Ecology, 89(7), 2008, pp. 1994–2004
Ó 2008 by the Ecological Society of America
DENSITY-DEPENDENT SETTLEMENT AND MORTALITY STRUCTURE
THE EARLIEST LIFE PHASES OF A CORAL POPULATION
MARK J. A. VERMEIJ1,3
1
AND
STUART A. SANDIN2
Cooperative Institute for Marine and Atmospheric Studies, Rosenstiel School for Marine and Atmospheric Sciences,
University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149 USA
2
Center for Marine Biodiversity and Conservation, Scripps Institution of Oceanography, 9500 Gilman Drive,
La Jolla, California 92093-0202 USA
Abstract. The local densities of heterospecifics and conspecifics are known to have
profound effects on the dynamics of many benthic species, including rates of settlement and
early post-settlement survivorship. We described the early life history of the Caribbean coral,
Siderastrea radians by tracking the population dynamics from recently settled planulae to
juveniles. Through three years of observation, settlement correlated with the abundance of
other benthic organisms, principally turf algae (negatively) and crustose coralline algae
(positively). In addition, adult density showed independent effects on coral settlement and
early post-settlement survivorship. Settlement rates increased across low levels of adult cover
and saturated at a maximum around 10% cover. Early post-settlement survivorship decreased
with adult cover, revealing structuring density dependence in coral settlers. The earliest life
stages of corals are defined by low survivorship, with survivorship increasing appreciably with
colony size. However, recent settlers (one-polyp individuals, ,1 year old) are more likely to
grow into two-polyp juveniles than older single polyps (.1 year old) that were delayed in their
development. The early benthic phase of corals is defined by a severe demographic bottleneck
for S. radians, with appreciable density-dependent and density-independent effects on
survivorship. For effective management and restoration of globally imperiled coral reefs, we
must focus more attention on this little studied, but dynamic, early life history period of corals.
Key words: coral reef; crustose coralline algae; Florida keys; invertebrates; mortality; population
dynamics; recruitment; settlement; Siderastrea radians; turf algae.
INTRODUCTION
The early life history period of marine species is
perhaps the most critical and dynamic, though least
understood, of all life stages. For benthic species,
settlement after the pelagic larval period is variable in
space and time and therefore is frequently deemed
unpredictable. Following arrival to the benthos a severe
survivorship bottleneck faces settling larvae, with
mortality frequently exceeding 99% especially in marine
invertebrates that lack parental care of young and have
small, little-developed larvae (Gosselin and Qian 1997,
Hunt and Scheibling 1997).
Settlement and subsequent mortality of benthicassociated species can be affected by characteristics of
the local environment in a density-independent manner.
For example, early survivorship of some species has
been shown to be affected by water temperature (Pepin
1991), salinity (Vermeij et al. 2006), sedimentation
(Gilmour 1999), and availability of refuges (Tupper
and Boutilier 1995, Anderson 2001) or preferred
Manuscript received 8 August 2007; revised 14 November
2007; accepted 20 November 2007. Corresponding Editor: R. B.
Aronson.
3 Present address: Department of Botany, University of
Hawaii at Manoa, 3190 Maile Way, Honolulu, Hawaii 96822
USA. E-mail:
[email protected]
settlement substrata (Harrington et al. 2004). Densitydependent effects also can increase or decrease population growth rates with increasing population size (Caley
et al. 1996). In a variety of taxa, increased conspecific
density can lead to an increase in rates of settlement.
Newly arriving individuals can demonstrate attraction
to sites where adults are already present (fish [Sweatman
1988]; barnacles [Chabot and Bourget 1988]; annelids
[Minchinton 1997]) or limited dispersal capabilities can
result in a positive relationship between the numbers of
settlers and reproductive adults at a local spatial scale
(Molofsky 1994, Hughes et al. 2000, Johst et al. 2002,
Vermeij 2005). In addition to the structuring effects on
settlement, increased conspecific density can lead to an
increase in per capita mortality probabilities for young
settlers (barnacles [Miron et al. 1999]; fish [Hixon and
Webster 2002]; algae [Reed 1990, Creed et. al. 1996]).
The agents of such direct density dependence can include
aggregating predators (Hixon and Carr 1997, Anderson
2001), competition for space (Roughgarden et al. 1985,
Carlon 2001), or distance-related disease transmission
(Bruckner et al. 1997, Lafferty et al. 2004).
Difficulties associated with studying early life history
stages in corals
Here we aim to dissect demographic patterns within
distinct phases of the early life history of a Caribbean
1994
July 2008
EARLY LIFE HISTORY DYNAMICS OF CORALS
scleractinian coral in order to provide insight into the
likely factors structuring coral population dynamics.
The conservation of coral reefs depends on a comprehensive understanding of the earliest life history
dynamics of corals. However, three issues have constrained such research. (1) The high topographic
complexity for which coral reefs are notorious makes
true coral settlers, i.e., planulae first settling to the
benthic environment, extremely difficult to find and to
quantify in situ. For example, coral recruits frequently
are defined operationally as individuals of small size
(e.g., ,5 cm2) in one-time censuses across space (see
notable exceptions in time-series sampling, e.g., Connell
et al. [1997], Hughes and Tanner [2000], Miller et al.
[2000]). These small individuals, however, are likely to
be more than one year old and hence will not reveal
insights into the earliest (and likely most dramatic)
demographic bottlenecks (Caley et al. 1996, Vermeij
2006). (2) Very small corals typically lack appreciable
morphological patterning to enable reliable taxonomic
identification in the field. Therefore repeated censuses
through time will reveal survivorship information of the
aggregated coral assemblage, but not species-specific
demographic information. (3) Coral settlement is
notoriously variable through time and across space
(e.g., Wallace 1985, Sammarco 1991, Connell et al. 1997,
Smith 1997, Dunstan and Johnson 1998, Hughes et al.
2000), thereby complicating statistically rigorous interpretation of early life history dynamics. Variability is
especially notable for broadcast spawning species whose
pelagic larvae are subject to the ephemeral transport and
delivery mechanisms defined by local oceanographic
conditions. An extensive body of research using
settlement tiles has successfully navigated a number of
these concerns to document spatial and temporal
patterns of coral recruitment (e.g., Sammarco 1991,
Smith 1992, 1997, Harriott 1999, Carlon 2001, Hughes
et al. 2002). However, detailed post-settlement survivorship studies through time remain little addressed.
A 60þ-year-old wreck off the coast of the Florida
Keys provided an experimental reef enabling the study
of the earliest life phases of Siderastrea radians, a
common Caribbean scleractinian coral. This study
system allowed us to accurately relate settlement and
early post-settlement mortality rates to characteristics of
the local environment in a Caribbean coral population,
namely densities of conspecifics and cover of two
dominant benthic algae.
MATERIALS
AND
METHODS
Study site
The study site was a shipwreck, the MV Benwood,
located on a shallow fore-reef in the Florida Keys
Natural Marine Sanctuary near Key Largo (Florida,
USA; wreck coordinates: 2583 0 700 N, 80819 0 5900 W). The
Benwood is a 110-m, metal-hulled freighter that has been
submerged since 1942. Its deck (approximately 110 3 10
m) is parallel to the water surface at 10 m depth. The
1995
metal deck is no longer exposed, but is encrusted
primarily with a 1–3 cm thick layer of crustose coralline
algae (CCA; mainly species of Porolithon and Paragoniolithon) on which other substrate types (predominately turf algae and adult S. radians colonies) can be
locally abundant.
The surfaces of the wreck offered a virtually twodimensional limestone substrate characterized by smallscale topographical complexity (millimeters to centimeters) upon which to reliably find all size classes of corals,
including recently settled planulae. The coral community of the wreck was dominated by S. radians, as the
single species comprised over 95% of the colonies
present. The remaining coral colonies included large
colonies of broadcast-spawning hard corals (.50 cm
diameter) and gorgonians (.30 cm height). Coral
assemblages dominated by S. radians were observed in
Jamaica and Bermuda as long as a century ago
(Duerden 1904 and references therein). More recent
surveys report a similar coral assemblage in many reef
flat environments (i.e., shallow, near-horizontal habitats) throughout the tropical Atlantic (Virgin Islands
[Lewis 1989], south Florida [Chiappone and Sullivan
1994], Colombia and Haiti [González-Cortés, unpublished data], Netherlands Antilles [Vermeij et al. 2007],
Cape Verde Islands [Moses et al. 2003]).
Focal species
The Caribbean coral species Siderastrea radians
(Pallas 1766; Family Siderastreidae) occurs on shallow
limestone pavements and ledges throughout the Tropical
Atlantic where it dominates coral communities in
naturally disturbed habitats (e.g., areas with high
sedimentation, temperature, and/or salinity fluctuations
[Lewis 1989, Lirman et al. 2002, Moses et al. 2003,
Vermeij et al. 2007]). The species is gonochoric and
females release competent planulae year-round (Soong
1991, Vermeij 2005). Colonies are reproductively active
at sizes larger than 2.23 cm2 and are thus considered to
be adults. Beyond this size, total planular production
increases exponentially with colony size (Vermeij 2005).
At a 0.25-m2 scale, 46% of the settling planulae were
produced by local adults (Vermeij 2005), suggesting that
S. radians populations are largely demographically
closed at this small spatial scale (sensu Hixon et al.
2002). Planulae of S. radians are relatively large
(maximum length 0.7 mm [Vermeij 2005]) and have a
ring of zooxanthellae at their oral pole, facing away
from the substrate after settlement. Both their large size
and the characteristic ring of zooxanthellae make S.
radians planulae easily identifiable during underwater
surveys (see Plate 1). Furthermore, none of the surviving
settlers that we initially identified as S. radians were
found to be misidentified after early growth.
S. radians is a brooding species with frequent
reproduction and a short typical dispersal distance
(Vermeij 2005). These life-history characteristics are
common to at least eight other Caribbean and several
1996
MARK J. A. VERMEIJ AND STUART A. SANDIN
Pacific coral species (Richmond and Hunter 1990) as
well as to species from various other invertebrate taxa
(Vermeij 2005). Populations of coral juveniles are
frequently dominated by brooding species (Smith 1997,
Carlon 2001, Edmunds 2004, Vermeij 2006). Moreover,
ecosystem changes on many Caribbean reefs are leading
to the systematic replacement of spawning species by
brooders in the coral assemblage (Knowlton 2001,
Aronson et al. 2005). As such, the study of the earliest
stages of colony formation and survivorship of a
brooding species will provide insights in the population
dynamics typical of a growing number of degraded or
recovering reef communities worldwide.
Survey protocols
Benthic community composition was measured in 28
fixed 50 3 50 cm quadrats at approximately three-month
intervals from May 2002 until March 2005. Quadrats
were haphazardly positioned at the start of the study
and spanned the range of adult coral cover on the wreck
(;1–30% benthic cover at a 0.25-m2 scale). Beginning on
the second survey, settlement rates of S. radians were
estimated by counting new arrivals to the quadrats. As
such, we define settlement as the initial sighting of a
settled planula (no tentacles) or early-metamorphosed
polyp (with tentacles) that had not started skeletal
formation (diameter ,1 mm) and had arrived at most
three months previous. Benthic composition was determined in each quadrat at all intervals for three dominant
functional groups: turf algae (height ,2 cm), crustose
coralline algae (CCA), and adult S. radians colonies.
Settlement rates were estimated from the appearance
of newly arrived individuals found from repeated
photographic censuses. Although larger coral colonies
(.0.20 cm2) can be reliably documented with highresolution photography, smaller colonies are missed
frequently. To find these smallest colonies and recent
settlers, all sediment was removed from each quadrat
prior to sampling by gently waving a plastic sheet above
its surface so sediments were blown away by the moving
water. If necessary, a turkey baster was used to further
remove remaining sediments. Occasionally, algal turfs
were dense and dissecting needles were used to search
through them for recruits without removing the algae
themselves. All settlers could be found with the naked
eye and no magnification tools were used.
To avoid under-sampling, small individuals (,0.20
cm2) were located and labeled in situ with temporary
rings (stainless steel washers) that were color-coded to
indicate the size of the encircled settler. A picture of the
quadrat was taken recording all small colonies (labeled
by rings) and larger colonies present. Colonies on all
photographs were traced manually and analyzed using
spatial analysis software (Scion Corporation, Frederick,
Maryland, USA), providing information of the size and
location of coral individuals of all sizes present in the
quadrat. The success rate of finding all individuals inside
Ecology, Vol. 89, No. 7
a quadrat, including the smallest of settlers, was .99.7%
(Vermeij 2005).
Note that, because settlers were sampled at threemonth intervals, we likely underestimated true settlement as individuals may have settled and died between
successive survey periods. Additionally, size variation in
settlers might arise as they could have been present on
the bottom between one day and three months before
being noted. Nevertheless, all settlers considered in this
study were less than three months old when first
observed and thus represent the earliest stage of coral
recruitment, i.e., when a planula first attaches to the
benthic substrate. For this stage, post-settlement survival (or mortality) was determined by tracking the survival
(or mortality) of an individual through successive
intervals. The maximum length of time that a known
settler could be observed was 27 months, i.e., the time
from the second survey until the end of the study. We
define juvenile colonies as those composed of at least
two polyps that have started skeletal formation and
share a skeletal wall. This definition reflects the
significant decline in instantaneous mortality probability
for two- (or more) polyp individuals (minimum size of
0.20 cm2) relative to one-polyp settlers (see Results).
The abundance of turf algae and CCA was estimated
by overlaying each quadrat with a Plexiglas sheet labeled
with 200 randomly placed circles measuring 1 mm in
diameter. The benthic type under each circle was
recorded, in sum providing an estimate of benthic
composition (reported as percent cover of the quadrat).
For more details on the sampling methodology, see
Vermeij (2005).
Analyses
Data on settlement and post-settlement dynamics
were collected from repeated measures in permanent
quadrats through time. Thus, from each quadrat we
have quantitative estimates of settlement rates as well as
survivorship and growth histories for each individual
coral settler and for colonies already present at the
commencement of the study.
Temporal patterns of settlement.—We tested the
relationship between the abundance of each of two
dominant benthic algal types (i.e., cover of CCA and
cover of turf algae) and coral settlement rates through
time. These analyses are founded on the observations
that each algal type affects coral settlement in strong and
distinct ways. CCA have been shown to promote coral
settlement by releasing substances that attract coral
planulae (Morse et al. 1988, Steneck and Testa 1997,
Heyward and Negri 1999) and by creating suitable
settlement habitat on which early post-settlement
mortality is relatively low (Harrington et al. 2004,
Vermeij 2006). In contrast, turf algae can negatively
affect recruitment by trapping settling planulae (Birrell
et al. 2005, Kuffner et al. 2006, Vermeij 2006) and,
indirectly, by displacing CCA (Airoldi 2000). To
investigate temporal patterns of both CCA and turf
July 2008
EARLY LIFE HISTORY DYNAMICS OF CORALS
algae in relation to coral settlement rates, we performed
Pearson’s correlation analyses comparing means across
quadrats for each algal type to mean coral recruitment
rate for each of the 11 time intervals.
Spatial patterns of early post-settlement survivorship.—
To gain further insights into the early life history
dynamics of S. radians, we investigated patterns of
settlement and survivorship across quadrats. This
approach was motivated by two observations. First,
temporal fluctuations of settlement rate and algal cover
revealed strong annual periodicity (see Results). By
comparing variability across space, we avoided potentially confounding seasonal correlations in the data.
Second, adult density (as measured by total coral cover
of all colonies greater than 2.23 cm2) varied little
through time in each quadrat (,1%). As such,
investigation of the effects of adult density on settlement
and survivorship could only be realized through
comparisons among quadrats.
The effect of adult density on the probability of settler
survivorship was estimated directly from repeated
censuses of each quadrat. Because of the colonial nature
of corals, we define ‘‘density’’ here as the total cover of
adult corals (loosely scaling with the total number of
polyps), not the number of individual colonies, in the
quadrat. Nearby adult density can have a negative effect
on early survival through intraspecific competition
(Roughgarden et al. 1985, Vermeij 2005) or through
transmission of pathogens (Bruckner et al. 1997). To
understand the pattern of settler survivorship, three
models of early post-settlement survival probability, qA,
as functions of adult coral cover, A, were generated: i)
constant (qA ¼ s0), (ii) negative linear (qA ¼ s0 s1A),
and (iii) inverse (qA ¼ s0/[1 þ s1A]), where s0 is the
density-independent term, s1 is the density-dependent
term, and their values are constrained such that 0 qA
1 for all observed A. As such, model (i) describes
density-independent survivorship and models (ii) and
(iii) describe linear and slower-than-linear, respectively,
density-dependent survivorship.
Survivorship of settlers was estimated directly by
tracking the fates of known settlers found in one
sampling interval until the subsequent interval. For
each quadrat we recorded a maximum of 10 survivorship periods, i.e., the maximum number of inter-census
intervals surveyed in this study, with which to test the
relative fits of the survivorship models (with fewer than
10 events if no settlers arrived to a quadrat in the
preceding time interval). Because settlers were spatially
nonoverlapping, we assumed that the survivorship of
each settler within a quadrat was independent of the fate
of neighboring settlers, with a quadrat-specific probability of survivorship defined by models (i–iii). As such, a
binomial error distribution was used to confront models
of survivorship probabilities with observed settler data.
The summed log likelihood of a particular model
describing the observed data given values of parameters
s0 and s1 was computed as folows:
Lðs0 ; s1 Þ ¼ ln
"
28 Y
10
Y
i¼1 j¼1
1997
binomðNi; j ; Si; j jqA;x Þ
#
ð1Þ
where binom is the binomial probability mass function,
Ni, j and Si, j are the numbers of settlers arriving and
surviving, respectively, for the subsequent three-month
interval in quadrat i at time period j, and qA,x is the per
capita probability of settlers surviving given quadratspecific coral cover A and survivorship model x described
as models (i–iii) above. A simulated annealing algorithm
was used to identify values of s0 and s1 (if appropriate) to
maximize the likelihood described in Eq. 1 for each model
of survivorship x. The statistical significance of the
density-dependent term s1 from each model ii and iii was
determined using likelihood ratio tests relative to the
nested model (model i), i.e., explicitly testing whether the
value of s1 is significantly different than 0. Note that if s1 is
not different than zero, then each model ii and iii reduces
to model i and, as such, model i is nested within each of the
competing models. The relative fit to the data of model ii
vs. iii was determined by assuming equal Bayesian prior
probabilities for the models. With two alternative
hypotheses represented by two models, M1 and M2, the
posterior probability that the poorer fitting model M2 is
correct rather than the better-fitting model M1 is
2(exp[MLM2]exp[MLM2]/[exp(MLM1) þ exp(MLM2)]),
where MLMj is the maximum log likelihood for model
j. Multiplying by two accounts for the fact that a priori
we did not know which model was better, which is
analogous to using a two-tailed statistical test. We say
that model M1 is better than model M2 if this posterior
probability is ,0.05 (for further details, see Hilborn and
Mangel 1997).
Spatial patterns of settlement rates.—Insights into the
combined effects of adult density on settlement (i.e.,
each in producing settlers and affecting post-settlement
mortality) were afforded by data on settlement rate
across quadrats. The number of settlers observed at each
time step is the product of the number of settlers arriving
in the quadrat and the probability of each settler
surviving until being counted. Because upward of 50%
of settlers can be produced by nearby adults (Vermeij
2005), the total number of settlers arriving in a quadrat
may approximate a positive linear function with adult
density (i.e., total cover of colonies larger than 2.23
cm2). Adult density, in contrast, can have a negative
effect on post-settlement survival, as explored above. We
tested for density-dependent effects on settlement by
comparing six models of settlement rate, xA , as
functions of adult density, A. Functional forms were
generated factorially from the products of two functions
of settler production, mA (constant [mA ¼ p0] and positive
linear [mA ¼ p0 þ p1A], where p0 and p1 are positive,
estimated constants such that mA 0 for all observed A),
and three functions of survival (i–iii), as above.
Assuming constant planular production with increasing
adult density, the functions simplify to the following:
1998
MARK J. A. VERMEIJ AND STUART A. SANDIN
ðivÞ xA ¼ x0
where x0 ¼ s0 p0
ðvÞ xA ¼ x0 x1 A
where x0 ¼ s0 p0 and
x1 ¼ s1 p0
ðviÞ xA ¼ x0 =ð1 þ x1 AÞ
where x0 ¼ s0 p0 and
x 1 ¼ s1
and assuming positive linear production with increasing
adult density, the functions simplify to
ðviiÞ xA ¼ x0 þ x1 A
where x0 ¼ s0 p0 and
x1 ¼ s0 p1
ðviiiÞ xA ¼ x0 þ x1 A x2 A2
where x0 ¼ s0 p0 ;
x1 ¼ s0 p1 s1 p0 ;
and x2 ¼ s1 p1
ðixÞ xA ¼ ðx0 þ x1 AÞ=ð1 þ x2 AÞ
where x0 ¼ s0 p0 ;
x1 ¼ s0 p1 ; and
x2 ¼ s1 :
The estimated constants x0, x1, and x2 represent the
intercept, linear term, and nonlinear term, respectively,
of the settlement rate functions relative to adult density,
and are constrained such that xA . 0 for all observed A.
Settlement rate was calculated for each quadrat as the
mean number of settlers found per census, thereby
averaging over annual fluctuations. The effects of adult
density on settlement rate were estimated by comparing
the relative fits of models (iv–ix) to the observed
settlement rates. Based on the observations that
settlement generally increased with adult density and
that models describing a negative relationship between
settlement and adult density fit poorly, we do not
present analyses for models (v and vi) in the Results. To
account for possible confounding effects of abundance
of algae on realized settlement rates, we tested all
combinations of additive, linear functions of quadratspecific mean cover of each turf algae and CCA on each
settlement model. Site-specific cover data did not
significantly violate assumptions of normality. As such,
data were assumed to be normally distributed around
expectations, and the most likely values of parameters
x0, x1, and x2 (as needed) for each model (iv–ix) were
found by maximum likelihood approaches analogous to
ordinary least squares minimization. The statistical
significance of the density-dependent linear and nonlinear terms (x1 and x2) and of the additive algal weightings
were determined using likelihood ratio tests relative to
the nested models. Non-nested models with equivalent
numbers of parameters were compared statistically using
the assumption of equal Bayesian prior probabilities, as
in the analysis of the post-settlement survivorship data
above.
Size-specific mortality probability.—The dependence
of mortality probability on the size of an individual
Ecology, Vol. 89, No. 7
colony was estimated from mortality data. A Poisson
distribution described the probability of a coral dying at
a given interval, with parameter kc describing the mean
time until mortality occurred for a colony in size class c.
Two functional forms for the rate parameter of a
Poisson mortality model were constructed: constant
mortality probability with size, kc ¼ m0, and log-linear
mortality probability with size, kc ¼ m0 þ m1ln(c).
Estimated constants m0 and m1 describe the sizeindependent and size-dependent terms, respectively, of
the mortality probability and are constrained such that
kc . 0 for all observed c.
The time until mortality was recorded for all coral
colonies present in each quadrat at the second census
(July 2002; 986 total colonies). Colonies were binned
into 13 logarithmic size classes (i.e., with the geometric
mean of colony area being 0.02, 0.04, 0.08, . . ., 40.96,
.40.96 cm2). If the colony died during one of the
subsequent 10 sampling intervals of the study, the
duration was noted. Otherwise, the time until mortality
occurred was recorded as greater than 29 months. The
summed log likelihood of a particular model describing
the observed data given values of parameters m0 and m1
was computed as follows:
Lðm0 ; m1 Þ
8
> kci ;x ki lnðkci ;x Þ þ lnðki !Þ
986 >
<
X
12
X
¼
>1
½kci ;x kj lnðkci ;x Þ þ lnðkj !Þ
i¼1 >
:
for ki 10
for ki . 10
j¼1
ð2Þ
where kci ;x is the expected mortality probability based on
function x (i.e., either constant or linear with colony
size, ci) and ki is the number of three-month time
intervals until mortality for individual i. The equations
of (2) correspond to the conditions of the colony dying
during the study or the colony surviving through the
duration of the entire study. As above, maximum
likelihood estimates were generated for each mortality
model. The significance of the size-dependent term of the
log-linear model was determined by a likelihood ratio
test.
RESULTS
Through time, the rate of coral settlement was
coupled strongly to the abundance of other benthic
organisms (Fig. 1). Settlement rates were correlated
positively with cover of CCA (Pearson’s r ¼ 0.75, P ,
0.01, n ¼ 11 time intervals) and negatively with cover of
turf algae (Pearson’s r ¼ 0.63, P , 0.04, n ¼ 11). The
temporal fluctuations had a distinct annual cycle with
settlement high during the boreal fall, tracking annual
fluctuations in the cover of CCA and turf algae.
Following settlement to the benthos, adult density
had a pronounced, negative effect on early postsettlement survivorship. The probability of a known
new settler (zero to three months post settlement) to
July 2008
EARLY LIFE HISTORY DYNAMICS OF CORALS
1999
dependent effects of adult cover on settler survival, yet
both analyses fail to discriminate between the negative
linear and inverse models (ii and iii, respectively) of
survivorship.
In addition to density-dependent effects of adult cover
on settlement and early post-settlement survival, the
best-fitting model describing settlement rate included
significant effects of CCA cover (positive) and turf algal
cover (negative; Table 1). The independent effects of
each algal type in the saturating model are consistent
with the results from the univariate correlations through
time (Fig. 1).
After successful recruitment to the benthos, the
mortality probabilities of juvenile S. radians remained
high, with over 75% of coral colonies up to 0.08 cm2
dying within the first year (Fig. 3). In contrast, larger
FIG. 1. Seasonal fluctuations in benthic organisms. (A)
Settlement (recruitment) rates of the Caribbean coral Siderastrea radians (number of newly observed settlers per 0.25 m2
per 3 months) between July 2002 and March 2005. (B)
Temporal fluctuations in benthic habitat composition, notably
of percent cover of crustose coralline algae (CCA; solid circles)
and turf algae (open circles). Data at each time are shown as the
mean of all 28 quadrats surveyed; error bars are 6 SE.
survive for the subsequent three months was best fit by a
negative linear function of conspecific adult cover (qA ¼
0.802 0.006A; n ¼ 28 quadrats; P , 0.001 relative to
constant model [MLi ¼ 1219.5, MLii ¼ 1187.7, where
MLx is the maximum log likelihood value of model x];
Fig. 2A). Despite the superior fit of the linear model (ii),
the fit could not be statistically distinguished from the
inverse model (iii) (MLiii ¼ 1189.2, P ¼ 0.35).
Adult density showed a general, positive effect on
local settlement rates (Fig. 2B). At increased adult
densities (.10% cover) the positive effect weakened to
essentially no effect, as indicated by the best fit of a
nonlinear saturating model relating observed settlement
to adult density (Table 1; Fig. 2B). The saturating model
(ix) is consistent with adult density leading to a positive,
linear production of settlers, each with a probability of
surviving the short period until the next census inversely
related to adult density. Notably, model ix is only
marginally better supported than the model (viii) with
linear production and negative linear survival (P ¼ 0.053
assuming equal Bayesian priors; Table 1). As such, each
the direct survivorship analyses (Fig. 2A) and the
indirect survivorship analyses (i.e., the product of
settlement and survivorship patterns as inferred from
models (viii–ix); Table 1) confirm the negative, density-
FIG. 2. Effects of adult coral cover on survivorship
probability and settlement rate (number of newly observed
settlers per 0.25 m2 per 3 months) of S. radians. (A) Observed
(solid circles) and predicted (line) survivorship probability for
early post-settlement corals for each quadrat. Survivorship is
defined here as the mean proportion of newly observed
individuals (beginning at 0–3 months post-settlement) that
survive for the following three months. Note that each data
point is the mean survivorship from the 10 surveys in one
quadrat. (B) Observed settlement rate is presented (solid circles)
with expected settlement rate (þ symbols) based on prediction
for best-fit model (ix). Model expectation includes quadratspecific effects: adult, CCA, and turf algal cover (see Table 1).
2000
MARK J. A. VERMEIJ AND STUART A. SANDIN
Ecology, Vol. 89, No. 7
TABLE 1. Best-fit parameter estimates and maximum likelihood value (ML) for competing models
of mean coral settlement rate.
Parameter estimate
Model and components
x0
x1
x2
iv) Constant, no adult effects, xA ¼ x0
No algae
21.97
CCA
11.28
Turf
35.73
CCA þ turf
33.63
vii) Linear adult effects, xA ¼ x0 þ x1A
No algae
18.93
CCA
12.78
Turf
31.96
CCA þ turf
55.30
xCCA
xturf
ML
0.62
0.59
99.82
98.65
96.87à
96.84
0.55
0.73
98.51
91.99ààà
95.97à
90.41ààà
0.56
0.41
95.36*
89.81ààà
92.10à
89.35àà
0.46
0.03
91.75***
88.66à
88.80à
86.28àà
0.19
0.03
0.34
0.84
0.26
1.36
viii) Quadratic adult effects, xA ¼ x0 þ x1A x2A2
No algae
12.32
1.97
CCA
14.40
2.01
Turf
25.26
1.95
CCA þ turf
36.19
1.99
ix) Saturating adult effects, xA ¼ (x0 þ x1A)/(1 þ x2A2)
No algae
24.93
46.03
CCA
63.18
44.82
Turf
9.04
60.23
CCA þ turf
20.59
6.52
0.49
0.89
0.06
0.04
0.06
0.03
1.62
3.00
1.58
0.41
0.44
0.65
0.23
0.33
Notes: Coral settlement rate was estimated in units of number of settlers per 0.25 m2 per 3
months, and adult coral, crustose coralline algae (CCA), and turf algal cover are all measured as
percent cover. The estimated constants x0, x1, and x2 represent the intercept, linear term, and
nonlinear term, respectively, of the settlement rate functions relative to adult density. The four
competing models of adult effects, f (x0, x1, x2, A), are outlined in Materials and methods:
Analsyses: Spatial patterns of settlement rates, with additive effects of two algal types as xA ¼
f (x0, x1, x2, A) þ (xCCA 3 [CCA cover]) þ (xturf 3 [turf algal cover]). The best supported model is in
boldface. Asterisks indicate significance of density-dependent model parameters (x1 and x2) relative
to the nested density-independent model (iv): * P , 0.05; ** P , 0.01; *** P , 0.001. Significant,
additive effects of algae on model fit, repeated for each model (iv, vii–ix), are indicated as follows:
à P , 0.05; àà P , 0.01; ààà P , 0.001.
FIG. 3. Survival durations for all individuals present in July 2002. The maximum life span was quantified as the total number of
months a recruit could have been present on the reef before its disappearance was noticed. The graph shows the fraction of each size
class that survived for a certain period of time as indicated by the key. Individual polyps start first polyp divisions at a colony size
around 0.2 cm2. Note the x-axis log scale.
July 2008
EARLY LIFE HISTORY DYNAMICS OF CORALS
colonies experienced much reduced mortality probabilities, with less than 5% of colonies larger than 2.5 cm2
dying within two years. This effect of size class on
mortality probability was described by strong statistical
support for a log-linear function with colony size (kc ¼
10.98 þ 2.24ln(c), P , 0.001 compared to sizeindependent mortality model).
Successful recruitment of S. radians settlers into the
juvenile and adult classes depends on the combination of
survivorship and colony growth. Fig. 4A combines these
two processes, revealing that on average less than 20% of
S. radians settlers successfully enter the two-polyp
juvenile class. Although growth into a two-polyp
individual can occur for upward of 27 months after
settlement, the probability of initial growth (i.e., from
one-polyp individuals to individuals of two or more
polyps) decreased with time since settlement (Fig. 4B).
For the first year after settlement, approximately 21% of
surviving settlers grew into two-polyp juveniles. In
contrast, less than 6% surviving as one-polyp individuals
for over one year made this growth step (74 of 350 new
[,3 months old] settlers vs. five of 84 one-year-old, onepolyp individuals; Fisher’s exact test, two-tailed, P ,
0.004). Because both age and size strongly determined
the fate of coral settlers, and because single polyps can
be present on the reef for over two years, it is important
to highlight both (1) the presence of one-polyp
individuals per se is not indicative of recent or effective
recruitment, and (2) processes structuring the early life
history of S. radians populations operate predominantly
on the smallest individuals, as mortality rates quickly
drop after the start of polyp divisions.
DISCUSSION
Our results show that the early life phase of
scleractinian corals is a dynamic period. Characteristics
of the local habitat, especially conspecific density,
greatly affect the rates of settlement and subsequent
survival of settled planulae. Settlement rates of S.
radians showed a saturating relationship with adult
cover. Across low levels of adult cover (;0–10%)
settlement increased rapidly, but settlement saturated
to a fairly constant rate at higher adult cover (Fig. 2B).
Importantly, the saturating relationship between total
settlement and coral cover translates to a negative
function of settlement per unit local coral cover
(analogous to per capita measures across adult polyp
densities) across all levels of cover, consistent with
models of density-dependent settlement (Caley et al.
1996). Additionally, post-settlement survivorship of
recently settled planulae was negatively related to adult
cover (Fig. 2A). Such conspecific density dependence
could act to limit the population growth rates of S.
radians at high density.
Density-dependent recruitment likely is effected by
the combination of local production (a positive function
of adult density) and very early post-settlement mortality (a negative function of adult density), occurring
2001
FIG. 4. Survival and growth of S. radians settlers. (A) The
overall survival rates of all recruits observed in this study and
the cumulative fraction of recruits that divided into two- (or
more) polyp individuals, as a function of time after initial
recruitment (i.e., age). (B) The effect of age on growth into
juvenile colonies. Bars represent the fraction of surviving
settlers of a particular age that had divided into two- (or more)
polyp colonies. Each fraction was computed based on
approximately 100 haphazardly selected individuals per age
class.
within days to weeks after settlement. The positive
effects of density are well described by the short
dispersal distances typical of S. radians planulae
(Vermeij 2005). As a species adapted to physically
disturbed habitats, S. radians appears to maintain the
capacity for rapid population expansion at small spatial
scales given arrival to an uncolonized environment. In
complement, negative density dependence sets a limit to
the local population growth. Vermeij (2005) proposed a
mechanism leading to negative effects of adult density
on early survivorship, namely Janzen-Connell effects in
which the survivorship of settlers increases with increasing distance from conspecific adult colonies (Janzen
1970, Connell 1971). Microbial associates on coral
surfaces are candidate pathogenic agents driving this
form of density dependence (Knowlton and Rohwer
2003), such that settlers close to adult colonies harboring
potential pathogens are more prone to infection than
those at greater distances. Independently of mechanism,
the negative density dependence inferred from settle-
2002
MARK J. A. VERMEIJ AND STUART A. SANDIN
Ecology, Vol. 89, No. 7
PLATE 1. Overview of various life stages of Siderastrea radians that were considered in this study. The two metal rings in the
top-left corner were used to indicate the position of recently settled planulae and non-metamorphosed settlers in our overview
pictures. Photo credit: M. Vermeij.
ment models and directly quantified in early postsettlement survivorship is an important dynamic affecting the regulation and ultimate density of S. radians
populations (Fig. 2; Hixon et al. 2002). The patterns of
both positive and negative density dependence documented here are accentuated by the high planular
production rates typical of S. radians and other
particularly opportunistic species. However, the mechanisms of such density dependence likely are extensible
to many other coral species, especially to other brooding
species with short dispersal distances.
Complementing conspecific density dependence, other
benthic organisms provided strong density-independent
effects on patterns of settlement. We observed that
increased cover of CCA increased local settlement rates
of S. radians, while increased cover of turf algae reduced
settlement (Fig. 1, Table 1). This result is consistent with
the independent roles of each algal type; while CCA
have been suggested to facilitate coral settlement by
providing suitable substrate (Harrington et al. 2004,
Vermeij 2005), turf algae are considered to be compet-
itors for space with settling corals (Birrell et al. 2005,
Vermeij 2005, 2006, Kuffner et al. 2006). The densityindependent effects of these two algal types reveal a
notable pathology relating coral population growth and
human disturbance. Human activities are known to
increase turf algal cover while reducing available CCA
cover through additions of nutrients and reductions of
grazing pressure through overfishing (Hatcher and
Larkum 1983, Belliveau and Paul 2002). As such, our
data suggest that the efficacy of coral settlement is
particularly prone to the systematic shifts in the algal
community (i.e., from CCA to turf algae) commonly
induced by human disturbance.
Given settlement to the benthos, we have shown that
one-polyp settlers have a brief temporal window during
which to grow and recruit to the juvenile two-polyp
stage. Mortality of settlers is high, especially while
individuals are in the smallest size classes (Fig. 3). In
addition to the effect of size on mortality, the
probability of growing into larger size classes is affected
by the age of the settler. Surviving one-polyp individuals
July 2008
EARLY LIFE HISTORY DYNAMICS OF CORALS
can be present on the benthos without growing into a
two-polyp individual for more than two years. However,
those that are less than one year old are at least three
times more likely to grow into a two-polyp juvenile than
their older counterparts (Fig. 4B). The combination of
these mortality and maturation processes indicates the
critical importance of early growth to ensure successful
recruitment to the adult population.
We have shown that density-dependent processes able
to structure populations act on the youngest age classes
of corals, i.e., before individuals become two-polyp
juveniles. By the time that a coral settler reaches one
year of age, most of the severe and structuring mortality
and demographic bottlenecks have passed. Notably,
most survey-based studies exploring early dynamics of
coral populations operationally define a ‘‘recruit’’ or
‘‘settler’’ as an individual ,3–5 cm2. In comparison, the
typical S. radians individual surviving one year is ,0.1
cm2 in size. Depending on species-specific growth rates,
tracking the fate of .1 cm2 colonies likely omits
reference to the most critical of early life history
dynamics. Because structuring mortality, both densityindependent and density-dependent, occurs well before
these larger colonies are counted, we may expect that
studies based on such definitions will identify tight
coupling between density of operational ‘‘recruits’’ and
future contribution to the adult population (see similar
concern for coral reef fish in Doherty and Fowler [1994]
and subsequent comments in Caley et al. [1996] and
others). Although this predictive ability is powerful for
some applications (e.g., monitoring programs and basic
inventories), focused investigations of the factors
dynamically affecting coral settlement and recruitment
will not be successful without specific reference to these
smallest size and age classes and the incorporation of
time.
ACKNOWLEDGMENTS
This work was carried out under permit no. FKNMS 2002032. C. Fasano, as well as many volunteers from RSMAS and
NOAA are thanked for their assistance during the fieldwork.
Funding was provided by NOAA Fisheries’ Coral Reef
Initiative and E. W. Scripps, Jr., Rolf P. M. Bak, Scott
Lilliputian Hamilton, and three anonymous reviewers provided
invaluable comments on earlier drafts.
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