Partitioning colony size variation into growth and partial mortality

Coral geometry and why it matters

Clonal organisms like reef building corals exhibit a wide variety of colony morphologies and geometric shapes which can have many physiological and ecological implications. Colony geometry can dictate the relationship between dimensions of volume, surface area, and length, and their associated growth parameters. For calcifying organisms, there is the added dimension of two distinct components of growth, biomass production and calcification. For reef building coral, basic geometric shapes can be used to model the inherent mathematical relationships between various growth parameters and how colony geometry determines which relationships are size-dependent or size-independent. Coral linear extension rates have traditionally been assumed to be size-independent. However, even with a constant calcification rate, extension rates can vary as a function of colony size by virtue of its geometry. Whether the ratio between mass and surface area remains constant or changes with colony size is the determining factor. For some geometric shapes, the coupling of biomass production (proportional to surface area productivity) and calcification (proportional to volume) can cause one aspect of growth to geometrically constrain the other. The nature of this relationship contributes to a species' life history strategy and has important ecological implications. At one extreme, thin diameter branching corals can maximize growth in surface area and resource acquisition potential, but this geometry requires high biomass production to cover the fast growth in surface area. At the other extreme, growth in large, hemispheroidal corals can be constrained by calcification. These corals grow surface area relatively slowly, thereby retaining a surplus capacity for biomass production which can be allocated towards other anabolic processes. For hemispheroidal corals, the rate of surface area growth rapidly decreases as colony size increases. This ontogenetic relationship underlies the success of microfragmentation used to accelerate restoration of coral cover. However, ontogenetic changes in surface area productivity only applies to certain coral geometries where surface area to volume ratios decrease with colony size.

Disturbance-Induced Changes in Population Size Structure Promote Coral Biodiversity

AbstractReef-building coral assemblages are typically species rich, yet the processes maintaining high biodiversity remain poorly understood. Disturbance has long been thought to promote coral species coexistence by reducing the strength of competition (i.e., the intermediate disturbance hypothesis [IDH]). However, such disturbance-induced effects are insufficient to inhibit competitive exclusion. Nevertheless, there are other mechanisms by which disturbance and, more generally, environmental variation can favor coexistence. Here, we develop a size-structured, stochastic coral competition model calibrated with field data from two common colony morphologies to investigate the effects of hydrodynamic disturbance on community dynamics. We show that fluctuations in wave action can promote coral species coexistence but that this occurs via interspecific differences in size-dependent mortality rather than solely via stochastic fluctuations in competition (i.e., free space availability). While this mechanism differs from that originally envisioned in the IDH, it is nonetheless a mechanism by which intermediate levels of disturbance do promote coexistence. Given the sensitivity of coexistence to disturbance frequency and intensity, anthropogenic changes in disturbance regimes are likely to affect coral assemblages in ways that are not predictable from single-population models.

Coral assemblages at higher latitudes favor short-term potential over long-term performance

The persistent exposure of coral assemblages to more variable abiotic regimes is assumed to augment their resilience to future climatic variability. Yet, while the determinants of coral population resilience across species remain unknown, we are unable to predict the winners and losers across reef ecosystems exposed to increasingly variable conditions. Using annual surveys of 3171 coral individuals across Australia and Japan (2016-2019), we explore spatial variation across the short- and long-term dynamics of competitive, stress-tolerant, and weedy assemblages to evaluate how abiotic variability mediates the structural composition of coral assemblages. We illustrate how, by promoting short-term potential over long-term performance, coral assemblages can reduce their vulnerability to stochastic environments. However, compared to stress-tolerant, and weedy assemblages, competitive coral taxa display a reduced capacity for elevating their short-term potential. Accordingly, future climatic shifts threaten the structural complexity of coral assemblages in variable environments, emulating the degradation expected across global tropical reefs.

Net effects of life-history traits explain persistent differences in abundance among similar species

Life-history traits are promising tools to predict species commonness and rarity because they influence a population's fitness in a given environment. Yet, species with similar traits can have vastly different abundances, challenging the prospect of robust trait-based predictions. Using long-term demographic monitoring, we show that coral populations with similar morphological and life-history traits show persistent (decade-long) differences in abundance. Morphological groups predicted species positions along two, well known life-history axes (the fast-slow continuum and size-specific fecundity). However, integral projection models revealed that density-independent population growth (λ) was more variable within morphological groups, and was consistently higher in dominant species relative to rare species. Within-group λ differences projected large abundance differences among similar species in short timeframes, and were generated by small but compounding variation in growth, survival, and reproduction. Our study shows that easily measured morphological traits predict demographic strategies, yet small life-history differences can accumulate into large differences in λ and abundance among similar species. Quantifying the net effects of multiple traits on population dynamics is therefore essential to anticipate species commonness and rarity.

Contrasting coral community structures between natural and artificial substrates at Koh Tao, Gulf of Thailand

Concrete cubic frames and decommissioned steel naval vessels have been deployed in Thailand liberally to act as artificial substrates for coral restoration and marine recreation. We assessed recruitment at such substrate types at Koh Tao, Gulf of Thailand, and compared the community structure of scleractinian corals between artificial substrates and nearby natural reefs. Our results from a sample of 2677 recruits from nine sites highlighted significant differences in community structure between both reef types. Investigations of variables including time since deployment, distance from the natural reef, and seafloor depth revealed only the latter as a possible influencing factor. The diversity of recruits could not be explained by dynamics in coral spawning, and were found to represent groups with lower structural complexity. Our results suggest that coral community structure on artificial and natural reefs differs and supports distinct ecological and functional roles.

Coral adaptation to climate change: Meta-analysis reveals high heritability across multiple traits

Anthropogenic climate change is a rapidly intensifying selection pressure on biodiversity across the globe and, particularly, on the world's coral reefs. The rate of adaptation to climate change is proportional to the amount of phenotypic variation that can be inherited by subsequent generations (i.e., narrow-sense heritability, h² ). Thus, traits that have higher heritability (e.g., h²  > 0.5) are likely to adapt to future conditions faster than traits with lower heritability (e.g., h²  < 0.1). Here, we synthesize 95 heritability estimates across 19 species of reef-building corals. Our meta-analysis reveals low heritability (h² < 0.25) of gene expression metrics, intermediate heritability (h²  = 0.25-0.50) of photochemistry, growth, and bleaching, and high heritability (h²  > 0.50) for metrics related to survival and immune responses. Some of these values are higher than typically observed in other taxa, such as survival and growth, while others were more comparable, such as gene expression and photochemistry. There was no detectable effect of temperature on heritability, but narrow-sense heritability estimates were generally lower than broad-sense estimates, indicative of significant non-additive genetic variation across traits. Trait heritability also varied depending on coral life stage, with bleaching and growth in juveniles generally having lower heritability compared to bleaching and growth in larvae and adults. These differences may be the result of previous stabilizing selection on juveniles or may be due to constrained evolution resulting from genetic trade-offs or genetic correlations between growth and thermotolerance. While we find no evidence that heritability decreases under temperature stress, explicit tests of the heritability of thermal tolerance itself-such as coral thermal reaction norm shape-are lacking. Nevertheless, our findings overall reveal high trait heritability for the majority of coral traits, suggesting corals may have a greater potential to adapt to climate change than has been assumed in recent evolutionary models.

Juvenile corals underpin coral reef carbonate production after disturbance

Sea-level rise is predicted to cause major damage to tropical coastlines. While coral reefs can act as natural barriers for ocean waves, their protection hinges on the ability of scleractinian corals to produce enough calcium carbonate (CaCO₃ ) to keep up with rising sea levels. As a consequence of intensifying disturbances, coral communities are changing rapidly, potentially reducing community-level CaCO₃ production. By combining colony-level physiology and long-term monitoring data, we show that reefs recovering from major disturbances can produce 40% more CaCO₃ than currently estimated due to the disproportionate contribution of juvenile corals. However, the buffering effect of highly productive juvenile corals is compromised by recruitment failures, which have been more frequently observed after large-scale, repeated bleaching events. While the size structure of corals can bolster a critical ecological function on reefs, climate change impacts on recruitment may undermine this buffering effect, thus further compromising the persistence of reefs and their provision of important ecosystem services.

Projected shifts in coral size structure in the Anthropocene

Changes in the size structure of coral populations have major consequences for population dynamics and community function, yet many coral reef monitoring projects do not record this critical feature. Consequently, our understanding of current and future trajectories in coral size structure, and the demographic processes underlying these changes, is still emerging. Here, we provide a conceptual summary of the benefits to be gained from more comprehensive attention to the size of coral colonies in reef monitoring projects, and we support our argument through the use of case-history examples and a simplified ecological model. We neither seek to review the available empirical data, or to rigorously explore causes and implications of changes in coral size, we seek to reveal the advantages to modifying ongoing programs to embrace the information inherent in changing coral colony size. Within this framework, we evaluate and forecast the mechanics and implications of changes in the population structure of corals that are transitioning from high to low abundance, and from large to small colonies, sometimes without striking effects on planar coral cover. Using two coral reef locations that have been sampled for coral size, we use demographic data to underscore the limitations of coral cover in understanding the causes and consequences of long-term declining coral size, and abundance. A stage-structured matrix model is used to evaluate the demographic causes of declining coral colony size and abundance, particularly with respect to the risks of extinction. The model revealed differential effects of mortality, growth and fecundity on coral size distributions. It also suggested that colony rarity and declining colony size in association with partial tissue mortality and chronic declines in fecundity, can lead to a demographic bottleneck with the potential to prolong the existence of coral populations when they are characterized by mostly very small colonies. Such bottlenecks could have ecological importance if they can delay extinction and provide time for human intervention to alleviate the environmental degradation driving reductions in coral abundance.