Effects of future climate on coral-coral competition

demonstrate evidence of loss of coloration or differences in Fv/Fm. It is possible that these results may have changed if the experiment had run over a longer period of time. Regardless, our results demonstrate that ecological interactions can be affected by environmental stress before common signs of physiological stress (e.g. loss of coloration and, potentially, subse- quent bleaching) are visible. It may be that a trade-off exists between an ability of a coral to respond to competition and its ability to withstand OA and warming as a coral re-allocates resources away from fending off competition to withstanding environmental stress. Our results highlight that predicting the effects of OA and warming on ecological interactions may require direct tests under predicted future conditions and that predicting outcomes based on physiology alone may be challenging. The goal of this study was to evaluate how the dual factors of OA and warming affected coral-coral interactions among some of the common mounding corals remaining on Carib- bean reefs. While it is not possible to make conclusions about the relative importance of the two environmental factors in this study, multiple studies have documented the immediate effect that temperature stress can have on corals–particularly during recent mass bleaching events [4]. Follow-up studies considering the relative importance of these two factors and eval- uating the generality of effects would be useful. Of particular interest would be enhanced understanding of the potential trade-off between environmental stress response and competi-

tion and the effect of any potential for these corals to acclimate and/or adapt to effects of a changing climate on ecological interactions. Some corals can increase their resilience to envi- ronmental stress over time through changes in gene expression [67]. Differential ability to acclimate to OA and/or warming would likely impact competitive interactions and should be an area of focus. Competitive interactions between P. astreoides and M. cavernosa or O. faveolata suggest a competitive hierarchy such as those found in earlier coral competitive interaction studies [29, 30]. However, similar to the model evaluated by Horwitz et al. [41], our findings also suggest that competitive hierarchies may change or become more variable under OA and warming. If these changes occur in nature as oceans warm and acidify, this may establish a new competi- tive relationship among remaining corals on reefs in the Caribbean. P. astreoides is among the most abundant corals in the Florida Keys and is more abundant than either O. faveolata or M. cavernosa [42, 68]. It was also relatively resistant to O. faveolata and M. cavernosa contact under present conditions; however, its advantage is compromised as warming and acidifica- tion increase. O. faveolata is the least abundant of the three species [42], but it and P. astreoides had similar effects on each other’s Fv/Fm (Fig 3), and O. faveolata caused larger lesions on P. astreoides than P. astreoides did on O. faveolata (Fig 5). These results suggest that the current

dominant coral on Caribbean reefs may become more compromised under predicted future conditions of OA and warming. Coral-coral interactions are less common on modern, degraded reefs; however, they still occur in areas where coral persists at higher cover [41]. Understanding and predicting the out- come of these interactions may be of increasing relevance as rising temperatures and increas- ing OA alter community dynamics of future reefs. Acknowledgments We thank LJ Houk and W Lee for their laboratory assistance. Research was conducted under collection permits: FKNMS-2015-078-A1 and FKNMS-2017-128. This is contribution #1141 of the Smithsonian Marine Station at Fort Pierce and contribution #202 from the Coastlines and Oceans Division of the Institute of Environment at Florida International University. PLOS ONE Climate change alters coral-coral competition PLOS ONE | https://doi.org/10.1371/journal.pone.0235465 August 13, 2020 12 / 16