Disease-driven mass mortality event leads to widespread extirpation and variable recovery potential of a marine predator across the eastern Pacific

Abstract

The prevalence of disease-driven mass mortality events is increasing, but our understanding of spatial variation in their magnitude, timing and triggers are often poorly resolved. Here, we use a novel range-wide dataset comprised 48 810 surveys to quantify how sea star wasting disease affected Pycnopodia helianthoides , the sunflower sea star, across its range from Baja California, Mexico to the Aleutian Islands, USA. We found that the outbreak occurred more rapidly, killed a greater percentage of the population and left fewer survivors in the southern half of the species's range. Pycnopodia now appears to be functionally extinct (greater than 99.2% declines) from Baja California, Mexico to Cape Flattery, Washington, USA and exhibited severe declines (greater than 87.8%) from the Salish Sea to the Gulf of Alaska. The importance of temperature in predicting Pycnopodia distribution rose more than fourfold after the outbreak, suggesting latitudinal variation in outbreak severity may stem from an interaction between disease severity and warmer waters. We found no evidence of population recovery in the years since the outbreak. Natural recovery in the southern half of the range is unlikely over the short term. Thus, assisted recovery will probably be required to restore the functional role of this predator on ecologically relevant time scales.

Impact of the extinct megaherbivore Steller’s sea cow (Hydrodamalis gigas) on kelp forest resilience

“…Filbee-Dexter and Scheibling [7] have suggested that the barrens state may be unstable because of predation, and that is supported by the positive impact of recovering P. helianthoides populations in our model. Unfortunately, evidence for actual widespread sea star recovery from SSWD is currently equivocal at best [46]. Nevertheless, our model results, which predict a delayed onset of transitions if Steller’s sea cow is present, also predict that recovery times would have been shorter in the presence of the sea cow.…”

“…The MHW, or blob perturbation, consisted of an abrupt increase of temperature 3.0 ° C in excess of the seasonal cycle, and lasted for 2 years (730 days), after which temperatures returned to the seasonal cycle. The onset of SSWD was also initiated 10 years after burn-in, and was implemented as an exponential decline of the P. helianthoides population, modeled from empirical data of population decline from central and northern California [19]. after which the sea star population was allowed to recover.…”

“…Rates of recruitment of M. pyrifera and C. corymbiferus were assumed to be 0.001d − 1 and 0.0001d − 1 respectively. These rates are derived from those presented by Detmer et al [19], but modified to maintain feasibility of the populations under the conditions of both spatial competition and predation in the current model.…”

“…The disease quickly devastated sunflower sea star populations, with the species disappearing entirely in large parts of its range. The rapid decline of the sea stars [19] was followed by widespread “outbreaks” of urchins, with increases in the numbers of actively foraging urchins and apparently local population sizes. These outbreaks resulted in the transition of numerous kelp forests to urchin barrens, e.g.…”

“…That situation that has persisted up to the present, although some recovery of kelp has been noted since summer 2021 along the coast of California [20]. There has been no documented recovery of the sunflower sea stars [19].…”