Comparative demography of two common scleractinian corals: Orbicella annularis and Porites astreoides

Introduction

Scleractinian corals are understood to be highly susceptible to environmental stress. However, species can vary widely in their capacity to withstand stressful environmental conditions such as high sea surface temperature, high sedimentation or epizootic events. Darling et al. (2012) categorized coral depending on their susceptibility to biological or physical disturbances. Paradoxically, in the classification proposed by Darling et al. (2012), Orbicella annularis (formerly known as Montastraea annularis) (Budd et al., 2012) is placed in the stress-tolerant category. There is evidence, however, indicating that the vital rates of this coral are sensitive to environmental stressors such as increased water temperature (Edmunds & Elahi, 2007; Hernández-Pacheco, Hernández-Delgado & Sabat, 2011) and is one of the most susceptible Caribbean corals to corallivorous snails (Potkamp, Vermeij & Hoeksema, 2017a, 2017b). It is, also, susceptible to coral diseases (Bruckner & Bruckner, 2006; Weil, Urreiztieta & Garzón-Ferreira, 2002; Weil, Cróquer & Urreiztieta, 2009) and is classified as “endangered” by the IUCN list (Carpenter et al., 2008). This apparent contradiction as to whether O. annularis is a “stress tolerant” species underlies our ignorance as to the biological basis of coral resilience to environmental stressors, and how to measure such resilience. On the other hand, Porites astreoides has been classified as a “weedy” coral species (Darling et al., 2012) and is considered as “least concern” by the IUCN list (Carpenter et al., 2008). This coral species is projected to increase its population size even when impacted with recurrent disturbance (Edmunds, 2010). Indeed, it is expected that coral cover in the Caribbean will be dominated by weedy species (Green, Edmunds & Carpenter, 2008). Based on the classification of Darling et al. (2012) we would expect similar demographic patterns on both species under similar conditions.

Documenting and understanding inter-species variability to environmental stress is key to predicting how corals will fare under the current and predicted climate change scenarios. For example, it has been suggested that coral reefs more resilient to climate change would be those dominated by coral species showing higher resistance to the effects of climate change rather than corals showing higher recovery from those effects (Coté & Darling, 2010). A powerful approach to evaluating the resistance of a species to environmental stress is by measuring the demographic or population level response of the species to various levels of environmental stress. Whether a coral population grows, declines or remains stable under a given environmental insult will depend on the effect of the environmental stressor on the colonies’ vital rates. In turn, vital rates are intimately related to the physiological state of corals (Anthony & Hoegh-Guldberg, 2003; Karim, Nakaema & Hidaka, 2015; Mayfield et al., 2012; McGinty, Pieczonka & Mydlarz, 2012). High temporal and spatial variability in vital rates should be indicative of a susceptible coral species, particularly if measured between sites or years that differ in the level of environmental stress to which colonies are subjected to. Resilient species, on the other hand, should exhibit little spatial and temporal variation in growth, survival or fecundity.

In this study, we examined the demography of O. annularis and P. astreoides from 2012 to 2014 at two localities in Northeastern Puerto Rico known to differ in benthic composition (cover). One of the reefs is dominated by non-calcareous algae, which is indicative of degraded conditions, whereas the other is characterized by a high coral cover which is characteristic of healthy coral reefs (Bruno et al., 2009; Done, 1992). The fact that both O. annularis and P. astreoides can be found inhabiting under these contrasting reefs conditions provides the opportunity to test whether this coral is indeed a stress-tolerant species. The working hypotheses are that if O. annularis is a stress-tolerant species, then its demographic behavior (1) should not vary considerably between localities, that is, we should observe no significant spatio-temporal difference in the vital rates and population growth rates; and (2) its demography would be comparable to that of a weedy species, P. astreoides. To test these hypotheses, we constructed and parameterized four size-based population matrices, one for each species, site and year. The matrices were used to (1) determine whether the studied populations were stable (λ = 1), increasing (λ > 1), or decreasing (λ < 1); (2) identify the life cycle transition(s) that accounted for the observed spatiotemporal differences in λ by performing life table response analyses; and (3) identify whether time or location have more effect on the fate of the colonies by using log-linear analyses. This study is the first to use demographic modeling to compare the population dynamics of O. annularis and P. astreoides in two contrasting coral reefs (regarding coral cover) to address how local stressors can affect their vital rates.

Materials and Methods

Field work was conducted at two sites within the Puerto Rico archipelago. Carlos Rosario Reef (18°19′29″N, 65°19′60″W) is located within the Canal Luis Peña No-Take Natural Reserve on the island municipality of Culebra, approximately 30 km east of the Puerto Rico mainland. This reef is a linear fringing reef covering an area of approximately 30 ha with depths ranging from sea level to 14 m. Palomino (18°21′3″N, 65°33′58″W) is located within the Arrecifes de la Cordillera Natural Reserve, 6 km east from the coastal municipality of Fajardo on the Puerto Rico mainland. Carlos Rosario reef has a northwest orientation and experiences low to moderate wave action and long-period swells during the winter. Palomino reef has a southwest orientation and leeward from Palomino Island. Thus, it experiences less wave action. The reefs show a contrasting biological structure with a significantly higher coral cover (F(3, 159) = 83.90; P = 0.01) in Carlos Rosario Reef (Fig. 1) as studied on September 2012. Based on these results, and because high coral cover is characteristic of healthier coral reefs (Bruno et al., 2009; Done, 1992), Carlos Rosario was considered as a healthier coral reef than Palomino, and thus, presumably, one that has experienced less stressful environmental conditions than Palomino.

Results

Density, cover and size structure

The number of colonies per m² of P. astreoides in Carlos Rosario (4.27 ± 0.36 SE) was significantly higher than for O. annularis (3.75 ± 0.34 SE) (Fig. 2). The same pattern was true in Palomino (P. astreoides: 1.98 ± 0.26 SE; O. annularis: 1.15 ± 0.31 SE; Fig. 2). However, no interaction was found between sites and species (Table 1). We did find significant differences when comparing the colony density (sites pooled) with P. astreoides showing higher densities of colonies (3.14 ± 0.24 SE) than O. annularis (2.47 ± 0.25 SE) (Table 1). We, also, found significant differences when comparing density within sites (species pooled) with Carlos Rosario showing higher values (4.01 ± 0.25 SE) than Palomino (1.57 ± 0.21 SE) (Table 1).

Table 1:

Two-way ANOVA for the colony density analysis.

Source of variation	df	Sum squares	Mean square	F	P
Species	1	28.372	28.372	4.466	0.036
Site	1	365.525	365.525	57.542	<0.001
Species × site	1	1.425	1.425	0.224	0.636
Residual	241	1530.909	6.352
Total	244	1925.363	7.891

DOI: 10.7717/peerj.3906/table-1

Mean colony size (cm²) of O. annularis in Carlos Rosario (58.47 ± 3.32 SE) was larger than P. astreoides (49.21 ± 2.70 SE) (Fig. 3). The same pattern was observed in Palomino, where mean colony size was 66.23 cm² (±7.10 SE) and 42.94 cm² (±3.70 SE) for O. annularis and P. astreoides, respectively (Fig. 3). Like the density data, no interaction was found between sites and size of species (Table 2) and no significant differences were found on mean colony size between sites (species pooled). We did find significant differences when comparing the size of species (sites pooled) with O. annularis showing larger colonies (62.35 ± 3.09 SE) than P. astreoides (46.08 ± 2.29 SE) (Table 2).

Table 2:

Two-way ANOVA for the size distribution analysis.

Source of variation	df	Sum squares	Mean square	F	P
Species	1	38253.1	38253.1	11.096	<0.001
Site	1	81.632	81.632	0.0237	0.878
Species × site	1	7110.766	7110.766	2.063	0.151
Residual	1,022	3,523,277	3447.433
Total	1,025	3,565,008	3478.056

DOI: 10.7717/peerj.3906/table-2

Population growth

During the first year of the study, populations of both species showed λ values below 1.0 (decreasing population). During the first year, populations of O. annularis exhibited λs below equilibrium at Carlos Rosario (0.817) and Palomino (0.694), followed by a decline at both sites during the second year (0.700 and 0.667, respectively). Populations of P. astreoides showed higher λs than O. annularis during the first census period at Carlos Rosario (0.898) and Palomino (0.894) with a decline at one of the sites (0.681 and 0.893) during the second census period. Significant differences (based on 95% CI) where observed when comparing populations of O. annularis between sites (Table 3). In contrast, λs did not vary considerably spatially for P. astreoides (Table 3). When comparing λs between species within sites O. annularis showed lower rates of population growth rates than P. astreoides at Palomino but not in Carlos Rosario. For the second year, λs decreased for both species and sites with a slight reduction found in Palomino, but no significant differences were found (based on 95% CI) (Table 3). O. annularis in C. Rosario showed a slightly higher λ value compared to Palomino but a lower λ value when compared to the first census. Contrastingly, P. astreoides in C. Rosario showed a notable decrease in λ than Palomino and compared to the first census.

Table 3:

Size class transition probabilities of small-sized colonies (S: 0–50 cm²), medium-sized colonies (M: 51–150 cm²), large-sized (L: >150 cm²) and proportion of dead (D) colonies of O. annularis and P. astreoides at two localities (Carlos Rosario, Culebra and Palomino, Fajardo) in Eastern Puerto Rico during two annual time periods.

Size class	2012–2013				2013–2014
	S	M	L		S	M	L
O. annularis
C. Rosario
S	0.673	0.066	0	S	0.363	0.066	0
M	0.163	0.666	0.142	M	0.090	0.166	0.090
L	0	0.166	0.500	L	0.151	0.333	0.636
D	0.163	0.100	0.357	D	0.393	0.433	0.272
λ	0.817 (95% CI 0.791–0.915)			λ	0.700 (95% CI 0.573–0.894)
Palomino
S	0.500	0.400	0	S	0.620	0.062	0.045
M	0.086	0.400	0.142	M	0	0.312	0.136
L	0.043	0.066	0.500	L	0.034	0.187	0.545
D	0.369	0.133	0.357	D	0.344	0.437	0.272
λ	0.694 (95% CI 0.686–0.825)			λ	0.667 (95% CI 0.636–0.883)
P. astreoides
C. Rosario
S	0.608	0.275	0.200	S	0.551	0.429	0.150
M	0.065	0.437	0.230	M	0	0.363	0.230
L	0	0.187	0.769	L	0	0.090	0.615
D	0.326	0.187	0	D	0.448	0.181	0.153
λ	0.898 (95% CI 0.837–0.982)			λ	0.681 (95% CI 0.642–0.909)
Palomino
S	0.755	0.466	0.503	S	0.512	0.482	0.189
M	0.066	0.350	0.312	M	0.051	0.214	0.090
L	0.022	0.250	0.250	L	0.1025	0	0.818
D	0.155	0.100	0.187	D	0.333	0.428	0.090
λ	0.894 (95% CI 0.824–0.934)			λ	0.893 (95% CI 0.777–1.000)

DOI: 10.7717/peerj.3906/table-3

Note:

λ = estimated population growth rate; numbers within () represent the lower and upper 95% CI.

LTRE analysis indicates that differences in the first year of the study in O. annularis growth rates between sites were mainly due to a lower proportion of retrogression (i.e., colonies that decreased in size) from medium to small-sized colonies (Rsm) and higher values of stasis (i.e., colonies that stayed in the same size class) of medium-sized (Smm) colonies (Fig. 4A) found in Carlos Rosario (Table 3). In the second year (2013–2014), differences were mainly due to the higher proportion of stasis of large-sized colonies (Sll; Fig. 4B) found in Carlos Rosario (Table 3).

Differences in λs observed in P. astreoides between Carlos Rosario and Palomino during the first year of study were mainly due to a high proportion of stasis of large-sized colonies (Sll; Fig. 4C) found in Carlos Rosario (Table 3). However, in the second year of the study, the reduction in λ for P. astreoides in Carlos Rosario was mainly caused by a decrease in the proportion of stasis of large-sized colonies (Sll; Fig. 4D).

In terms of differences in λs between census periods, populations of O. annularis in Carlos Rosario showed a decrease from 0.80 to 0.70 (Table 3). The higher growth rates experienced by the populations during 2012–2013 was most probably due to a higher survival rate of medium-sized colonies that remained in the same size category (Fig. 5A). The decrease in λs between census periods found in P. astreoides populations in Carlos Rosario from 0.89 to 0.68 (Table 3) was mainly due to a higher survival rate of large-sized colonies that remained in the same size category during the first year (Fig. 5C).

Discussion

The objectives of this study included a comparison of the vital rates of two common scleractinians in Caribbean coral reefs that are thought to be stress-resistant (Darling et al., 2012): O. annularis and P. astreoides. In general, we found that for the studied populations (1) stasis of small-sized colonies was the life cycle transition that contributes the most to local population growth rates; (2) colony shrinkage was more common than colony growth; (3) colony fate was more dependent on spatial rather than on time variability; and (4) population growth rates were below equilibrium.

In comparisons of size progression (e.g., growth) and shrinkage (e.g., size reduction) of colonies during the whole census period, both species showed higher rates of size retrogression, particularly in Palomino, the location classified as degraded. These results are consistent with previous studies concluding that partial mortality is more prevalent in reefs with “suboptimal” environmental conditions (Nugues & Roberts, 2003; Wielgus et al., 2010; Dikou & van Woesik, 2006). The dominance of small colonies (positive skewing) within coral populations is a common response to environmental stress episodes (Hughes & Tanner, 2000; Edmunds & Elahi, 2007). Positive skewing found on O. annularis may be due to episodes such as disease, physiological stress, or algal overgrowth that reduce colony size by partial mortality or fission (Edmunds & Elahi, 2007). Demographic analysis and modeling have indicated that the vital rates of O. annularis are very sensitive to bleaching, and that interval of major events of less than 17 years will result in population reduction that will seriously compromise the viability of O. annularis populations (Hernández-Pacheco, Hernández-Delgado & Sabat, 2011). However, in our two-year census study, no major disturbances such as hurricanes, bleaching, predator outbreaks, or epizootic events directly impacted the two study sites. The only perturbing event during the study period was a heavy rainfall event (356 cm, an anomaly of 215% in relation to mean monthly rainfall) during June 2013 (Hernández-Delgado et al., 2014), three months before the first survey. Low salinity may weaken the coral physiological performance (Kerswell & Jones, 2003) with conceivable costs to colony growth. Therefore, our study suggests that even in the absence of a major disturbance, the vital rates of P. astreoides and O. annularis can be susceptible to minor or moderate local variations in environmental parameters. This pattern concurs with a previous study about the population vital rates of Acropora cervicornis within the same localities (Mercado-Molina et al., 2015). Even with these environmental conditions, λs in both sites were <1 perhaps due to the zero-recruitment found for O. annularis.

Results of the log-linear analysis for both species suggest that location is a better predictor of colony fate than time. Mercado-Molina et al. (2015), working in the same area, found comparable results for the threatened coral A. cervicornis. In the current study, the log-linear analysis showed that large-sized colonies of O. annularis were not sensitive to the effect of location or time. This contrast with previous studies where the largest effect of O. annularis population decline have been attributed to the mortality of large colonies (Hughes & Tanner, 2000; Edmunds & Elahi, 2007). On the other hand, in P. astreoides none of the size classes was clearly vulnerable to spatio-temporal variability. However, it was observed that both small and large colonies had a high probability to survive and remain within their size class. This observation coincides with a previous demographic study showing that stasis is the most common demographic transition in P. astreoides (Edmunds, 2010).

The observed spatio-temporal similarity in the pattern of demographic transitions can lead us to conclude that both species behave in similar ways from the perspective of their demography, which is consistent with our hypothesis, that if O. annularis is a stress-tolerant species, then its demography would be comparable to that of a stress-tolerant species, P. astreoides. Nevertheless, to be considered demographically resilient, a population should exhibit low spatial and temporal variability of its vital rates. For instance, both species showed considerable spatio-temporal variability in rates of demographic transitions as well as in λ values. The drastic decline in population growth rates experienced by the two species at Carlos Rosario was somehow surprisingly, not only because it occurred during a period with no major disturbances but also because the site seems to have appropriate environmental conditions for coral development (e.g., clear water, low algal cover). Interestingly, Mercado-Molina et al. (2015) also documented a considerable reduction in population growth rates in the coral A. cervicornis at Luis Peña reef, which is adjacent and has similar environmental conditions to Carlos Rosario. A possible explanation is that corals at Carlos Rosario were more susceptible to even slight changes in the prevailing environmental conditions, i.e., factors that are not necessarily considered major disturbances, such as an increase in heavy rainfall (as mentioned above). Corals that have survived poor conditions tend to be stronger immunologically and may be adapted to high environmental variability (Baker et al., 2004; Baker, Glynn & Riegl, 2008).

After the mass bleaching in 2005, O. annularis colonies within the Carlos Rosario site suffered a population decline but recovered three years after the event (Hernández-Pacheco, Hernández-Delgado & Sabat, 2011). A similar pattern of recovery was found in Tektite Reef in St. Johns (Edmunds, 2015). Edmunds (2015) proposed that O. annularis shows ecological resilience because of the increased coral cover within the Tektite Reef and projected a continued population growth in years to come if no disturbance impacts this site. He concluded that this resilience was found mainly because of the deep habitat (14 m) in which these colonies are found. At this depth colonies would be alleviated from the effects of elevated temperature bleaching due to lower light intensity. Contrastingly in our study, O. annularis populations showed a decrease in growth even with no major environmental conditions affecting these colonies. However, our study did not include a deep transect and all colonies sampled were at depths shallower than 8 m.

We found the lower intrinsic rate of population growth (λ) for P. astreoides colonies (0.68–0.89) rates than previously reported for other Caribbean reefs (0.95–1.02) (Edmunds, 2010). However, similar λ values (0.89) have been reported in restored coral reef sites in Florida (Lirman & Miller, 2003). Low and decreasing population growth in P. astreoides was unexpected since an increase in abundance of these colonies has been projected over the next 100 years even with recurrent disturbance events and variable recruitment (Green, Edmunds & Carpenter, 2008; Edmunds, 2010). Conversely, we found O. annularis colonies showing λs <1 (0.69–0.81) during the two years this study was performed. Similar values have been found for these colonies (0.58–0.67) in other Caribbean reefs (Edmunds & Elahi, 2007).

Population growth of P. astreoides was 0.89 for both sites in the first year and for Palomino in the second year. In the second year, the population in Carlos Rosario showed λ of 0.68, the lowest for these colonies. If we compare the recruitment rates of this species within both sites, colonies in Carlos Rosario showed a significant lower recruitment rate. Perhaps the lower λ values found on this site is because population growth of P. astreoides is susceptible to variations in recruitment rates (Lirman & Miller, 2003). Recruitment rates in Carlos Rosario are similar to what Lirman & Miller (2003) found in Florida (0.30 m² y⁻¹). Such recruitment rates are considered moderate for this species. The recruitment rate found in Palomino (0.62 m² y⁻¹) may be considered as high for P. astreoides which may result in an increase in population abundance in the near future (Lirman & Miller, 2003). However, λs of P. astreoides were below 1 even with the high recruitment in Palomino. Previous studies have shown even higher recruitment rates for these colonies (0.98–1.64 individuals 0.25 m⁻²) in St Johns (Edmunds, 2010). Perhaps the population growth below equilibrium found in the current study for P. astreoides is because of highly variable recruitment rates (Lirman & Miller, 2003).

The comparable variability in λs found in our study within both sites suggests that “weedy” (P. astreoides) and “stress tolerant” (O. annularis) (Darling et al., 2012) corals may behave similarly in terms of their demographics under a scenario of no major stressful event. The higher recruitment rates for P. astreoides found in the “degraded” Palomino may support other studies highlighting the ability of this species to exploit areas that are normally not suitable for planulae settlement (in terms of macroalgal cover). Similar survival rates of both species within both sites suggest comparable demographic resilience of both coral species in a “degraded” and “healthy” site and that O. annularis may survive in stressful environments (in terms of macroalgal cover) such as Palomino. Perhaps the O. annularis colonies in this low coral/high macroalgal cover conditions have invested more energy into survival and have adapted to these stressful conditions.

In terms of coral reef management, we propose that coral colonies surviving, but most important recruiting, in sites with high macroalgal/low coral cover (such as Palomino) are an important aspect of future resiliency in coral reef environments. Palomino site is one of the most visited sites by recreational vessels within the Eastern part of Puerto Rico and is used by a major hotel which takes its clients to visit the island as well (Shivlani, 2009) showing high anthropogenic impacts in its marine ecosystems (Hernández-Delgado et al., 2012). Carlos Rosario reef, the high coral/low macroalgal cover site, is located within a marine protected area and 30 km apart from the main island of Puerto Rico. However, we observed similar patterns of vital rates and population growth rates of both species within both locations and even higher recruitment of P. astreoides in Palomino. These findings concur with the recent theory that locally impacted reefs might be less sensitive to global stressors than remote, more pristine, reefs (Bruno & Valdivia, 2016; Coté & Darling, 2010) because corals that survive in impacted reefs may be more resistant to climate change effects. Bruno & Valdivia (2016) found no correlation between coral/macroalgal cover with isolation from local anthropogenic stressors around the world. We need to look at the coral population dynamics within the “degraded” coral reefs and start asking questions to understand better how to deal with the issue of promoting resilience in the coral reef environment (Coté & Darling, 2010).

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