Recent changes in mussel beds following mass mortality of a keystone marine predator

Marine habitat-forming species increase biodiversity and enhance ecosystem function, but they are highly threatened by local and global stressors that are increasing worldwide. We evaluated the relative roles and combined effects of biotic (i.e. predatory whelk and sea star abundance) and abiotic correlates (i.e. seawater temperature, wave height) on the long-term dynamics of habitat-forming mussel ( Mytilus californianus ) beds at 2 rocky intertidal sites in Monterey Bay, California, from 2014 to 2024. We found that mussel cover has doubled since 2014, following a sea star mass mortality event in 2013 from sea star wasting disease (SSWD), and the lower limit of mussel beds has shifted to lower tidal heights, supporting the hypothesized mussel expansion from decreased predation. However, long-term monitoring of an intertidal keystone species, the ochre sea star Pisaster ochraceus (1954-2024), highlights that sea star abundance has been declining since long before the SSWD of 2013. Changes in mussel abundance and distribution appear to be driven by a combination of factors, including mussel recruitment and the 2014-2016 North Pacific marine heatwave, in addition to decreased predation pressure. As more mass mortality and extreme weather events are expected under warming conditions in the coming decades, long-term monitoring addressing the impact of multiple stressors on marine communities is necessary to inform predictions about future change.