Decadal decline of functional diversity despite increasing taxonomic and phylogenetic diversity of coral reefs under chronic urbanisation stress

Coral reefs provide a multitude of ecosystem functions owing to the high levels of biodiversity they host. Coral species, as the foundation of shallow-water reefs, differ in their contributions toward the functioning of the ecosystem due in part to the disparate phylogenetic histories of scleractinian lineages. Understanding the spatial patterns and temporal trajectories of these biodiversity facets, as well as their interrelationships, is critical for more targeted conservation strategies in the face of widespread habitat degradation and climate change. Here, we analyse long-term benthic data spanning 1986 through 2020 on coral reefs in Singapore, which have been impacted by decades of urbanisation-related and thermal stressors, to test for differences between coral biodiversity facets—specifically, taxonomic, phylogenetic (evolutionary relatedness between species), and functional (occupancy of functional trait space) richness and diversity. Analyses show that taxonomic and phylogenetic richness and diversity measures increased over the 35-year period despite declines during major bleaching events. Yet, while taxonomic and phylogenetic diversity increased, functional richness and diversity declined over the same period. Community-weighted trait measures indicate a shift towards dominance of more stress-tolerant traits such as slower growth rates, smaller corallite sizes, and massive colony forms. Together, these trends highlight the effects of chronic urban stressors alongside major bleaching events impacting reef assemblages. Critically, such assemblage shifts and functional diversity declines were masked by increasing taxonomic diversity, which is most commonly assessed, and could erode ecosystem resilience. The temporal decoupling of the biodiversity facets examined here underscore the need for more comprehensive monitoring of reefs through a combination of trait-based approaches alongside traditional field surveys at finer taxonomic resolution.