• Energy Research

AbstractThermal-stress events have changed the structure, biodiversity, and functioning of coral reefs. But how these disturbances affect the dynamics of individual coral colonies remains unclear. By tracking the fate of 1069 individual Acropora and massive Porites coral colonies for up to 5 years, spanning three bleaching events, we reveal striking genus-level differences in their demographic response to bleaching (mortality, growth, and recruitment). Although Acropora colonies were locally extirpated, substantial local recruitment and fast growth revealed a marked capacity for apparent recovery. By contrast, almost all massive Porites colonies survived and the majority grew in area; yet no new colonies were detected over the 5 years. Our results highlight contrasting dynamics of boom-and-bust vs. protracted declines in two major coral groups. These dangerous demographics emphasise the need for caution when documenting the susceptibility and perceived resistance or recovery of corals to disturbances.

AbstractGlobal climate change is altering community composition across many ecosystems due to nonrandom species turnover, typically characterized by the loss of specialist species and increasing similarity of biological communities across spatial scales. As anthropogenic disturbances continue to alter species composition globally, there is a growing need to identify how species responses influence the establishment of distinct assemblages, such that management actions may be appropriately assigned. Here, we use trait‐based analyses to compare temporal changes in five complementary indices of reef fish assemblage structure among six taxonomically distinct coral reef habitats exposed to a system‐wide thermal stress event. Our results revealed increased taxonomic and functional similarity of previously distinct reef fish assemblages following mass coral bleaching, with changes characterized by subtle, but significant, shifts toward predominance of small‐bodied, algal‐farming habitat generalists. Furthermore, while the taxonomic or functional richness of fish assemblages did not change across all habitats, an increase in functional originality indicated an overall loss of functional redundancy. We also found that prebleaching coral composition better predicted changes in fish assemblage structure than the magnitude of coral loss. These results emphasize how measures of alpha diversity can mask important changes in the structure and functioning of ecosystems as assemblages reorganize. Our findings also highlight the role of coral species composition in structuring communities and influencing the diversity of responses of reef fishes to disturbance. As new coral species configurations emerge, their desirability will hinge upon the composition of associated species and their capacity to maintain key ecological processes in spite of ongoing disturbances.

For species with complex life histories such as scleractinian corals, processes occurring early in life can greatly influence the number of individuals entering the adult population. A plethora of studies have examined settlement patterns of coral larvae, mostly on artificial substrata, and the composition of adult corals across multiple spatial and temporal scales. However, relatively few studies have examined the spatial distribution of small (≤50 mm diameter) sexually immature corals on natural reef substrata. We, therefore, quantified the variation in the abundance, composition and size of juvenile corals (≤50 mm diameter) among 27 sites, nine reefs, and three latitudes spanning over 1000 km on Australia's Great Barrier Reef. Overall, 2801 juveniles were recorded with a mean density of 6.9 (±0.3 SE) ind.m(-2), with Acropora, Pocillopora, and Porites accounting for 84.1% of all juvenile corals surveyed. Size-class structure, orientation on the substrate and taxonomic composition of juvenile corals varied significantly among latitudinal sectors. The abundance of juvenile corals varied both within (6-13 ind.m(-2)) and among reefs (2.8-11.1 ind.m(-2)) but was fairly similar among latitudes (6.1-8.2 ind.m(-2)), despite marked latitudinal variation in larval supply and settlement rates previously found at this scale. Furthermore, the density of juvenile corals was negatively correlated with the biomass of scraping and excavating parrotfishes across all sites, revealing a potentially important role of parrotfishes in determining distribution patterns of juvenile corals on the Great Barrier Reef. While numerous studies have advocated the importance of parrotfishes for clearing space on the substrate to facilitate coral settlement, our results suggest that at high biomass they may have a detrimental effect on juvenile coral assemblages. There is, however, a clear need to directly quantify rates of mortality and growth of juvenile corals to understand the relative importance of these mechanisms in shaping juvenile, and consequently adult, coral assemblages.

In several Pacific Island countries and territories (PICTs), rapid population growth and inadequate management of coastal fish habitats and stocks is causing a gap to emerge between the amount of fish recommended for good nutrition and sustainable harvests from coastal fisheries. The effects of ocean warming and acidification on coral reefs, and the effects of climate change on mangrove and seagrass habitats, are expected to widen this gap. To optimise the contributions of small-scale fisheries to food security in PICTs, adaptations are needed to minimise and fill the gap. Key measures to minimise the gap include community-based approaches to: manage catchment vegetation to reduce sedimentation; maintain the structural complexity of fish habitats; allow landward migration of mangroves as sea level rises; sustain recruitment and production of demersal fish by managing ‘source’ populations; and diversify fishing methods to increase catches of species favoured by climate change. The main adaptions to help fill the gap in fish supply include: transferring some fishing effort from coral reefs to tuna and other large pelagic fish by scaling-up the use of nearshore fish aggregating devices; developing fisheries for small pelagic species; and extending the shelf life of catches by improving post-harvest methods. Modelling the effects of climate change on the distribution of yellowfin tuna, skipjack tuna, wahoo and mahi mahi, indicates that these species are likely to remain abundant enough to implement these adaptations in most PICTs until 2050. We conclude by outlining the policies needed to support the recommended adaptations.

Coral settlement is a key ecological process in the maintenance, recovery and resili- ence of coral reef ecosystems. Coral reefs in the Persian Gulf survive in one of the world's most extreme environments, yet there remains limited knowledge of the role of coral settlement consid- ered critical for maintaining population dynamics. Spatial and temporal patterns of coral settle- ment were examined at 6 sites in Dubai, United Arab Emirates, using settlement tiles deployed and collected every 3 mo from 2009 to 2011 following coral community surveys. Settlement was highly seasonal with the highest settlement rates between June and August (3.2 spat tile �1 ± 0.21 SE), when summer sea temperatures approached 35°C. There was a smaller settlement pulse between September and November, but no settlement between December and May. Settlement was observed 1 to 4 mo after the major spawning season (April and May), suggesting either delayed settlement of larvae, or spatial and/or taxonomic disparity between studies of reproduc- tion versus settlement. Settlement rates varied significantly among sites, but spatial variation was consistent between the 2 years of the study, suggesting strong effects of local environmental con- ditions or local coral assemblages. Poritidae and Acroporidae comprised 27 and 11% of the spat respectively, there were no Pocilloporidae and the most abundant coral spat (61%) were from other, not identifiable, families. These data indicate that observed long-term shifts in the commu- nity structure of adult coral assemblages are being reinforced through a combination of settlement and post-settlement processes, such that there is limited scope for recovery of former Acropora - dominated coral assemblages in the Persian Gulf.

Even in the absence of major disturbances (e.g., cyclones, bleaching), corals are subject to high levels of partial or whole-colony mortality, often caused by chronic and small-scale disturbances. Depending on levels of background mortality, these chronic disturbances may undermine individual fitness and have significant consequences on the ability of colonies to withstand subsequent acute disturbances or environmental change. This study quantified intraspecific variations in physiological condition (measured based on total lipid content and zooxanthellae density) through time in adult colonies of two common and widespread coral species (Acropora spathulata and Pocillopora damicornis), subject to different levels of biological and physical disturbances along the most disturbed reef habitat, the crest. Marked intraspecific variation in the physiological condition of A. spathulata was clearly linked to differences in local disturbance regimes and habitat. Specifically, zooxanthellae density decreased (r2 = 26, df = 5,42, p<0.02, B = -121255, p = 0.03) and total lipid content increased (r2 = 14, df = 5,42, p = 0.01, B = 0.9, p = 0.01) with increasing distance from exposed crests. Moreover, zooxanthellae density was strongly and negatively correlated with the individual level of partial mortality (r2 = 26, df = 5,42, p<0.02, B = -7386077, p = 0.01). Conversely, P. damicornis exhibited very limited intraspecific variation in physiological condition, despite marked differences in levels of partial mortality. This is the first study to relate intraspecific variation in the condition of corals to localized differences in chronic disturbance regimes. The next step is to ascertain whether these differences have further ramifications for susceptibility to periodic acute disturbances, such as climate-induced coral bleaching.

Increasing incidence of major disturbances is contributing to extensive and widespread coral loss, thereby undermining the biodiversity, structure and function of reef ecosystems. The composition of coral assemblages is already changing due to selective effects of recurrent disturbances, combined with marked differences in the underlying life-history dynamics of corals, which affects their recovery. This study quantifies the effects of varying disturbance regimes on two groups of corals with divergent life histories: short-lived species with rapid growth (bushy and tabular Acropora) and long-lived species with slow growth (massive and columnar Porites). Inter-decadal shifts in the coral assemblages across four locations suggest that a high frequency of moderate disturbances favours Porites, whereas infrequent, but severe disturbances favour rapidly replenishing Acropora. Using empirical modelling, we expand these observations to show that Acropora continues to dominate so long as the interval between major disturbances is > 2 years. The only disturbance regime we considered that favoured Porites was high frequency (2-year recurrence) of moderate disturbance, whereas high frequency of severe disturbances led to local extirpation of both Acropora and Porites. Our results show that increasing incidence of major disturbances will not necessarily lead to selective loss of species that are most susceptible to disturbance, as long as these species can continue to colonise vacant space and grow quickly in the aftermath of such disturbances. This study highlights the need to consider the sensitivity of taxa to changes in both disturbance frequency and severity when forecasting changes in the composition of coral assemblages under new disturbance regimes.