l reefs sustain a range of key ecosystem services to human wellbeing, including food security, storm protection, and economic benefits relevant to hundreds of millions of people around the globe (Moberg and Folke 1999; Foale et al. 2013; Norström et al. 2016; Woodhead et al. 2019). However, rising ocean temperatures linked to increased anthropogenic emissions of greenhouse gasses threatens the persistence of these ecosystems worldwide (Hughes et al. 2017).
Global warming is unequivocally the key driver of coral reef declines throughout the tropics. Pulse events such as marine heatwaves are widely documented to induce bleaching of corals, a process where photosynthetic endosymbionts are expelled from the cnidarian host (Warner et al. 1999; Fitt et al. 2001; Douglas 2003; Boilard et al. 2020; Suggett and Smith 2020). Bleaching is occurring over large spatial scales, resulting in mass mortality of entire coral colonies (Hughes et al. 2018a, 2018b). Additionally, the continued rise in ocean temperatures are preventing coral reefs from recovering before further pulse events occur (Hughes et al. 2018a; Harrison et al. 2019). Moreover, rising ocean temperatures also inhibit the recruitment on coral reefs by causing mortality to juvenile corals (Hughes et al. 2019), highlighting the multifaceted process of coral reef decline via global warming. Thus, global warming will continue to transform coral reefs into taxonomically, physically and functionally more homogenous environments (Hughes et al. 2018b), reducing biodiversity and impacting ecosystem function (Pratchett et al. 2011; Oliver et al. 2015; Brandl et al. 2019)
As global warming continues to devastate coral reefs across the globe, monopolisation of empty spaces where reef corals previously resided can occur rapidly. Primarily, this has led to the proliferation of macroalgae after a pulse event (Hughes et al. 2007; Graham et al. 2013; Bozec et al. 2019; Fulton et al. 2019). However, other taxa may also monopolise space previously inhabited by hard corals, such as sponges (Bell et al. 2013; Pawlik et al. 2016; Lesser and Slattery 2020) and soft corals (Inoue et al. 2013). Yet, these taxa do not provide equal ecological complexity to support biodiversity and provision of ecosystem services as reef building corals (Friedlander and Parrish 1998; Hughes et al. 2017; Woodhead et al. 2019). Furthermore, a combination of biotic interactions and abiotic effects can prevent taxa from monopolising uninhabited space for a period of time, resulting in an increased prevalence of sand or rock across the reef scape, further reducing habitat heterogeneity (Alvarez-Filip et al. 2009). Finally, other non-living benthic components such as coral rubble can inhabit reef space, a clear indication of hard coral mortality, and thus substratum homogenisation. These changes in benthic and taxonomic compositions of coral reefs ultimately represent a phase shifts of coral reefs, which are becoming more common under global warming in the Pacific (Ledlie et al. 2007; Bozec et al. 2019), along the great Barrier Reef (Hughes et al. 2007) and especially in the Atlantic ocean (Roff and Mumby 2012).
While global warming persists for inducing phase shifts on coral reefs, resistance driven by biotic processes, and refuge from warming, are two key factors which can mitigate the transition of coral reefs from coral dominated environments. For example, biodiversity can maintain ecosystem function for coral reefs under multiple stressors, including global warming (Benkwitt et al. 2020). The increased biodiversity of key functional taxa, such as reef fish and reef building corals, varies biogeographically (McWilliam et al. 2018; Siqueira et al. 2019) with the Caribbean showing reduced functional redundancy of both corals (McWilliam et al. 2018) and reef fish (Siqueira et al. 2019). Reef fish which are critical for macroalgae excavation are especially salient for preventing phase shifts (Hughes et al. 2007; Graham et al. 2013; Brandl et al. 2019). Regarding refuge, the reciprocal relationship between light and heat for inducing large scale bleaching can be moderated by turbidity under rising ocean temperatures (Sully and van Woesik 2020), which is a spatially variable factor. Furthermore, internal waves offering refuge via cooling during marine heatwaves have been found to mitigate bleaching (Wall et al. 2015; Storlazzi et al. 2020), thus preventing a major cause of coral mortality. Ultimately, the spatial variability in resistance and resilience of coral reefs to global warming is a result of multifaceted processes, thus highlighting the need to continuingly explore regions which may show resistance to compositional change under anthropogenic heating.
Given the generalised devastating effects of global warming on coral reefs for coral reproduction, transformation of benthic assemblages (Hughes et al. 2018b), loss of coral cover structural complexity (Magel et al. 2019), and subsequent loss of ecosystem services (Woodhead et al. 2019; Eddy et al. 2021), identification of regions where coral reefs are either buffered from, or resistant/resilient to the effects of global warming is critical. Here, we thus compare the relationship between global warming and compositional change at coral reef systems from the Caribbean, located off the Honduras coast, to reefs in the centre of the coral triangle, in the Wakatobi National Park, Indonesia. Using 9 years of reef monitoring surveys across 2 distinct biogeographic realms, we aim to identify generic patterns of global warming on reef benthic components.