Temporal dynamics of Devonian reef communities: Insights into natural phase shifts and long-term resilience in the face of environmental variability

Living coral cover is rapidly decreasing due to global climate change and local human impacts. Response to the ongoing transformation of coral reefs requires a deep understanding of the mechanisms that regulate community assembly and sustain biodiversity in these systems. The goal of this study is to test the persistence of Devonian reef communities across 70 sea-level fluctuation cycles. Abundance data were collected for in situ coral and calcareous sponge taxa found along 164 transects across two field localities within the Canning Basin, Western Australia. The Bray-Curtis dissimilarity index was used to quantify the dissimilarity in taxonomic composition among transects. Despite the notable persistence of reef communities over numerous cycles (ranging from 22 to 29 cycles) spanning hundreds of thousands of years, significant taxonomic shifts unfolded over millions of years, primarily driven by the diversity and composition of coral assemblages. This indicates that while reef communities exhibit persistence over intermediate time scales, they undergo changes in taxonomic composition over extended periods. The observed shifts in taxonomic composition likely reflect the profound changes in Earth-surface systems that occurred during the Devonian (i.e., fluctuations in global sea-surface temperatures and sea-level). Therefore, sustained abiotic processes can act to disrupt community structure over extended time scales as the threshold for community persistence is exceeded. This study helps establish baseline community composition dynamics in systems devoid of human influence and thus deepens our understanding of naturally occurring phase shifts. Understanding the factors that contribute to the persistence of community assembly across multiple scales is crucial for conserving coral reef ecosystems in the face of ongoing environmental changes.