title: Phanerozoic reef community responses to global environmental perturbations
Creators: name: Dimitrijević, Danijela
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: subject: reefs, reef crises, corals, traits, Phanerozoic, mass extinctions, global warming

description: Reef communities are incredibly diverse, with a wide variety of life forms contributing to reef construction and providing habitats for different reef-dwelling organisms. Throughout their long geological history, reef communities have been restructured by events such as mass extinctions, reef crises, and global warming. This has led to alternating periods of growth and decline in reefs. Modern coral reefs are now considered the most vulnerable ecosystems on our planet. This dissertation aims to examine the geological record of reefs and corals, as well as their responses to past environmental challenges, in order to understand the potential for coral reefs to survive in the face of escalating threats. The extent to which reef crises have induced substantial changes in reef community composition still remains uncertain. More importantly, it is unclear if the fate of millions of reef inhabitants depends on the survival of reef-building communities and if reef crises in the geological record caused cascading effects on reef dwellers. Cascading effects are a series of secondary extinctions caused by the primary extinction of a keystone species. As reef builders provide a three-dimensional structure for numerous species that depend on it for habitat, shelter, resources, and breeding grounds, they can be considered as true keystone organisms. Indeed, reef builders and reef dwellers were intimately linked over the past 500 million years, but their patterns were not fully concordant with cascading effects regarding extinctions. However, in terms of reef crises, there was a cascading effect on reef communities. These findings underscore the intricate relationship between reef builders and reef dwellers and render the cascading effects of reef crises plausible. This study draws attention to the risk of losing many more species than just corals when coral reefs vanish. Even though coral reefs have declined within short geological timeframes, they have consistently managed to rebound. This may be attributed to the existence of refugia, which are stable and resilient habitats where organisms can seek protection from environmental stressors, survive, and then expand once conditions become less stressful. In the case of reefs, refugia could reduce the risk of extinction and serve as a source of propagules to repopulate shallow waters, contributing to the long-term preservation of coral diversity. This research investigated the role of refugia for tropical coral reefs in deeper, turbid, and mid-latitude regions across warming-induced reef crises. After periods of extreme warming, the proportion of reefs in deeper (blue) mesophotic environments increased, and coral occurrences predominantly shifted toward turbid mesophotic environments. Additionally, both coral reefs and reef corals moved their ranges towards higher latitudes. This suggests that subtropical, blue, and brown mesophotic environments may have provided immediate refuge for shallow-water coral species seeking to escape warming seawater. This offers some hope for coral reefs in the face of present- day climate change; however, complete recovery can take millions of years. Trait-based approaches can aid in understanding the ecological impacts of past crises and predict how reef-building corals and reef ecosystems will respond to climate change in the future. Thus, fossil coral traits were compiled in the Ancient Reef Trait Database (ARTD). Since 2020, more than 15 morphological traits of scleractinian corals were collected at the specimen level from published literature. Analyses of traits related to photosymbiotic efficacy (such as corallite sizes, corallite integration, and branching degree) showed significant changes and contractions in coral trait space during hyperthermal events, while the trait space remained stable during background times. This could indicate a potential collapse of photosymbiosis in the aftermath of global warming events. These findings highlight the value of trait-based approaches in quantifying the ecological impacts of reef crises and global warming in the past, as well as in predicting the responses of reef-building corals and reef ecosystems to climate change in the future. The trait data from the ART database was also used to investigate the correlation between the size of corallites and corals' ability to engage in photosymbiosis over the last 250 million years. The comprehensive analysis revealed a decreasing trend in the average corallite width of colonial corals when weighted by occurrences depicting ecological patterns. This suggests that corals with smaller corallites became more dominant over time, indicating a tighter symbiotic relationship over the last 250 million years. This dependency on photosymbionts may have given reef corals a competitive advantage in the shallow, tropical, and nutrient-poor seas they predominantly inhabit today.