I
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.