Abstract
Anthropocene coral reefs are faced with increasingly severe marine heatwaves and mass coral bleaching mortality events. The ensuing demographic changes to coral assemblages can have long-term impacts on reef community organisation. Thus, understanding the dynamics of subtropical scleractinian coral populations is essential to predict their recovery or extinction post-disturbance. Here we present a 10-yr demographic assessment of a subtropical endemic coral, Pocillopora aliciae (Schmidt-Roach et al. in Zootaxa 3626:576–582, 2013) from the Solitary Islands Marine Park, eastern Australia, paired with long-term temperature records. These coral populations are regularly affected by storms, undergo seasonal thermal variability, and are increasingly impacted by severe marine heatwaves. We examined the demographic processes governing the persistence of these populations using inference from size-frequency distributions based on log-transformed planar area measurements of 7196 coral colonies. Specifically, the size-frequency distribution mean, coefficient of variation, skewness, kurtosis, and coral density were applied to describe population dynamics. Generalised Linear Mixed Effects Models were used to determine temporal trends and test demographic responses to heat stress. Temporal variation in size-frequency distributions revealed various population processes, from recruitment pulses and cohort growth, to bleaching impacts and temperature dependencies. Sporadic recruitment pulses likely support population persistence, illustrated in 2010 by strong positively skewed size-frequency distributions and the highest density of juvenile corals measured during the study. Increasing mean colony size over the following 6 yr indicates further cohort growth of these recruits. Severe heat stress in 2016 resulted in mass bleaching mortality and a 51% decline in coral density. Moderate heat stress in the following years was associated with suppressed P. aliciae recruitment and a lack of early recovery, marked by an exponential decrease of juvenile density (i.e. recruitment) with increasing heat stress. Here, population reliance on sporadic recruitment and susceptibility to heat stress underpin the vulnerability of subtropical coral assemblages to climate change.
Highlights
Marine habitats are under increasing pressure in the Anthropocene due to cumulative stress from multiple local and global stressors (Pandolfi et al 2003; Hughes et al 2017a, b; Duarte et al 2020)
During 2016, when coral bleaching occurred at all study sites, 221 days were hotter than baseline mean temperatures but only 69 days were cooler
We investigated demographic patterns of a subtropical endemic coral over a 10-yr period and linked these patterns to in situ accumulated thermal stress
Summary
Marine habitats are under increasing pressure in the Anthropocene due to cumulative stress from multiple local and global stressors (Pandolfi et al 2003; Hughes et al 2017a, b; Duarte et al 2020). Populations can react abruptly to stress events, and often follow the disturbance dynamics model (Gilmour 2004; Crabbe 2009; Leray et al 2012): while some populations decline, others proliferate in the newly available ecological niche space. These dynamics are influenced by speciesspecific life history strategies and differential responses to stress events. Long-term studies on population dynamics and vital rates are numerous in terrestrial ecology, they are lacking in marine and coral reef ecology (Edmunds and Riegl 2020; Pisapia et al 2020). In the context of climate change impacts on coral reefs, addressing population dynamics of marine species is an important scientific and monitoring priority (Edmunds and Riegl 2020; Pisapia et al 2020)
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