COMMUNITY COMPOSITION DYNAMICS OF UPPER DEVONIAN (~360 MA) REEFS FOUND IN THE CANNING BASIN, WESTERN AUSTRALIA

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 75 sea-level fluctuation cycles spanning ~3.9 million years. 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 amongst transects. Reef communities remained remarkably persistent for ~1 million years before experiencing a unidirectional shift in community structure. Three community types were identified, each of which differ in the diversity and composition of coral assemblages. These findings suggest that reef communities are persistent over intermediate time scales ranging from hundreds to thousands of years. However, over extended time scales spanning millions of years, reef communities display increased variability. This variability likely reflects 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. Results highlight the importance of deterministic processes on community persistence. 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 various scales is crucial for conserving coral reef ecosystems in the face of ongoing environmental changes.