Download Relative importance of recruitment and other causes of variation in

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~.~..~
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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)