Functional extinction of a genus of canopy-forming macroalgae (Cystophora spp.) across Western Australia

Abstract

Declines of canopy-forming macroalgae in response to a variety of anthropogenic stressors are increasingly prevalent in temperate latitudes, with most research efforts focusing on kelp forests. In contrast, comparatively little is known about marine forests formed by fucoid macroalgae, despite them being more diverse and globally widespread. Here, I examine the biogeography of the second-largest genus of fucoids globally (Cystophora), which is endemic to Australasia. To do so, I use a combination of field surveys, records from the literature, anecdotal evidence, and herbaria collections spanning a period of > 150 years. Despite the sampling effort quadrupling in contemporary times, most historically common species were found to be absent or exceedingly rare across their rear (warm) range edge, suggesting their functional extinction. Three species experienced apparent functional contractions across > 250 km of coastline, with some losing approximately 8% of their global distribution. These losses are among the largest reported for any forest-forming species in the Australian continent. Reasons for Cystophora spp. decline are unknown, but likely involve gradual warming, marine heatwaves, and rapid urbanization. Increasing human impacts and further warming in the region threaten several species with further extirpation, some of which are endemic to the area and play unique ecological roles.