Effects of future climate on coral-coral competition

Ocean acidification (OA) and elevated temperatures are exacerbating this decline [4, 5]. Over the next 100 years, average sea surface temperatures are expected to increase 1–2˚C, and ocean pH levels are predicted to decrease by 0.3–0.5 units under the IPCC “business as usual” sce- nario [8, 9]. The 2015 and 2016 mass bleaching events in the tropics brought on by higher than average water temperatures demonstrated that the effects of climate change are already dra- matically and rapidly impacting coral reefs [4], with a recent, five-fold increase in the fre- quency of mass bleaching events [10]. These rapid changes emphasize the need to better understand and predict the effects of climate change on these valued systems. Given the foundational role of corals in tropical reef ecosystems, the effects of OA and/or warming on coral physiology have been commonly investigated [11]. Numerous previous studies have focused solely on the effects of either OA or warming on species physiology and interactions with fewer studies focused on the combined effects of OA and warming even though they will be joint stressors [12]. OA reduces calcification and growth rates [13, 14], causes expulsion of dinoflagellate endosymbionts (Symbiodiniaceae) [15], and, in worst cases, leads to coral death [16, 17]. At the same time, the magnitude of the physiological responses to OA and warming can vary among taxonomic groups, populations, and locations [15, 18–21]. Some populations of certain species, such as the coral Porites astreoides, have been shown to be

able to withstand environmental stressors [22]. Further, meta-analysis and modeling studies have found that responses to OA and warming vary among taxonomic groups and species [21, 23]. These complicated, context-dependent responses to OA and warming make predicting the potential outcomes of interactions between species difficult and highlight the need for direct investigations of how OA and warming impact coral-coral interactions [23]. Corals compete for space on coral reefs with many competitors including algae [24], sponges [25, 26], soft corals [27], ascidians [28], and other hard corals [29–31]. Of these inter- actions, the effects of OA and warming on coral-coral interactions have rarely been investi- gated. This may be due to declines in coral cover and increases in algal cover that have led to an increase in coral-algal interactions on many reefs [12, 32]. However, regions or habitat patches with high coral cover can still be found across the globe [33]. Moreover, given the right conditions, some reefs have been shown to rapidly recover from low to high coral cover [34, 35]. These results demonstrate that in certain locations, given proper management strate- gies, coral-coral interactions are not uncommon. Corals compete via numerous mechanisms, but in the short-term, competition is com- monly via mesenterial filaments used to digest neighboring corals’ tissues [29, 30]. In some cases, initial interactions can reverse when losing colonies form larger sweeper tentacles in areas of contact and reverse the competitive interaction [36, 37]. Coral communities can be

shaped by competitive hierarchies [29, 30], with uninterrupted competition leading to changes in coral diversity [38]. A limited number of studies focused on understanding the effects of OA on coral-coral interactions have found mixed results. In some cases, OA affected coral growth (i.e. linear extension) without affecting competition [39] or led to the early extrusion of mesenterial filaments [40]. In another case, OA significantly impacted intraspecific competi- tive interactions in five species while leaving strong interspecific competitive interactions unaf- fected. A follow-up model suggested that these changes in competition can shift competitive hierarchies and lower overall coral cover [41]. These studies indicate that the effects of OA on coral-coral competition are mixed and suggest the need for further study. Moreover, the simultaneous effects of increased carbon dioxide and temperature on coral-coral competition have not been evaluated. This study assessed how the combined stressors of OA and warming affect competition among common mounding corals in the Caribbean (P. astreoides versus O. faveolata and M. cavernosa). These species are among the most common corals in the Florida Keys, are PLOS ONE Climate change alters coral-coral competition PLOS ONE | https://doi.org/10.1371/journal.pone.0235465 August 13, 2020 2 / 16