of the tank and videotaped unhatched brine shrimp eggs illuminated in this narrow slit of light with a high-resolution video camera with a macro lens. The slit of light was generated by light projected from above the tank through a narrow slit cut in an opaque piece of plexiglass. The slit of light was focused to illuminate a section of the water column -5 mm across. The video camera took images through the side of the tank at a right angle to the illuminated slit. The field of view was approximately 1.5 cm wide X 1 cm tall, and the camera was focused on the center of the narrow illu- minated slit at different positions along the working section of the tank. This technique allowed us to follow particles (brine shrimp eggs) in a small section of the water column. By shooting images in a dark room, we could follow illuminated particles that traveled in this 0.5 X 1.5 X 1.0-cm region of the water column. The width of thc field of view (i.e., the dimension particles traveled) was measured precisely with a scale placed in the field of view prior to the measurement trials. The scale was removed from the water while particle ve- locities were measured. Velocities were calculated from the number of video frames it took individual particles to traverse the known width of the field of view. By positioning the light slit at different positions in the tank, we could characterize the flow field and test for artifacts associated with interactions with the wall of the tank.

To examine feeding success in hummocked barna- cles, we compared the relative feeding success of sol- itary barnacles to crowded barnacles from two posi- tions in hummocks: near the peak and in the trough between hummocks. Settlement tiles with barnacles were placed into the flow tank in a recessed area of the bottom so that the tops of the settlement tiles were flush with the bottom of the tank. We used brine shrimp eggs

as food and quantified capture rates from videotapes made by a high-resolution macro video camera, which shot images through the plexiglass sides of the tank. We counted the number of brine shrimp eggs captured in 20-min intervals by each barnacle. We ran feeding experiments at two flow tank velocities (8 and 15 cmls, measured in the centcr of the tank with a Marsh McBirney electromagnetic current meter). For all feed- ing experiments, we used only barnacles within 3 cm of the midline of the flume, even though wall effects on velocity and capture rates were restricted to areas within 1 cm of the walls (see Results). For each treat- ment combination (barnacle position and flow speed) we quantified particle capture for 20 individuals. Be- fore each feeding trial, brine shrimp densities were re- set by filtering the water to remove all brine shrimp eggs, and the water was then restocked with a fixed mass of eggs (10 g). There was little depletion of brine shrimp eggs in the 20-min feeding trials.

Hunznzock d e ~ , e l o p m e ~ ~ t in $eld populations

S . balanoides recruitment varied markedly both within and among sites (Fig. 2). Densities were ana- lyzed with a repeated-measurement ANOVA (site and tidal height were fixed main effects and sample date was the repeated factor) on log-transformed densities. Neither site nor tidal height had significant main effects on recruit densities, but there was a highly significant interaction between site and tidal height (Table 1). Re-cruit densities also diffcred significantly over time (Ta- ble I), and there was a highly significant interaction between sample date, site, and tidal height.

Significant interactions in the recruitment data (Table I) were largely caused by strong density-dependent mortality. Initial recruit densities were similar at high and low tidal heights at each site (P > 0.10, Scheffe test each site), but varied markedly among sites (Fig. 2). Initial recruit densities were nearly twice as high at Portsmouth as at either of the other sites. Over time. however, recruit densities declined markedly at all sites due to intense competition for primary substrate space (Fig. 2). The magnitudc of the decline scaled with ini- tial density. Declines were greatest at Portsmouth, where growth rates and initial recruit densities were both high (Sanford et al. 1994). No low intertidal re- cruits survived, and < I % of the high intertidal recruits survived at Portsmouth over the summer (Fig. 2). At Mt. Hope, where initial recruit densities and growth rates were intermediate, no low intertidal recruits sur- vived the summer, but high intertidal recruit survivor- ship was >60% (Fig. 2). Finally, at Seal Cove where barnacle growth rates and recruitment wcre lowest, the survivorship of S. balanoides recruits at both high and low tidal heights was >50% (Fig. 2). Thus both within and among sites, barnacle survivorship was the highest in habitats with the lowest growth rates and lowest

MARK D. BERTNESS ET AL Ecology, Vol. 79. No. 4

-E-- HIGH INTERTIDAL ZONE

-+- LOW INTERTIDAL ZONE

PORTSMOUTH

MAY JUNE JULY MAY JUNE JULY

FIG.2. Se~nibalanus balanoides recruit densities (left) and hummock development (percentage of individuals in hutntnocks, right) at the study sites. For each site. data are given for high and low intertidal zone quadrats. Each point represents the mean i I s~ of 10 quadrats. Error bars are not shown when smaller than the symbol. See Tables 1 and 2 for statistics.

recruit densities. Most mortality was clearly the result of intraspecific crowding leading to hummocks that subsequently broke free from the substratum (Barnes and Powell 1950, Wethey 1983b, Bertness 1989). Bar-nacle mortality due to predators and physical stress was

TABLE1. Repeated-measures ANOVA (five sample dates) of log-transformed densities of Semibalanu~ balanoides at three sites and two tidal heights.

Source df F P

Site 2, 2 2.2 0.32 Tidal height 1, 2 5.5 0.14 Site X tidal height 2, 42 10.1 0.0003

Dates 4, 168 36.6 <0.0001 Dates X site 8, 2 1.9 0.20 Dates X tidal height 4, 2 1.1 0.40 Dates X site X height 8. 168 28.7 <0.0001

rare at all sites at the tidal heights examined (see also Bertness 1989, Bertness et al. 1991, Sanford et al. 1994).

The frequency of hummocks within and among sites in the recruitment quadrats (Fig. 2) was analyzed with a site X tidal height repeated-measures ANOVA on arcsine-transformed hummock percentages. Qualitative patterns were identical to those for recruit density (Ta- ble 2). The main effects of site and tidal height were not significant, whereas the site by tidal height inter- action was highly significant. Hummock frequency also varied significantly over time, and there was a highly significant datc X site X tidal height interaction.

Interactions in the frequency of hummocking data (Table 2) were a result of density-induced hummock- ing, varying in extent and timing among sites and tidal heights. Hummocking was more pronounced at low than at high tidal heights at all sites (Fig. 2). Among

1387 June 1998 GROUP BENEFITS IN BARNACLES

TABLE2. Repeated-measures ANOVA (five sample dates) on arcsine-transformed percentage cover of hummocks at three sites and two tidal heights.

Source d f F P

Site 2, 2 2.1 0.32 Tidal height 1, 2 5.5 0.14 Site X tidal height 2, 42 10.1 0.0003

Dates 4, 168 36.6 <0.0001 Dates X site 8, 168 1.9 0.20 Dates X tidal height 4, 168 1.1 0.40 Dates X site X height 8, 168 28.7 <0.0001

sites, hummocking was most pronounced and occurred earliest at Portsmouth, where recruitment and growth were the highest, and least pronounced at Seal Cove, where both recruitment and growth rates were low (Fig. 2). At Portsmouth, hummocks were common at both

--B- SEAL COVE LOW DENSITY

-8- SEAL COVE HlGH DENSITY

+ PORTSMOUTH LOW DENSITY

--O- PORTSMOUTH HlGH DENSITY

a MAY JUNE JULY

FIG. 3. Results of S. btrlanoides recruit transplant exper- iment where high and low (<1 recruit/cm2) recruit densities were transplanted to high and low growth rate sites (Ports- mouth and Seal Cove, respectively) to examine the effect of growth rates on density-dependent mortality and hummock- ing. Each point represents the mean f 1 SE of 10 samples. Error bars are not shown when they are smaller than the symbols. See Tables 3 and 4 for statistics.

TABLE 3. Repeated-measures ANOVA on arcsine-trans-formed survivorship patterns for transplanted barnacles at two recruit densities and two sites: Portsmouth (high growth rates) and Seal Cove (low growth rates).

Source df F

Site 1, 1 1.4 Density 1, 1 1.7 Site X density 1. 21 54.9

Date 5 , 1 0 5 225.4 Date X site 5, 105 0.9 Date X density 5, 105 1.7 Date X site X density 5, 105 14.5

high and low tidal heights in May, at least 1 mo before they were observed at other sites (Fig. 2). Early hum- mocking and hummock breakage, however, resulted in >95% mortality of Portsmouth recruits by July. At Mt. Hope and Seal Cove, hummocking began much later, and with the exception of low tidal heights at Mt. Hope, hummock breakage was minimal. At Seal Cove, hum- mocks were not observed until late June and never led to marked mortality (Fig. 2).

Transplanting plates with high and low recruit den- sities to high and low growth rate sites demonstrated the importancc of growth rates in barnacle hummock- ing and subsequent mortality (Fig. 3). Recruit survi- vorship and hummock frequency data (both arcsine- transformed) from the transplant experiment were an- alyzed with repeated-measures ANOVA, with site and recruit density as main effects and sample datcs the repeated factor. Analyses of both the survivorship and percentage cover of hummocks data yielded the same conclusions (Tables 3 and 4). There were no significant effects of site or recruit density, but there was a highly significant interaction between these two factors. In addition, both the survivorship and percentage cover of hummocks varied significantly among sample dates and had a significant sample date X site X density interaction. The significant interactions, including site and recruit density, followed from the different timing of hummock formation; the ensuing increases in mor- tality followed from the different combinations of site and initial density treatments. The survivorship of sol- itary recruits was >70% and similar among sites. In

TABLE 4. Repeated-measures ANOVA on arcsine-trans-formed percentage cover of hummocks for transplanted barnacles at two recruit densities and two sites: Portsmouth (high growth rates) and Seal Cove (low growth rates).

Source df F P s i te 1, 1 0.9 0.51 ~~~~i~~ 1, 1 17.9 0.15 si te x densit" 1. 27 27.4 0.0001

DateDate site

5, 135 5, 135

201.51.0

0.00010.50

Date XDate densitysite density 5, 135 5 , 135

0.8 476.4

0.58 0.0001

1388 MARK D. BERTNESS ET AL. Ecology. Vol. 79. No. 4

X SOLITARY HUMMOCK TOPS [? BETWEEN HUMMOCKS W

HIGH LOW HIGH LOW HIGH LOW

TIDAL HEIGHT

FIG.4. Morphology and size of S. balanoides recruits at low and high tidal heights at the study sites in early June. Data are given for solitary individuals (not in physical contact with neighboring barnacles). individuals on the tops of hummocks, and individuals between hummocks. Each bar represents the mean i1 SE of 12-16 individuals. Solitary individuals were not available at Portsmouth.

contrast, high-density transplants fared quite differ- ently at the two sites. At Portsmouth (the high growth rate site), all high- density recruits died by July. where- as at Seal Cove (the low growth rate site) >35% of the high-density recruits survived the summer (Fig. 3). Hummock development at high densities also occurred earlier and was more intense at the high-growth site, Portsmouth, than at the low-growth site, Seal Cove (Fig. 3).

Size arid rnorplzology qf recruits

Analysis of the size and morphology of recruits (Fig. 4) as a function of site, tidal height, and position (i.e., on hummocks. in troughs between hummocks, or sol- itary) was complicated because we were unable to sam- ple solitary individuals at Portsmouth. Consequently, we analyzed data from Mt. Hope and Seal Cove where we had a complete data set with a three-way (site X tidal height X position) ANOVA. Data from Ports- mouth were analyzed separately with a two-way (tidal height X position) ANOVA, considering only crowded

individuals, on hummocks and in troughs. Recruit mor- phology was quantified using a morphological index (shell heightlbasal diameter) which reflects changes in morphology (increased relative height) associated with barnacle crowding (Bertness 1989).

At Mt. Hope and Seal Cove the morphological index varied among positions, tidal heights, and sites (Fig. 4, Table 5). Individuals on hummocks were relatively taller than individuals between hummocks. whereas solitary individuals were the shortest. Morphological variation among hummocked and solitary individuals also differed among sites and tidal heights (significant site X position and tidal height X position interactions, Fig. 4). There was much greater morphological mod- ification at Mt. Hope than at Seal Cove, although the rankings (hummocks > troughs > solitary) were iden- tical at both sites. A similar pattern was seen among tidal heights. There were also larger differences among solitary and crowded barnacles in the low zone than in the high zone, although the ranking of morphological indices was the same.

June 1998 GROUP BENEFITS IN BARNACLES

T I I B L ~5 . Summary of ANOVA (P valucsi on barnacle size and morphology data presented in Fig. 4. Mt. Hope and Seal Cove data were analyzed wit11 a three-way site X tidal height (high or low) X location (on or between hummocks, solitary) ANOVA. Portsmouth data were analyzed separately with a two-way tidal height X location ANOVA. since solitary individuals were not available.

hforphological Source of variation df

Mt. Hopelseal Cove Site Tidal height Position Site X tidal height Site X position Tidal height X position Site X height X position

Portsmouth Tidal height Position Tidal height X position

Tissue mass of recruits was greater at Mt. Hope than Seal Cove and greater at low than high tidal heights (Fig. 4, Table 5). In addition, tissue mass of recruits on hummocks was greater than recruits bctween hum- mocks, which were greater than solitary recruits. There wcre two significant interaction terms in the analysis. The tissue mass of solitary, hummock, and trough in- dividuals varied among tidal heights, but only in mag- nitude, not direction. The diffcrences among the three types of individuals were larger in the low zone than in the high zone. There was also a significant three-- way interaction between type of individual, tidal height, and site. Again, the pattern of tissue mass among barnacle types was consistent (hummocks > troughs > solitary), but the magnitudes of differences varied significantly among site and tidal height com- binations.

At Mt. Hope and Seal Cove, site, tidal height, and position of individual all directly affected shell growth, with only one significant interaction--tidal height by location (Fig. 4, Table 5). As was the case for tissue mass, shell mass was higher at Mt. Hope than Seal Cove and greater at lower tidal heights (Fig. 4). In contrast to tissue mass, however, although individuals near the peaks of hummocks had the greatest shell mass. solitary individuals had greatcr shell mass than individuals between hummocks (Fig. 4, Table 5). This suggests that solitary individuals invest more in struc- tural support costs than crowded individuals, which share walls with neighbors (also see Wu 1980). The tidal height X position of individual interaction was due to differcnces in magnitude, not pattern. The rank- ing of the three types of individuals was consistent among tidal heights, but the magnitude of shell-mass differences was larger at low than high tidal heights.

Morphological patterns at Portsmouth were quali-tatively similar to those at Mt. Hope and Seal Cove, although individuals at Portsmouth were substantially larger (2-3X) than individuals at the other sites (see

index Shell mass Tissue mass

also Bertness et a1 1991, Sanford et a1 1994). Mor- phology varied with both tidal height and position of individual, with a significant tidal height X position of individual interaction (Fig. 4, Table 5). Individuals on hummocks and at low tidal heights were taller than individuals between hummocks and at high tidal heights. As with all of the above significant interac- tions, the tidal height X position of individual inter- action was duc to differences in magnitude rather than pattern. Barnacles on hummocks were always taller than those in troughs between hummocks, but the mag- nitude of this difference was larger at low than at high tidal heights.

Tissue mass at Portsmouth was also higher at low than high tidal heights (Fig. 4), and individuals 011 hum-mocks were larger than individuals between hummocks (Table S), with no significant interaction between tidal height and position of individual. Shell mass at Ports- mouth also varied with tidal height and position of individual, with no significant interaction (Table 5). Shell mass at Portsmouth was higher at low tidal heights than at high tidal heights. They were also high- est for individuals near the peaks of hummocks (Fig. 4).

Where do I1~1nrnocks form?

Examination of small-scale patterns of recruit den- sities and sizes on settlement tiles revealed that within dense assemblages of recruits, hummocks develop in areas of locally-elevated recruit densities, and were not associated with initial patterns of recruit size (Table 6). Points that became hummock peaks 2 mo after settle- ment were locations with higher initial recruitment den- sities just after metamorphosis than points that even- tually became the troughs between hummocks. Spatial variation in the size of initial recruits, however, did not predict where hummocks would form. Recruits at points that became hummock peaks 2 mo later were similar in size to recruits at points which became

--

1390 MARK D. BERTNESS ET AL. Ecolog). Vol 79. Yo 4

TARLF6. Patterns of initial recruitmellt densities and av- erage recruit sizes at points that subsequently became hurn- mock peaks or troughs between hummocks. Data are means and standard errors. P values are from one-way ANOVA. - --

Recruit density Recruit (no. recruits1 diameter

Point cm2) (mm)

El entual hummock peaks (11 = 75) 1 1 6 (0 26) 1 72 (0 05)

Eventual troughs betv een humlnocks ( 1 1 = 45) 9 9 (0221 1 83 ( 0 0 6 )

P < 0 0 0 0 1 P = O 0 8

troughs between hummocks. The trend toward indi- viduals that became hummocks being smaller than those that became individuals in troughs (Table 6) like-ly reflects the early effects of crowding and is just the opposite of the prediction that initial recruit size ad- vantages can trigger hummmock formation (Barnes and Powell 1950).

The measured velocities in the flow tank used showed the typical increase in velocity as function of distance from the bottom of the tank (Fig. 5). All of the velocity depth profiles were indistinguishable from one another, except for the single profile taken 0.5 cm from the wall of the tank. This profile had a similar pattern of velocity with depth, but the velocities were lower than the other profiles at all depths. By following barnacle feeding only on individuals within 2 cm of the center of the tank, we could therefore avoid most artifacts due to interactions with the tank wall.

Feeding rates of barnacles in different locations (on hummock peaks, in hummock troughs, or solitary) dif-

2 c 1

---- 3 5 7 9 1 1 1 3 1 5

Distance from Bottom (cm)

FIG.5. Particle velocity as a function of position in the recirculating flume. Particle velocities were measured from video images of brine shrimp eggs moving through the field of view. Each curve represents the obscr\led profile of ve-locity with depth at different distances from the flow tank wall. All curves are statistically indistinguishable except the 0.5-cm curve, which has consistently lower velocities at all depths.

,

O Hummock Peak Solitary Between Hummocks

FIG.6. Feeding rates by barnacles in different locations (peak of hummock. trough between hummock. solitary) at two velocities in a recirculating flow tank. See Tablc 7 for analyses.

fered substantially (Fig. 6 , Table 7) at both velocities (8 and 15 cmis). Barnacles on hummocks had the high- est capture rates, barnacles in troughs between hum- mocks had the lowest capture rates, and solitary bar- nacles had consistently intermediate capture rates (Ta-. ble 7).

Two factors affected the differences in barnacle feed- ing rates. The first is differences in velocities. Water velocities were easily visualized by examining the ve- locities of brine shrimp eggs in our feeding trials. The larger barnacles at the peaks of hummocks fed in water moving at consistently higher velocities than barnacles in the troughs between hummocks. This is partly due to their larger size, which elevates their feeding struc- tures into more rapidly flowing water (see Fig. 5), and partly due to patterns of flow over the hummocks which we observed in the flow tank. Water tended to skim across the peaks of the hummocks at both flow veloc- ities, creating extremely low velocities between the hummocks (see Denny 1988 for a discussion of this phenomenon). As a result, particle fluxes reaching bar- nacles in troughs between hummocks appeared to be consistently small. The second factor contributing to the differences in barnacle feeding rates was diffcr-ences in the size of the cirral nets of the different bar- nacles. We measured the cirsal net lengths of 20 ran-domly chosen individuals of each type from our video records and found that barnacles on hummocks had the

TABLE7 . Analysis of capture rateu from laboratory fceding trials. Results are from a two-way ANOVA with barnacle location (hunimock peak vs. hummock trough vs. solitary) and free stream velocity (8 cmis vs. 15 cmis) as main ef- fects.

Source df F P

Locat~on 2, 28 33.7 0 0001 Veloclty 1, 28 14.1 0.0008 Location X velocity 2. 28 1.9 0.17

1391 June 1998 GROUP BENEFITS IN BARNACLES

largest cirral nets (mean length = 21.2 -C 1.6 mm), followed by solitary barnacles (mean length = 19.8 + 0.7 mm). Barnacles in troughs between hummocks had significantly smaller cirral nets than either o f these two barnacle types (mean length = 15.9 +- 1.3 mm).I f cirral length is added as a covariate to the analysis in Table 7 , it explains a significant amount o f variation ( P < 0.02) , but the effects o f velocity and barnacle type (solitary or hummocked) remain significant even after removing the influence o f variation in cirral net size.

The results o f this study show that while high re- cruitment in acorn barnacles can lead to heavy mor-tality, crowding can also benefit barnacles by enhanc- ing feeding success and minimizing skeletal support costs. Thus, our results suggest that interactions among high densities o f acorn barnacle recruits are best thought o f as a balance between positive and negative interactions. The relative strength o f the beneficial and detrimental effects o f crowding, however, must be mea- sured to determine the net effect on barnacle success.

Acorn barnacle benthic dynamics

High recruitment and growth rates both intensify competition for primary substrate space and can lead to crowding, hummocking, and mass mortality in bar- nacles (Barnes and Powell 1950, Wethey 1984b, Bert-ness 1989). Our study emphasizes the role that indi- vidual growth rates can play in causing barnacle hum- mocking and density-dependent mortality. At low tidal heights, barnacles are submerged longer and have high- er growth rates than at higher tidal heights (Sanford et al. 1994). As a result hummocking, and the subsequent mortality o f hummocked individuals, is more common at low than high tidal heights (Fig. 2 ) . Among sites, hummocking is also more common and pronounced at sites with higher growth rates. O f our field sites, bar- nacle growth is higher by nearly an order o f magnitude at Portsmouth than at either Mt. Hope or Seal Cove (Bertness et al. 1991, Sanford et al. 1994). Portsmouth is located at a constriction in Narragansett Bay and is thus exposed to high free-stream flow speeds that in- crease the delivery o f food to filter-feeding barnacles (Sanford et al. 1994). As a result o f high growth rates, barnacle crowding, hummocking, and mortality are dramatically elevated at Portsmouth in contrast to our other study sites (Figs. 2 and 3) .

High recruitment densities are also clearly necessary for barnacle hummocking (Barnes and Powell 1950, Wethey 1984a, Bertness 1989). At large spatial scales, oceanographic features that dictate the transport o f lar- vae are important determinants o f recruitment numbers (Roughgarden et al. 1987, Farrell et al. 1991, Gaines and Bertness 1992, Botsford et al. 1994). On smaller spatial scales larval settlement preferences for conspe- cifics (Knight-Jones 1953, Wethey 1984a), substrate-types (Chabot and Bourget 1988, Raimondi 1988) and

tidal heights (Bertness et al. 1991) are important de- terminants o f recruit densities. Small-scale hydrody- namic forces, however, can also strongly influence lar- val supply (Eckman 1983, Butman 1987). In particular, increased free-stream flow speed increases larval flux- es, leading to elevated recruitment in habitats exposed to higher velocities (Gaines and Bertness 1992).

In the absence o f other factors that can limit barnacle survival, such as predation (Paine 1966) ar.d distur-bance (Dayton 1971, Sousa 1979),predictable variation in recruitment and growth can lead to equally predict- able patterns in the benthic dynamics o f acorn barna- cles (Barnes and Powell 1950, Sanford et al. 1994). At sites with high recruitment and growth, intense com- petition for space can routinely lead to extreme crowd- ing, hummock development, and heavy mortality (Barnes and Powell 1950). In contrast, at sites with lower recruitment and growth rates, competition for space is less intense and mortality due to crowding is greatly reduced (Sanford et al. 1994). As a result, bar- nacle survivorship, reproduction, and fitness are likely often higher at sites with low recruitment and growth rates than at sites with extremely high recruitment and high individual barnacle growth rates.

Although crowding leads to high barnacle mortality in productive habitats, crowding can clearly also ben- efit barnacles. Barnacle crowding can increase survi- vorship by buffering individuals from thermal stress (Bertness 1989),increase reproductive output, since the morphology o f crowded barnacles permits higher egg- carrying capacities (Wethey 1984b),and decrease skel- etal support costs (Wu 1980). Our work suggests that crowding can also benefit individual barnacles by in- creasing feeding efficiency. Thus, in habitats where barnacle growth rates are relatively low and crowding does not lead to heavy mortality, crowding may have net positive effects on benthic barnacle populations. This is likely the case in many open-coast, high inter- tidal habitats in New England (M. D. Bertness, per-sonal observation). In these habitats, low recruitment and growth rates lead to barnacle hummocks that per- sist for at least 3-4 yr. In this situation, where hum- mocking does not lead to high mortality, the benefits o f crowding likely outweigh the costs.

Hummock development

Our examination o f hummock development in field populations revealed that hummocks occur under high- recruitment and high-growth conditions. This does not explain, however, why hummocks occur at specific lo- cations within dense stands o f recruits. Barnes and Powell (1950) suggested that barnacles on elevated ar- eas on settlement surfaces have a feeding advantage over their neighbors, which leads to increased growth and the nucleation o f a hummock, This scenario, how- ever, does not explain the common observation that hummocks form as readily on uniform surfaces as they do on irregular surfaces with topographic relief (Barnes

1392 MARK D. BERTNESS ET AL. Ecology. Vol. 79, No. 4

and Powell 1950, Wu 1980). On small spatial scales we found that hummocks form at locations with locally elcvated initial recruit densities (Table 6). This suggests that substrate features that enhance larval settlement, such as cracks and crevices (Chabot and Bourget 1988, Raimondi 1988), gregarious settlement or settlement with conspecifics (Knight-Jones 1953, Wethey 1 9 8 4 ~ ) or hydrodynamic forces (Butman 1987) may trigger early recruit crowding (Bourget and Crisp 1975) and initiate hummock development. Once hummocks be- gin, the feeding advantage of hummocked individuals relative to their neighbors likely enhances further hum- mock development. Therefore, small differences in ini- tial settlement densities get magnificd by subsequent advantages of crowding.

Our data on hummock development also do not ex- plain the observation that hummocks often occur in relatively regular spatial patterns (Barnes and Powell 1950, Wu 1980, Wethey 1984b). Wc hypothesize that the regular spacing of hummocks is the product of feed- ing interactions among crowded individuals once hum- mocking has begun. Once a hummock is initiated, in- dividuals on hummocks have enhanced feeding success and growth in comparison to individuals between hum- mocks (Fig. 6). This positive-feedback effect on growth rates must accentuate the effects of small differences in initial recruitment densities. We suggest that feeding suppression of barnacles adjacent to hummocks leads to spatial patterns in the growth of crowded barnacles and ultimately yields regular spatial patterns in hum- mock structures.

Costs a n d benejits of barnacle crowding

Our flow-tank studies reveal that barnacles on hum- mocks have enhanced particle capture rates compared with barnacles between hummocks (Fig. 6). This is likely due to hummocked individuals being elevated above the surface and exposed to higher fluid velocities and particle fluxes than individuals between hum-mocks. We also found that solitary barnacles have high- er particle capture rates than crowdcd individuals be- tween hummocks. These patterns in particle capture are consistent with our observations of differential growth patterns in the field (Fig. 4).

Whereas most examinations of interactions in dense assemblages of sessile filter feeders have focused on competitive effects among neighbors (Buss 1979, Oka- mura 1984, Bertness and Grosholz 1985, Peterson and Black 1987), group benefits in filter feeding may not be uncommon. In the calcareous tube worm Eudistylia. Merz (1984) found that clumps of individuals modify local flow patterns and enhance particle flux to clump members in comparison to solitary individuals. In an- other polychaete species, Carey (1983) showed that polychaete tubes modify local flows and enhance filter feeding. More recently, FrCchette et al. (1989) have also shown that mussel beds increase turbulent mixing and increase food supplies to mussels in dense aggre-

gations. Phoronid neighbors (Johnson 1990) and the design of bryozoan colonies (Lidgard 1985) also have been shown potentially to enhance filter-feeding suc-cess. Shimeta and Jumars (1991) have argued on the- oretical grounds that morphologies that elevate filter feeders above the surface potentially enhance food sup- ply by exposing individuals to higher particle fluxes. We suggest that when crowding among sessile filter feeders leads to tall individuals that are elevated above the surface in comparison to solitary individuals, feed- ing advantages may be a common consequence.

In contrast to shell growth, which directly mirrored patterns of particle capture, solitary barnacles had less tissue growth than crowded individuals either on or between hummocks (Fig. 4). The most parsimonious explanation for this result is that crowded barnacles share structural support with neighbors, need to expend less energy on skeletal support, and thus have more resources for tissue growth. The shell walls of solitary barnacles are generally 2-5 times thicker than those of crowded barnacles of similar size or age (M. D. Bert-ness, unp~tblished data, also see Wu 1980). Wu et al. (1977) and Wethey (1984b) have previously argued that crowded barnacles benefit from reduced skeletal sup- port costs in comparison to solitary individuals. Similar suggestions that crowding reduces structural support costs and offsets some of the competitive costs of neighbors have been made for tunicates (Paine and Su- chanek 1983), macroalgae (Holbrook et al. 1991), and vascular plants (Harley and Bertness 1996).

Our work contributes to growing evidence that co- operative interactions are more common in sessile or- ganisms than is generally appreciated. We suggest that interactions among sessile space holders are best en- visioned as a balance between negative and positive interaction components and that whether interactions are on balance positive or negative is context dependent and controlled by local physical and biotic conditions.

We thank D. Bermudez. E. Sanford, D. Carrier. T. Goslow, and S. Swartz for field help and advice, and K. Benoit for drawing Fig. 1 . J . Bruno, G. Leonard, R. Grosherg, and two anonymous reviewers made helpful suggestions on the manu- script. Our work was supported by the Andrew Mellon FOUII- datioli and the Biological Oceanography Program of the Na- tional Science Foundation.

Barnes, H.. and H. T. Powell. 1950. The development, gen- eral morphology, and subsequent elimination of barnacle populations aster a heavy initial settlement. Journal of An- imal Ecology 19:175-179.

Bertness, M. D. 1989. Positive and negative density-depen- dent mot-tality and the population structure of Sernibcr[at~us bnla~zoidesin a sheltered bay habitat. Ecology 70:257-268.

Bertness, M . D., and A. M. Ellison. 1987. Determinants of pattern in a New England salt marsh plant community. Ecological Monographs 57: 129-147.

Bertness. M. D. . S. D. Gaines, D. Bermudez, and E. Sanford. 199 1. Extreme spatial \ ariation in the growth and repro-

1393 June 1998 GROUP BENEFITS IN BARNACLES

tfuctive output of the acorn barnacle Semihnlnnus hc~ln t~- oide.5. Marine Ecology Progress Series 75:91-100.

Bcrtness, M. D., and T. Grosholz. 1985. Population dynamics of the ribbed mussel, C;eukensia den~is.sri: the costs and benefits of a clumped distribution. Oecologia (Berlin) 67: 192-204.

Best. B. A. 1988. Passive suspension feeding in a sea pen: effects of ainbient flo\v on volume flow rate and feeding efficiency. Biological Bulletin 175:332-342.

Botsford, L. W., C. L. Moloney. A. Hastings, J. L. Largier, T. M. Powell, K. Higgins, J. F. Quinn. 1994. The influence of spatially and temporally varying oceanographic condi- tions on meroplanktonic metapopulations. Deep Sea Re- search 41: 107-145.

Bourget, E., and D. J. Crisp. 1975. Early changes in the shell form of Bnlc~nus bellanoides (I.) Journal of Experimental Marine Biology and Ecology 17:221-237.

Brokaw, N. V. L. 1985. Treefalls. regrowth, and community structure in tropical forests. Pages 53-71 in S. T. Pickett. and P. S. U7hite, editors. The ecology of natural disturbance and patch dynamics. Academic Press, New York, New York, USA.

Buss, L. W. 1979. Bryozoan overgrowth interactions-the interdependency of cornpetition for space and food. Naturc 281:475-477.

Butman, C. A. 1987. Larval settlement of soft-sediment in- vertebrates: spatial scales of pattern explained by active habitat selection and the emerging role of hydrodynamical processes. Oceanography and Marine Biology Annual Re- view 25:113-165.

Cancino. J. M., and R. N. Hughes. 1987. The effect of water no\v on growth and reproduction of Celleporellc~ 1z))alinn (L.) ( B r y o ~ o a : Cheilostornata). Journal of Experimental Marine Biology and Ecology 112:109-130.

Carey, D. A. 1983. Particle resuspension in the benthic boundary layer induced by flow around polychaete tubes. Canadian Journal of Fisheries and Aquatic Sciences 40: 301-308.

Chabot. R.. and E. Bourget. 1988. Influence of substratum heterogeneity and settled barnacle density on the settlement of cyprid larvae. Marine Biology 97:45-56.

Dayton, P. K. 197 1. Competition, disturbance and commu- nity organization: the provision and subsequent utilization of space in rocky intertidal community. Ecological Mono- graphs 41:351-389.

Denny, M. W. 1988. Biology and mechanics of the wave swept environment. Princeton University Press, Princeton, New Jersey, USA.

Eckman, J. E. 1983. Hydrodynamic processes affecting ben- thic recruitment. Liinnology and Oceanography 18:241-257.

Eckman, J. G. , C. 11. Peterson, and J. A. Cahalan. 1989. Effects of flow speed, turbulence, and orientation 011 growth of ju\enile bay scallops Argopectin ii-i-ndians. Journal of Experimental Marine Biology and Ecology 132:123-140.

Ellison. A. M. 1987. Density-dependent dynainics of Snli-cornin europnea monocultures. Ecology 68:737-741.

, 1989. Morphological determinants of self-thinning in plant monocultures and a proposal concerning the role of self-thinning in plant evolution. Oikos 54:287-293.

Ellison, A. M. , and D. IIar\.ell. 1989. Size hierarchies in ;2;Ienzbri~nip~~i.ornernbrc~nncen:do colonial animals follow the same roles as plants'? Oikos 553349-355.

Farrell, T., D. Bracher. and J. Roughgarden. 1991. Cross- shelf transport causes recruitment to intertidal populations in central California. Lirnnology and Oceanography 36: 279-288.

Frechette, M.. C. A. Butman, and W. R. Geyer. 1989. The importance of boundary-layer flows in supplying phyto-

plankton to the benthic suspension feeder. Myti1~i.s edulis (L.). Liinnology and Oceanography 34(1): 19-36.

Gaines. S. D.. and M. D. Bertness. 1992. Dispersal of ju- veniles and variable recruitment in marine species. Nature 360:579-580.

Gaines. S. D., and J. Roughgarden. 1985. Larval settlement rate: a leading determinant of structure in an ecological coinniunity of the marine intertidal zone. Proceedings of the National Acadeiny of Science USA 82:3707-3711.

Harger, J. R.. and D. E. Ladenberger. 1971. The effect of storms as a density-dependent mortality factor on popu- lations of sea mussels. Veliger 14:195-201.

Harley, C.. and M. D. Bertness. 1996. United we stand: group dependent architecture in marsh grasses. Functional Ecol- ogy 10:654-661.

Holbrook. N. M.. M. W. Dennv. and M. A. R. Koehl. 1991. Intertidal trees: consequences of aggregation on the me- chanical and photosynthetic properties of sea-palms Pos-tel.5ic1 pc~l1n~iej'ii.i7~i.s. Journal of Experimental Marine Bi- ology and Ecology 146:39-67.

Johnson, A. 1990. Flow around phoronids: consequences of a neighbor to suspension feeders. Liinnology and Ocean- ography 35: 1395-1401.

Knight-Jones. E. U7. 1953. Laboratory experiments on gre- gariousness during setting in Ouln/zzts hnlnnoides and other barnacles. Journal of Experimental Biology 30:584-598.

LaBarbera. M. 1984. Feeding currents and particle capture mechanisms in suspension feeding animals. American Zo- ologist 24:71-84.

Leck. M. A., V. T. Parker, and R. Simpson. 1989. Ecology of soil seed Banks. Academic Press. New York, New York. USA.

Lidgard. S. 1985. Zooid and colony growth in encrusting cheilostome I>ryozoans. Palaeontology 28:255-291.

Merz, R. A. 1984. Self-generated versus en\~ironmentally produced feeding currents: a comparison for the sabellid polychaete Eudistylia vnncouveri. Biological Bulletin 167: 200-209.

Niklas, K. J . 1992. Plant biomechanics-an engineering ap- proach to plant form and function. University of Chicago Press, Chicago, Illinois, U S 4 .

Okamura. B. 1984. The effects of ambient flow velocity. colony size. and upstream colonies on the feeding success of Bryozoa. I . Rug~ila .stolonifem Ryland, an arborescent species. Journal of Experimental Marine Biology and Ecol- ogy 83:179-193.

. 1992. Microhabitat variation and patterns of colony growth and feeding in a marine bryozoan. Ecology 73: 1502-1513.

Paine, R. T. 1966. Food \veb complexity and species diver- sity. American Naturalist 100:65-75.

Paine. R. T., and T. 11. Suchanek. 1983. Co~lvergence of ecological processes between independently evolved com- petitive dominants. Evolution 37:821-83 1.

Patterson, M. R. 1984. Patterns of whole colony prey capture in the octocoral, Alcyoni~inz sideriunz. Biological Bulletin (Woods Hole) 167:613-629.

Peterson. C. H., and R. Black. 1987. Resource depletion by active suspension feeders on tidal flats: influence of local density and tidal elevation. Lirnnology and Oceanography 32: 143-166.

Platt, W. J. 1975. The colonization and formation of equi- librium plant species associations on badger disturbances in a tall-grass prairie. Ecological Monographs 45:285-305.

Pullen. J . . and M. LaBarbera. 1991. Modes of feeding in aggregations of barnacles and the shape of aggregations. Biological Bulletin 181:442-452.

Raimondi, P. T. 1988. Settlement cues and the determination of the vertical limit of an intertidal barnacle. Ecology 69: 400-407.

1394 MARK D. BERTNESS ET AL. Ecology, Vol. 79. Ko. 4

Roughgarden, J. S., S. D. Gaines, and S. Pacala. 1987. Sup- ply side ecology: the role of physical transport processes. Pages 459-489 in Organization of communities: past and present. Proceedings of the British Ecological Society. Blackwell Scientific, London. UK.

Rubenstein, D. I., and M. A. R. Koehl. 1977. The mecha- nisms of filter feeding: some theoretical considerations. American Naturalist 111:981-984.

Sanford. E.. D. Bermudez, M. D. Bertness, and S. D. Gaines. 1994. Flow, food supply and acorn barnacle population dynamics. Marine Ecology Progress Series 104:49-62.

Sebens, K. P. 1984. Water flow and coral colony size: in- terhabitat comparisons of the octocoral Alcyoniunr sider-ium. Proceedings of the National Academy of Science U S 4 81:5473-5477.

Shimeta. J., and P. A. Jurnars. 1991. Physical mechanisms and rates of particle capture by suspension feeders. Ocean- ography and Marine Biology Annual Review 29:191-257.

Sinnott, E. W. 1960. Plant morphogenesis. McGraw Hill, Ne\v York, Ne\v York. USA.

Sousa, W. P. 1979. Disturbance in marine intertidal boulder fields: the non-equilibrium maintenance of species diver- sity. Ecology 60: 3225-1 239.

Vogel, S. 1981. Life in moving fluids. Princeton University Press, Princeton, New Jersey, USA.

Wethey. D. S . 1983n. Geographical limits and local zonation: the barnacles Serrzibnlc~nusand Chthnnrulus in New En- gland. Biological Bulletin (U700ds Hole) 165:330-341.

1983b. Intrapopulation variation in growth of sessile organisms: natural populations of the intertidal barnacle Bc11c1nu.s ba1anoide.s. Oikos 40: 14-23.

. 1984a. Spatial patterns in barnacle settlement: day to day changes during the settlement season. Journal of the Marine Biological Association U.K. 64:687-698.

. 1984b. Effects of cro\vding on fecundity in barna- cles: semi ha la nu.^ balanoide.~, Ba1anu.s hulanoides, Balanus glandula, and CI1t1zanrulu.s dali. Canadian Journal of Zo- ology 62:1788-1795.

Wu, R. S. S. 1980. Effects of crowding on the energetics of the barnacle Balairu.~ glandula Darwin. Canadian Journal of Zoology 58:559-566.

U7u, R. S. S.. C. D. Levings. and D. J. Randall. 1977. Dif- ferences in energy partitioning between crowded and un- crowded individual barnacles, Canadian Journal of Zool- ogy 55:643-647.

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Making Mountains out of Barnacles: The Dynamics of Acorn Barnacle HummockingMark D. Bertness; Steven D. Gaines; Su Ming YehEcology, Vol. 79, No. 4. (Jun., 1998), pp. 1382-1394.Stable URL:

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Literature Cited

The Development, General Morphology and Subsequent Elimination of Barnacle Populations,Balanus crenatus and B. Balanoides, After a Heavy Initial SettlementH. Barnes; H. T. PowellThe Journal of Animal Ecology, Vol. 19, No. 2. (Nov., 1950), pp. 175-179.Stable URL:

http://links.jstor.org/sici?sici=0021-8790%28195011%2919%3A2%3C175%3ATDGMAS%3E2.0.CO%3B2-E

Intraspecific Competition and Facilitation in a Northern Acorn Barnacle PopulationMark D. BertnessEcology, Vol. 70, No. 1. (Feb., 1989), pp. 257-268.Stable URL:

http://links.jstor.org/sici?sici=0012-9658%28198902%2970%3A1%3C257%3AICAFIA%3E2.0.CO%3B2-R

Determinants of Pattern in a New England Salt Marsh Plant CommunityMark D. Bertness; Aaron M. EllisonEcological Monographs, Vol. 57, No. 2. (Jun., 1987), pp. 129-147.Stable URL:

http://links.jstor.org/sici?sici=0012-9615%28198706%2957%3A2%3C129%3ADOPIAN%3E2.0.CO%3B2-U

http://www.jstor.org

LINKED CITATIONS- Page 1 of 5 -

Passive Suspension Feeding in a Sea Pen: Effects of Ambient Flow on Volume Flow Rate andFiltering EfficiencyBarbara A. BestBiological Bulletin, Vol. 175, No. 3. (Dec., 1988), pp. 332-342.Stable URL:

http://links.jstor.org/sici?sici=0006-3185%28198812%29175%3A3%3C332%3APSFIAS%3E2.0.CO%3B2-1

Competition, Disturbance, and Community Organization: The Provision and SubsequentUtilization of Space in a Rocky Intertidal CommunityPaul K. DaytonEcological Monographs, Vol. 41, No. 4. (Autumn, 1971), pp. 351-389.Stable URL:

http://links.jstor.org/sici?sici=0012-9615%28197123%2941%3A4%3C351%3ACDACOT%3E2.0.CO%3B2-L

Hydrodynamic Processes Affecting Benthic RecruitmentJames E. EckmanLimnology and Oceanography, Vol. 28, No. 2. (Mar., 1983), pp. 241-257.Stable URL:

http://links.jstor.org/sici?sici=0024-3590%28198303%2928%3A2%3C241%3AHPABR%3E2.0.CO%3B2-%23

Density-Dependent Dynamics of Salicornia Europaea MonoculturesAaron M. EllisonEcology, Vol. 68, No. 3. (Jun., 1987), pp. 737-741.Stable URL:

http://links.jstor.org/sici?sici=0012-9658%28198706%2968%3A3%3C737%3ADDOSEM%3E2.0.CO%3B2-6

Cross-Shelf Transport Causes Recruitment to Intertidal Populations in Central CaliforniaTerence M. Farrell; David Bracher; Jonathan RoughgardenLimnology and Oceanography, Vol. 36, No. 2. (Mar., 1991), pp. 279-288.Stable URL:

http://links.jstor.org/sici?sici=0024-3590%28199103%2936%3A2%3C279%3ACTCRTI%3E2.0.CO%3B2-8

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Larval Settlement Rate: A Leading Determinant of Structure in an Ecological Community ofthe Marine Intertidal ZoneS. Gaines; J. RoughgardenProceedings of the National Academy of Sciences of the United States of America, Vol. 82, No. 11.(Jun. 1, 1985), pp. 3707-3711.Stable URL:

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Structural Interdependence: An Ecological Consequence of Morphological Responses toCrowding in Marsh PlantsC. D. G. Harley; M. D. BertnessFunctional Ecology, Vol. 10, No. 5. (Oct., 1996), pp. 654-661.Stable URL:

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Flow Around Phoronids: Consequences of a Neighbor to Suspension FeedersAmy S. JohnsonLimnology and Oceanography, Vol. 35, No. 6. (Sep., 1990), pp. 1395-1401.Stable URL:

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Self-Generated versus Environmentally Produced Feeding Currents: A Comparison for theSabellid Polychaete Eudistylia vancouveriRachel Ann MerzBiological Bulletin, Vol. 167, No. 1. (Aug., 1984), pp. 200-209.Stable URL:

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Microhabitat Variation and Patterns of Colony Growth and Feeding in a Marine BryozoanBeth OkamuraEcology, Vol. 73, No. 4. (Aug., 1992), pp. 1502-1513.Stable URL:

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Food Web Complexity and Species DiversityRobert T. PaineThe American Naturalist, Vol. 100, No. 910. (Jan. - Feb., 1966), pp. 65-75.Stable URL:

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Convergence of Ecological Processes Between Independently Evolved Competitive Dominants:A Tunicate-Mussel ComparisonR. T. Paine; T. H. SuchanekEvolution, Vol. 37, No. 4. (Jul., 1983), pp. 821-831.Stable URL:

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Resource Depletion by Active Suspension Feeders on Tidal Flats: Influence of Local Densityand Tidal ElevationCharles H. Peterson; R. BlackLimnology and Oceanography, Vol. 32, No. 1. (Jan., 1987), pp. 143-166.Stable URL:

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The Colonization and Formation of Equilibrium Plant Species Associations on BadgerDisturbances in a Tall-Grass PrairieWilliam J. PlattEcological Monographs, Vol. 45, No. 3. (Summer, 1975), pp. 285-305.Stable URL:

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Modes of Feeding in Aggregations of Barnacles and the Shape of AggregationsJulie Pullen; Michael LaBarberaBiological Bulletin, Vol. 181, No. 3. (Dec., 1991), pp. 442-452.Stable URL:

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Settlement Cues and Determination of the Vertical Limit of an Intertidal BarnaclePeter T. RaimondiEcology, Vol. 69, No. 2. (Apr., 1988), pp. 400-407.Stable URL:

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The Mechanisms of Filter Feeding: Some Theoretical ConsiderationsDaniel I. Rubenstein; M. A. R. KoehlThe American Naturalist, Vol. 111, No. 981. (Sep. - Oct., 1977), pp. 981-994.Stable URL:

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Water Flow and Coral Colony Size: Interhabitat Comparisons of the Octocoral AlcyoniumsideriumK. P. SebensProceedings of the National Academy of Sciences of the United States of America, Vol. 81, No. 17,[Part 1: Biological Sciences]. (Sep. 1, 1984), pp. 5473-5477.Stable URL:

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Disturbance in Marine Intertidal Boulder Fields: The Nonequilibrium Maintenance of SpeciesDiversityWayne P. SousaEcology, Vol. 60, No. 6. (Dec., 1979), pp. 1225-1239.Stable URL:

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