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. However, how to conserve coral reefs remains unclear, since those without local anthropogenic disturbances often seem equally or more susceptible to thermal stress as impacted ones. We disentangle this apparent paradox, revealing that the relationship between reef disturbance and heatwave impacts depends upon the scale of biological organization. We show that a tropical heatwave of globally unprecedented duration (~1 year) culminated in an 89% loss of hard coral cover. At the community level, losses depended on pre-heatwave community structure, with undisturbed sites, which were dominated by competitive corals, undergoing the greatest losses. In contrast, at the species level, survivorship of individual corals typically declined as local disturbance intensified. Our study reveals both that prolonged heatwaves projected under climate change will still have winners and losers and that local disturbance can impair survival of coral species even under such extreme conditions.

Tipping points and interactive effects of chronic human disturbance and acute heat stress on coral diversity

“…In coral reef communities, few studies have examined the effects of both local anthropogenic impacts and heatwaves [43], but those that have variously reported synergistic, antagonistic and additive effects [68][69][70]. For example, past studies on Kenyan reefs examining coral cover as the response reported no co-tolerance between a heat stress event and local anthropogenic stress [5,71], whereas a study in the Philippines showed that locations which were more affected by local anthropogenic stress provided a refuge for corals during a heatwave [72], presumably owing to co-tolerance.…”

“…Specifically, we collected coral diversity data from 17 shallow forereef sites on Kiritimati, (also known as Christmas Island; Republic of Kiribati; electronic supplementary material, figure S1), a large atoll (150 km perimeter) in the central equatorial Pacific. Surveyed sites are part of a long-term monitoring programme [43], and span a significant gradient of local stress owing to the concentration of Kiritimati's local population (approx. 6500 people [44]) and infrastructure on the northwest side of the atoll.…”

“…Local anthropogenic stress within Kiritimati's reef environment has been previously quantified with an integrated measure that encompasses the nearby human population and local fishing pressure for each site [43,45,46]. Human population on the atoll serves as an accurate proxy for the local stressors of dredging, sewage runoff and pollution, which are probably royalsocietypublishing.org/journal/rspb Proc.…”

“…R. Soc. B 290: 20230209 the primary local stressors for corals [43,47]. We applied a square root transformation for all examinations of local anthropogenic stress, to account for the right skew in the data.…”

“…To date, the 2015/2016 El Niño event was the longest and most intense of any El Niño event on record, leading to the highest cumulative heat stress inflicted on any coral reef to date [49]. This marine heatwave caused Kiritimati's reefs to experience continuous heat stress between May 2015 and April 2016, peaking at approximately 25°C-weeks 'degree heating weeks' (DHW), and culminating in a loss of 89% of the atoll's coral cover [43]. Heat stress was consistent across the shallow forereef sites surveyed for this study, with DHW magnitudes during the 2015/2016 El Niño event differing by only 1°C (range: 24.3-25.3°C) around the atoll [49].…”