Introduction
The capacity of corals to adapt to climate change is among the Earth’s most pressing environmental needs (Hughes et al. 2017). Localized studies have shown that corals acclimatize or adapt by changing protein expressions (Palumbi et al. 2014), switching of symbionts in variable sea-surface temperature (SST) environments (Boulette et al,. 2016; Safaie et al. 2018; Sully et al. 2019), and after experiencing warm-SST anomalies (Guest et al. 2012; McClanahan 2017; Hughes et al. 2019a). However, the resistance (=exposure/sensitivity) of scleractinian corals at large spatial scales of variability is only slowly being understood (Sully et al. 2019). Here, we show that resistance of corals to heat stress is geographically variable and likely driven by the differential environmental histories.
Resistance is a measure of system change when exposed to stress, and a key component of coral reef resilience or the ability of a reef to resist and survive a disturbance (McClanahan et al. 2012). Under the increasing impacts of climate change, coral bleaching is an early and obvious indicator of thermal stress (Hughes et al. 2018). Bleaching is less often considered for its potential to inform resistance, whereby some reefs may bleach less than expected based on historical and current environmental conditions or exposure to thermal stress (Fig. 1a). Ultimately, resistance should be influenced by variation in geographical and evolutionary history and associated genotypic diversity, adaptation to thermal stress, and taxonomic composition (Edmunds & Gates 2008; Palumbi et al. 2014; Sully et al. 2019). These patterns remain untested across large biogeographic scales but will be critical for calibrating future climate impact models that are currently based on current and projected thermal exposure (Couce et al. 2013; Freeman et al. 2013; McManus et al. 2019).
Here, we combine satellite SST observations with a globally coordinated effort of coral bleaching surveys to evaluate coral resistance to heat stress across a large geographic gradient in 2016 (Fig. 1a). We evaluated resistance through two components: exposure to extreme heat stress and the resulting sensitivity of ecological communities to that exposure. We define exposure as the degree, duration, and extent of perturbations beyond background levels. As coral exposure to heat and light perturbations increases with climate change, reef resilience is threatened but also potentially attenuated by variable coral sensitivity and subsequent resistance (Sully et al. 2019). To evaluate coral reef exposure, we considered two models derived from NOAA AVHRR satellite measurements that provided proxies for a number of essential ocean variables that are used to estimate stress to corals (Eakin et al. 2010; Maina et al. 2008; 2011; Muller-Kager et al. 2018). We used two models: (1) a single metric of temperature stress - the cumulative incremental sum of SSTs above local summer SSTs (CTA), and (2) a multivariate metric of climate exposure (CE), which is a weighted index combining heat, light, and water flow variables and strongly associated with field observations of coral bleaching (Maina et al. 2008). Additionally, coral reef field biologists have developed field measurements to estimate coral bleaching and therefore ecological sensitivity. Sensitivity can take various forms, but here we define and document it as the percentage of corals that lost color, a proxy for the decline in light-absorbing algal symbiont densities, which can lead to coral mortality (Fitt et al. 2001; McClanahan et al. 2001).