Transformation of coral communities subjected to an unprecedented heatwave is modulated by local disturbance

Abstract

Corals are imminently threatened by climate change-amplified marine heatwaves. Yet how to conserve reef ecosystems faced with this threat remains unclear, since protected reefs often seem equally or more susceptible to thermal stress as unprotected ones. Here, we disentangle this apparent paradox, revealing that the relationship between reef disturbance and heatwave impacts depends upon the focal scale of biological organization. We document a heatwave of unprecedented duration that culminated in an 89% loss of coral cover. At the community level, losses hinged on pre-heatwave community structure, with sites dominated by competitive corals—which were predominantly protected from local disturbance—undergoing the greatest losses. In contrast, at the species level, survivorship of individual coral colonies typically decreased as local disturbance intensified, illustrating that underlying chronic disturbances can impair resilience to thermal stress at this scale. Our study advances understanding of the relationship between climate change and local disturbance, knowledge of which is crucial for coral conservation this century.

Marine heatwaves threaten cryptic coral diversity and erode associations among coevolving partners

“…2). Past work has found that human disturbance can have either negative ( 41 , 49 ) or positive effects ( 80 , 81 ) on coral thermotolerance or survivorship through heatwaves and this has been linked to disturbance-driven changes in the symbiont composition ( 41 , 81 ) or density ( 80 ). Here, we found negative impacts of human disturbance irrespective of symbiont community composition (figs.…”

“…Human uses, including wastewater runoff, subsistence fishing, and a large pier, are densely concentrated in this area, while other parts of the atoll experience substantially less human disturbance. The intensity of chronic local human disturbance at each site has previously been quantified, using two spatial data sources: (i) human population densities and (ii) fishing pressure ( 41 , 49 ). First, as a proxy for immediate point-source inputs from villages into the marine environment such as pollution and sewage runoff, a geographic buffer (in ArcGIS) was generated to determine human population size within 2 km of each site.…”

“…For example, a colony with 50% mortality but only 10% bleaching of remaining tissue would be assigned a “3/4” and a “5/6” for the Baum et al. ( 49 ) and McClanahan et al. ( 88 ) methods, respectively.…”

“…Given challenges associated with classifying bleaching into discrete categories, we used two different schemes to ensure that results were robust to methodological choices. We used the categories of Baum et al (49) and Claar et al (41)-hereafter method 1-which includes three categories from healthy to severely (>80%) bleached: (i) no bleaching, (ii) small patches of bleaching <5 cm or paling to 50% of colony area, (iii) bleaching in patches >5 cm or more than 50% of colony pale, and (iv) severe or complete bleaching (>80%) or entire colony pale. We also used the categories of McClanahan (88)-hereafter method 2 -which includes six categories from healthy to completely (100%) bleached: (i) no bleaching, (ii) paling or discoloration, (iii) 0 to 20% bleaching, (iv) 21 to 50% bleaching, (v) 51 to 80% bleaching, and (vi) 81 to 100% bleaching.…”

“…Given that many colonies had already experienced partial bleaching-induced mortality by March 2016 (due to the extreme length of the heatwave), we assumed that partial mortality of bleached colonies was the result of bleaching and therefore grouped partial colony mortality and bleaching together for both bleaching metrics. For example, a colony with 50% mortality but only 10% bleaching of remaining tissue would be assigned a "3/4" and a "5/6" for the Baum et al (49)…”