Local anthropogenic stress does not exacerbate coral bleaching under global climate change

Abstract

Aim: Rising ocean temperatures are widely recognized as the dominant driver behind the rapid degradation of coral reefs via the process of coral bleaching (the expulsion of photosynthetic endosymbionts, which reveals the coral skeleton). However, bleaching of hard corals is often assumed to be further aggravated by the effect of local-scale stressors from anthropogenic activity, accelerating coral reef decline where these stressors are stronger. Despite the importance of this hypothesis, the interaction between climate change and local stressors for driving coral bleaching has only been investigated in a handful of studies, with no large scale (regional or global) test conducted thus far. We investigate the impact of human population density (HPD) – a proxy for local stressors – in both protected and non-protected marine regions, and their interaction under heat stress as drivers of coral bleaching. Location: Global. Time period: 2002–2018. Major taxa studied: Scleractinia corals. Methods: Using 9,170 coral reef surveys worldwide, we performed Bayesian modelling to assess the probability of coral bleaching in response to local-scale stressors in interaction with global warming. Results: Local HPD does not exacerbate coral bleaching, either independently or under thermal stress from climate change. Rather, the relationship between HPD and temperature stress appears weakly antagonistic for coral bleaching, contradicting the expectation that HPD increases sensitivity of corals to bleaching under thermal stress. Main conclusions: Local HPD does not interact with global warming for degrading coral reefs. However, regional variation in bleaching patterns exists. Consequently, bleaching will continue to occur on most coral reefs globally regardless of local HPD. Thus, it is likely that even isolated, well-protected, coral reefs will continue to decline because of climate warming-induced bleaching. Therefore, tackling the source of global warming remains the most effective way to mitigate coral reef decline via coral bleaching.