What happened to a subtropical coral population after severe heat stress?

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In the Anthropocene, coral reefs face increasingly severe marine heatwaves and mass coral bleaching mortality. The resulting demographic changes in coral assemblages can have long-term effects on reef community organisation. Understanding the dynamics of subtropical scleractinian coral populations is therefore essential for predicting their recovery or extinction after disturbance.

Researchers conducted a 10-year demographic assessment of the subtropical endemic coral Pocillopora aliciae in the Solitary Islands Marine Park, eastern Australia, alongside long-term temperature records. These populations are regularly affected by storms, experience seasonal temperature variability and are increasingly affected by severe marine heatwaves. To examine the demographic processes governing their persistence, the researchers inferred population changes from size-frequency distributions based on log-transformed planar area measurements of 7,196 coral colonies. They used the mean, coefficient of variation, skewness and kurtosis of those distributions, as well as coral density, to describe population dynamics. Generalised Linear Mixed Effects Models were used to determine trends over time and test demographic responses to heat stress.

Changes in size-frequency distributions over time revealed population processes ranging from recruitment pulses and cohort growth to bleaching impacts and temperature dependencies. Sporadic recruitment pulses likely support population persistence: in 2010, the distributions were strongly positively skewed and juvenile coral density was the highest measured during the study. The increase in mean colony size over the following six years indicates further cohort growth among those recruits. Severe heat stress in 2016 caused mass bleaching mortality and a 51% decline in coral density. Moderate heat stress in subsequent years was associated with suppressed P. aliciae recruitment and a lack of early recovery, with juvenile density, a measure of recruitment, decreasing exponentially as heat stress increased. The researchers concluded that reliance on sporadic recruitment and susceptibility to heat stress underpin the vulnerability of subtropical coral assemblages to climate change.