Metapopulation-scale resilience to disease-induced mass mortality in a keystone predator: From stasis to instability

Abstract

Mass mortality from disease epidemics challenges the resistance and resilience of populations and communities. Assessing the impacts and consequences of such events is crucially dependent on long-term datasets. In 2013–2016, sea star wasting disease (SSWD) caused population-wide crashes of the archetypal keystone species, the sea star Pisaster ochraceus, along the North American west coast. We used two long-term datasets to assess the resilience of Pisaster populations to this perturbation in Oregon: a 16-year time series (2007–2023) of annual predation rate at seven sites and a 23-year time series (2001–2024) of density and size of Pisaster at eight sites. In spring 2015, a novel and massive Pisaster recruitment event occurred at all sites, averaging 3.00 ± 0.57 recruits m⁻² (±SE), an 8100% increase compared to Pre-SSWD. Elevated but spatiotemporally variable recruitment has persisted over the subsequent decade. As expected, SSWD drove predation rates to near-zero. This persisted for 3–8 years depending on site, and as of 2024, predation rates at two sites remain unrecovered. Before SSWD, population size structure was relatively stable, consisting mostly of large adults with virtually no recruitment. After the outbreak, density, average size, and biomass density declined at most sites, while SSWD persisted at low levels, averaging ~4% symptomatic per year. As of the Current period (2021–2024), density and biomass density had recovered at all sites and often overshot prior levels, but average body size recovered at only three of seven sites. However, the 2014 crash and the post-2014 recruitment events apparently destabilized the populations; density remains more variable among years at all but two sites. To test the return of populations to Pre-SSWD structure, we developed a novel ‘recovery index’ based on density and size structure and found that only half the sites had recovered. This index tests the classic “ball and cup” stability heuristic and suggests that the epidemic shifted the prior adult-dominated state into an alternate persistent state characterized by pulses of recruits and juveniles. However, the system is clearly recovering, and if a causal link exists between the adult crash and the recruitment boom, this metapopulation may be resilient to mass mortality.