Functional Extinction of a Genus of Canopy-Forming Macroalgae (CystophoraSpp.) Across its Warm Range Edge

Declines of canopy-forming algae are increasingly prevalent in temperate latitudes, although most losses have occurred across densely populated areas. Here, I use a combination of field surveys, anecdotal evidence, records from the literature and herbaria collections, to document the biogeography of the canopy-forming genus Cystophora across the sparsely populated end of its distribution in Western Australia. Although historically common, most species were found to be absent or exceedingly rare across their rear (warm) range edge, suggesting their functional extinction. Out of five species, three experienced range contractions >500 km, with some losing approximately 8% of their global distribution. Reasons for their decline are unknown, but likely involve gradual warming, marine heatwaves and rapid urbanization. Increasing human impacts and further warming in the region threatens several species with further extirpation, some of which are endemic to the area and play unique ecological roles.