Sea urchin herbivory drives subcontinental similarity of tropicalizing reefs

Global warming is reshaping the composition and functioning of temperate reef communities and resulting in the emergence of habitats that resemble those often found on more equatorward reefs. In Tasmania, Australia, such 'tropicalization' has caused the loss of kelp bed habitats and the emergence of urchin barrens, which were historically found on more equatorward reefs in New South Wales (NSW). Despite these regions now sharing both kelp beds and urchin barrens, the extent of similarity in community composition and functioning across habitats and regions is unresolved. Here, we conducted underwater visual census and used experimental macroalgal assays to quantify and contrast community composition (taxonomic and functional) and herbivory, between kelp beds and barrens in two regions (Tasmania and NSW) separated by ∼1000 km. Ultimately, these comparisons revealed that the greatest dissimilarity generally lay between habitat types, rather than regions. Moreover, the sea urchin, Centrostephanus rodgersii, emerged as the most important species driving between habitat dissimilarity, and when present, its biomass was strongly related to the extent of macroalgal assay removal. Overall, these results underscore the similarities in community composition and herbivory that can arise within similar habitats across biogeographical scales.