Spatial and temporal patterns in echinoderm species and trait composition in the subtropical-to-temperate transition zone of eastern Australia

Subtropical biogeographic transition zones are undergoing large-scale marine community re-assembly due to ocean warming. These zones host a dynamic combination of tropical, subtropical and temperate taxa, many of which exist at their range limits. However, only some of these taxa undergo range shifts, making it difficult to predict future ecosystem changes. Traits associated with life-history, biology and ecology may aid in predicting which taxa are likely to redistribute. Considering the importance of echinoderms on reefs globally, we present the first study of echinoderm trait composition and diversity on reefs in a subtropical biogeographic transition zone. Focusing on coral-dominated habitats in eastern Australia (28-31°S) over a decade (2010-2019) of significant warming and marine heatwaves, we show that echinoderm diversity (31 taxa) varied spatially, with tropical taxa more prevalent at lower latitudes and subtropical taxa more abundant offshore. Functional richness and divergence of sea urchins increased poleward, reflecting the high prevalence of subtropical taxa with traits such as barrens habitat formation, winter spawning and long-lived planktotrophic larvae. Echinoderm biodiversity patterns, community temperature index and functional composition remained stable during this warming period, highlighting the importance of biogeographic transition zones as sentinels for determining the traits that promote stability or change in warming oceans.