Individual hard coral colonies from four representative reef sites around Little Cayman were surveyed yearly between 2010 and 2015, a period of non-disturbance between two elevated seawater temperature anomalies. Photographic censuses produced 7069 annual transitions that were used to describe the demographics (size class frequencies, abundance, area cover) and population dynamics under non-disturbance environmental conditions. Agariciids, Porites asteroides, and Siderastrea radians have replaced acroporids as the predominant massive corals. Recruitment rates were generally low (<1 colony per m 2), except for a fourfold recruitment pulse of S. radians that occurred in 2011. On average, 42% of coral recruits survived their first year but only 10% lived longer than four years. Temporal comparisons allowed correction factors to be calculated for in-situ methods that overestimate recruitment of colonies ≤2 cm in diameter and overlook larger colonies. Size class transitions included growth (~33%), stasis (~33%), partial mortality (10% - 33%), and whole colony mortality, which decreased with increasing colony size (typically <10% for colonies with surface areas >30 cm 2). Transition matrices indicated that Little Cayman assemblages have declining hard coral populations ( λ < 1) but as stable size class distributions progress toward higher proportions of colonies with >150 cm 2 surface areas, live area cover may remain relatively stable. Projection models indicated that downward population trends would be exacerbated even by mild disturbance (5% - 10% mortality) scenarios. The fate of hard corals on Little Cayman’s reefs was determined to be heavily dependent on the health and transitions of agariciid colonies. Conservation strategies that currently focus on restoration of Caribbean acroporids should be expanded to include agariciids, which were previously considered “weeds”.
Since the 1970s, Caribbean corals have been subjected to regional and local mass mortality events associated with disease outbreaks, temperature-induced bleaching, hurricanes, and anthropogenic stressors, which have resulted in a decline in average coral cover from 35% to 16% [
Little Cayman is an ideal location within the Caribbean to study the population dynamics of hard coral populations because it lacks many of the anthropogenic stressors to which other reefs are exposed. Little Cayman is located 120 km northeast of Grand Cayman, 145 km south of Cuba, and 10 km southwest of Cayman Brac (
Little Cayman has a small resident population (<200 people) and lacks agriculture, freshwater input rivers or streams, commercial fishing, industry, and ports; providing a rare opportunity to decouple the impacts that anthropogenic stressors (e.g. nutrient run off, sedimentation, pesticides, pollution, resource extraction, development) and environmental stressors have on the health of coral reefs.
Live coral cover in Little Cayman has rebounded from its low point of 14% in 2004 to an average of 25%, among the highest in the Caribbean, despite sharing a similar disturbance history with reefs region wide [
The study reported herein took place annually from 2010 through 2015 during non-disturbance environmental conditions (i.e. between the two most recent elevated temperature anomalies and during a period when no hurricanes, disease outbreaks, or other mortality events occurred). The objectives of this study were to 1) describe the demographics and dynamics of the hard coral communities around Little Cayman, 2) use the vital rates, based on temporal comparisons of individual colonies, to develop size class transition probability matrices, and 3) project the recovery potential of these coral assemblages for several disturbances scenarios.
Scleractinian coral populations were surveyed annually along the reef crests at four locations around Little Cayman for six consecutive years, between 2010 and 2015, during the non-disturbance period between two elevated seawater temperature anomalies (i.e. annual surveys began one year after the 2009 bleaching event and ended two months prior to first signs of coral paling in 2015) (
Station | Site Name | Depth (m) | Latitude | Longitude | MPA |
---|---|---|---|---|---|
CORL | Coral City | 11 - 13 | N19.68075 | W80.02330 | No |
GRUN | Grundy’s Garden | 9 - 12 | N19.65733 | W80.08955 | Yes |
ICON | ICON Reef | 11 - 12 | N19.69960 | W80.06058 | Yes |
PAUL | Paul’s Anchors | 11 - 15 | N19.69443 | W80.06943 | Yes |
MPA = Marine Protection Area.
Each scleractinian coral which appeared as a whole colony within a given belt transect was traced using the Area Analysis function in Coral Point Count (CPCe) [
Hard corals were grouped into eight size-dependent classifications (“SC”) (
Size Class | Area Cover (cm2) | Approx Radius (cm) |
---|---|---|
SC1 | <4 | <1 |
SC2 | 4 - 12 | 2 |
SC3 | 12 - 30 | 3 |
SC4 | 30 - 50 | 4 |
SC5 | 50 - 75 | 5 |
SC6 | 75 - 100 | 6 |
SC7 | 100 - 150 | 7 |
SC8 | >150 | >8 |
Hard coral colonies were grouped into eight size classes based on their measured area cover and estimated radii (assuming circular colonies, A = πr2). SC = size class.
S. radians colonies would be combined into the smallest size class (SC1) and their growth and partial mortality rates would be masked as size class stability which could, in turn, mask the overall vitality rates of Little Cayman’s corals. Therefore, size classes based on 1 cm radius increments were used for the analysis described herein.
Size class transition matrices were developed for the two most abundant taxa (agariciids and Porites asteroides) and for all hard corals pooled together to represent Little Cayman overall. (S. radians, the third most abundant taxa, was comprised of small colonies in SC1-3 only; therefore, the respective size class transitions were not developed separately.) The use of eight size classes resulted in 8 × 8 matrices in which each element represents the mean probability of moving from a starting size class or “state” (column) to ending size class or “fate” (row) [
An 8 × 8 matrix has eight eigenvalues, λi, or solutions to the matrix. The dominant eigenvalue (i.e. the largest, positive eigenvalues that is a real number) is the growth rate of the size class-structure population [
Projections were modeled through 2040 as idealized, best-case scenario forecasts of massive coral populations [
A total of 7069 annual transitions were recorded during the six-year study. The sample population was comprised of 23 taxa groups (
Abundant | Common | Rare |
---|---|---|
Agariciids | Diploria strigosa | Dendrogyra cylindricus |
Porites asteroides | D. labrynthiformis | Eusmilia fastigiata |
Siderastrea radians | Dichocoenia stokesi | Favia fragum |
P. furcata | Montastrea cavernosa | Isophyllia sinuosa |
S. siderea | Mycetophyllia spp. | Madracis dectactis |
Orbicella faveolata | Orbicella annularis | Manicina areolata |
O. franksii | Meandrina spp. | |
Stephanocoenia intercepta | Solenastrea bournoni | |
Scolymia spp. |
Abundant = Greater than 100 individual colonies recorded within the belt transects during one or more years; Common = Between 5 and 100 colonies; Rare = Less than 5 colonies.
present, and may be prevalent in certain areas around the island, but were not recorded within the belt transects.
Agariciids, Porites asteroides, and Siderastrea radians collectively comprised >68% of the hard coral communities.
70% of all colonies within the study area were smaller than 30 cm2 (SC1-SC3). Each of the larger size classes (S4-S8) contributed less than 10% to the total population each year (
Recruits were observed within the monitoring stations for all recorded taxa except D. cylindricus, D. labrynthiformis, F. fragum, I. sinosa, and Meandrina spp.; however, recruits of these taxa were observed elsewhere around Little Cayman during the study period. A recruitment pulse of S. radians occurred in 2011 during which recruit abundance was fourfold greater than during each subsequent year of the study (4 recruits per m2 compared to 1 recruit per m2) (
Previous studies have estimated coral recruitment in Little Cayman by counting colonies that are ≤2 cm in diameter [
“weedy” agariciids and P. furcata also had recruits as large as SC6 (diameters ≤ 12 cm): among the SC4-6 agariciid and P. furcata colonies, 5% (± 1%) and 20% (±5%) were recruits, respectively. No recruits were observed in SC7-8 for any of the taxa. Based on the results of this study, the authors recommend that recruit estimation procedures which use size as a proxy for newly settled corals be modified to 1) count colonies in size classes 1 - 3 and apply the correction factors provided in
Recruits from all taxa and size classes experienced low survival: 42% of
Taxa Group | SC1 | SC2 | SC3 | SC4-6 |
---|---|---|---|---|
AGAR | 71% (±5%) | 43% (±10%) | 19% (±3%) | 5% (±1%) |
PAST | 63% (±8%) | 28% (±5%) | 6% (±3%) | -0- |
PFUR | 68% (±19%) | 59% (±27%) | 41% (±14%) | 20% (±5%) |
SRAD | 54% (±14%) | 27% (±9%) | 10% (±12%) | -0- |
SSID | 68% (±8%) | 26% (±14%) | 6% (±8%) | -0- |
OTHERS | 55% (±12%) | 36% (±16%) | 7% (±6%) | -0- |
Five-year mean percentages (± STD) of colonies within each size class that are recruits. Mean percentages may be used as correction factors for in-situ recruit estimation surveys. AGAR = agariciids; PAST = P. asteroides; PFUR = P. furcata; SRAD = S. radians; SSID = S. siderea; OTHERS = 13 common and rare taxa of massive corals pooled together.
Taxa Group | Trend | Equation | r2 |
---|---|---|---|
AGAR | Linear | −0.1016x + 0.6585 | 0.9962 |
PAST | Linear | −0.1151x + 0.6112 | 0.9939 |
PFUR | Polynomial | −0.0433x3 + 0.348x2 ? 0.9175x + 1.0392 | 1.0000 |
SRAD | Polynomial | −0.0172x3 + 0.1777x2 ? 0.6474x + 0.9013 | 1.0000 |
SSID | Polynomial | −0.0461x3 + 0.4501x2 ? 1.3889x + 1.4848 | 1.0000 |
ALL | Polynomial | 0.0223x2 ? 0.2192x + 0.6165 | 1.0000 |
Survival trends of recruits. AGAR = agariciids; PAST = P. asteroides; PFUR = P. furcata; SRAD = S. radians; SSID = S. siderea; ALL = all taxa of massive corals pooled together.
recruits survived their first year; 27% survived to their second year; 16% survived to their third year; and 10% survived to their fourth year (
Approximately one-third of the SC1-SC7 colonies grew between monitoring periods regardless of size class (
Approximately one-third of the SC1-SC7 colonies remained within the same size class. However, 60% - 100% of those colonies which achieved surface areas >150 cm2 (SC8) had size class stability, depending on the taxa group.
Partial mortality varied by size class. Approximately one-third of the SC4-SC7 colonies decreased in size. Among the smaller SC2-SC3 and larger SC8 colonies, 10% - 15% experienced partial mortality and transitioned into a smaller size class. Occasionally, shrinkage of four size classes was recorded; however, 74% of the shrinking colonies transitioned to the next smaller size class.
Between one-third and half of SC1 colonies die each year, largely due to the high mortality of recruits. The probabilities of whole colony mortality decrease with increasing colony size and are typically <10% for those colonies with surface areas >30 cm2 (SC4-SC8). An exception is the apparent increase in mortality for P. asteroides, which increases from negligible mortality for SC6-SC7 colonies to 20% mortality for SC8 colonies. This study was conducted during a period of non-disturbance environmental conditions (e.g. no temperature anomalies, disease outbreaks, extreme storms); therefore, the cause(s) for whole colony mortality require(s) further study.
The stable size class distributions (i.e. the eigenvectors associated with the dominant eigenvalues), dominant eigenvalues and damping ratios were determined for agariciids, P. asteroides, and all 23 hard coral taxa pooled together (
Sensitivities and elasticities are measures of perturbation analyses that quantify the relative contribution of each vital rate to the population growth by a specific
A. Agariciids | ||||||||
---|---|---|---|---|---|---|---|---|
SC1 | SC2 | SC3 | SC4 | SC5 | SC6 | SC7 | SC8 | |
SC1 | 0.177 | 0.066 | 0.007 | 0.006 | 0 | 0 | 0 | 0 |
SC2 | 0.304 | 0.304 | 0.148 | 0.042 | 0.017 | 0.012 | 0 | 0.009 |
SC3 | 0.104 | 0.253 | 0.352 | 0.188 | 0.111 | 0.028 | 0.036 | 0.029 |
SC4 | 0.005 | 0.059 | 0.230 | 0.241 | 0.214 | 0.103 | 0.035 | 0.015 |
SC5 | 0.011 | 0.028 | 0.070 | 0.233 | 0.241 | 0.155 | 0.238 | 0.025 |
SC6 | 0 | 0.004 | 0.024 | 0.113 | 0.222 | 0.167 | 0.116 | 0.053 |
SC7 | 0 | 0.002 | 0.009 | 0.076 | 0.079 | 0.276 | 0.259 | 0.109 |
SC8 | 0 | 0 | 0.012 | 0.031 | 0.062 | 0.165 | 0.316 | 0.752 |
s | 0.601 | 0.716 | 0.852 | 0.930 | 0.946 | 0.906 | 1.000 | 0.992 |
d | 0.399 | 0.284 | 0.148 | 0.070 | 0.054 | 0.094 | 0 | 0.008 |
B. Porites asteroides | ||||||||
---|---|---|---|---|---|---|---|---|
SC1 | SC2 | SC3 | SC4 | SC5 | SC6 | SC7 | SC8 | |
SC1 | 0.320 | 0.077 | 0.007 | 0 | 0 | 0 | 0 | 0 |
SC2 | 0.264 | 0.431 | 0.107 | 0.010 | 0 | 0.066 | 0 | 0 |
SC3 | 0.065 | 0.242 | 0.543 | 0.291 | 0.121 | 0 | 0 | 0 |
SC4 | 0 | 0.027 | 0.179 | 0.335 | 0.118 | 0.124 | 0.050 | 0 |
SC5 | 0 | 0.003 | 0.055 | 0.179 | 0.384 | 0.224 | 0.050 | 0 |
SC6 | 0 | 0 | 0.003 | 0.087 | 0.159 | 0.067 | 0.200 | 0.033 |
SC7 | 0 | 0 | 0 | 0.074 | 0.080 | 0.195 | 0.200 | 0.150 |
SC8 | 0 | 0 | 0 | 0 | 0.040 | 0.324 | 0.500 | 0.617 |
s | 0.649 | 0.780 | 0.894 | 0.976 | 0.902 | 1.000 | 1.000 | 0.800 |
d | 0.351 | 0.220 | 0.106 | 0.023 | 0.098 | 0 | 0 | 0.200 |
C. All 23 taxa pooled together | ||||||||
---|---|---|---|---|---|---|---|---|
SC1 | SC2 | SC3 | SC4 | SC5 | SC6 | SC7 | SC8 | |
SC1 | 0.344 | 0.096 | 0.014 | 0.002 | 0 | 0 | 0 | 0 |
SC2 | 0.158 | 0.372 | 0.127 | 0.030 | 0.011 | 0.023 | 0 | 0.002 |
SC3 | 0.029 | 0.213 | 0.392 | 0.212 | 0.082 | 0.029 | 0.016 | 0.006 |
SC4 | 0.002 | 0.036 | 0.213 | 0.291 | 0.198 | 0.117 | 0.048 | 0.008 |
SC5 | 0.001 | 0.009 | 0.080 | 0.226 | 0.257 | 0.135 | 0.108 | 0.010 |
SC6 | 0 | 0.002 | 0.022 | 0.100 | 0.187 | 0.181 | 0.151 | 0.019 |
SC7 | 0 | 0.001 | 0.009 | 0.067 | 0.142 | 0.281 | 0.204 | 0.080 |
SC8 | 0 | 0.001 | 0.007 | 0.021 | 0.071 | 0.199 | 0.468 | 0.864 |
s | 0.534 | 0.730 | 0.864 | 0.949 | 0.948 | 0.965 | 0.995 | 0.989 |
d | 0.466 | 0.270 | 0.136 | 0.051 | 0.052 | 0.035 | 0.005 | 0.012 |
Mean probabilities from 2010-11, 2011-12, 2012-13, 2013-14, and 2014-15 annual comparisons. Columns depict starting state, rows depict ending fate according to Equation (1). s = probability of colony survival (the sum of the transition probabilities in the respective column); d = probability of whole colony death (1 ? s).
AGAR | PAST | ALL | |
---|---|---|---|
Stable SC Distributions | |||
SC1 | 12% | 1% | 1% |
SC2 | 21% | 6% | 2% |
SC3 | 25% | 18% | 5% |
SC4 | 13% | 11% | 6% |
SC5 | 11% | 11% | 6% |
SC6 | 6% | 8% | 6% |
SC7 | 4% | 12% | 10% |
SC8 | 8% | 33% | 64% |
Dominant Eigenvalue | |||
λ | 0.94 | 0.88 | 0.97 |
Damping Ratio | |||
Λ1/|λ2| | 1.36 | 1.16 | 1.29 |
Stable size class distributions and eigenvalues from mean annual transition probability matrices.
amount and by a specific proportion, respectively [
The mean annual size class transition probability matrices were used to project populations within the monitoring stations through 2040 (
1) The number of P. asteroides colonies is projected to decline by 23% over the 13 years following this study until a stable population distribution is reached. The largest changes in the population structure are projected to be a 16% decline in SC1, a 10% decline in SC2, and a 10% increase in SC8. The mid-sized colonies (SC3-SC7) are projected to increase by ≤4% each. The trend toward larger colonies is projected to nearly balance with the decline in the number of colonies, resulting in a ≤1% increase in the total area cover of P. asteroides in 2040 compared to 2015 (assuming the mean surface area of SC8 corals remains unchanged within this timeframe).
2) The number of agariciid colonies is projected to decline by 11% for six years, followed by a gradual 13-year recovery before stabilizing at 93% of
the starting population. The largest changes are projected to be 6% - 9% decreases in SC1-SC3 and a 14% increase in SC8. The total area cover of agariciids is projected to be 2% higher in 2040 than in 2015.
3) The number of hard corals (all taxa pooled together) is projected to decline by 13% for six years, and to require 19 years to fully recover to the population size recorded at the beginning of this study in 2010. The largest change is projected to be a 22% increase in SC8 colonies (
Projections were recalculated for the hardcoral population within the monitoring stations under three mild disturbance scenarios (
Agariciids, Porites asteroides, and Siderastrea radians have replaced acroporids as the predominant massive corals on the shallow reefs around Little Cayman. These resilient and resistant corals have characteristics of “long-term winners” [
Agariciids are the most dynamic of the hard coral taxa: recruit survival is relatively high compared to other taxa and colonies are capable of transitioning as many as six size classes up or down in a single year. Those colonies that reach SC4-SC8 (≥30 cm2) have a ≥ 90% chance of survival to the next year. These life history traits are critical because agariciids around Little Cayman are susceptible to bleaching [
Porites asteroides are characterized as persistent because they are slow to change: two-thirds of the colonies either remain in the same size class or transition (grow/shrink) only one size class each year. Previous studies in St. John, U.S. Virgin Islands recorded similar size class transition probabilities among this species [
Siderastrea radians are the most prolific recruiters during this study; however, the high recruit settlement is accompanied by the lowest recruit survival among the taxa. The recruitment pulse that occurred in 2011 did not result in a significant increase in coral area cover. S. radians colonies remain small, rarely exceeding SC2 (12 cm2) and together contribute only 1% of the live area cover.
The ecological succession of these taxa, along with S. siderea, following the loss of acroporids has been observed on reefs throughout the Caribbean [
Coral communities have been shown to require 10 - 30 years to recover after a significant mortality event (e.g. [
Little information pertaining to hard coral vital rates in the Cayman Islands have been published to date. The population dynamics reported in this study may be used as baseline comparisons when conducting reef health surveys, when reporting the effects of disturbances on coral communities, or when developing predictive ecological models. With declining populations during non-disturbance conditions, Little Cayman’s reefs are at risk of severe degradation should a large proportion of the SC8 colonies become compromised due to natural or anthropogenic stresses. Current recruitment levels cannot replace losses associated with minor disturbance events (e.g. 5% - 10% mortality).
The fate of hard corals on Little Cayman’s reefs was determined to be heavily dependent on the health and transitions of agariciid colonies. Conservation strategies that currently focus on Acropora cervicornis and A. palmata restoration e.g. [
The authors thank the Cayman Islands Department of Environment for issuing the permit to install the monitoring station markers. The Central Caribbean Marine Institute (CCMI) allowed the authors to collect underwater images during its undergraduate field programs and annual monitoring dive trips. The National Science Foundation provided partial funding for travel to/from Little Cayman and field support during the 2014-2016 Research Experience for Undergraduates (Grant OCE-1358600).
Foster, K.A. and Foster, G. (2018) Demographics and Population Dynamics Project the Future of Hard Coral Assemblages in Little Cayman. Open Journal of Marine Science, 8, 196-213. https://doi.org/10.4236/ojms.2018.81010