Sea star wasting disease: System-wide collapse of the keystone predator Pisaster ochraceus
In marine environments, disease frequency and variety has risen in recent decades. Among echinoderms, “wasting” disease has occurred in the past but events were typically quite localized and short-lived, making investigations of vectors, disease expression, and ecological impacts difficult to rigorously study. In 2013-15, Sea Star Wasting Disease hit the sea star Pisaster ochraceus, the original keystone species, across its North American west coast range in one of the most spatially and temporally extensive disease events ever recorded for a marine species. What was the impact of this event on this important predator? We documented wasting’s impact on Pisaster populations and how it varied it space and time by quantifying the type and frequency of wasting symptoms, the impact on sea star abundance, and effects on population structure. We studied nine sites along the Oregon coast, the last region to be afflicted after Washington, British Columbia and California, in spring and summer 2014. Six symptoms were tracked, ranging from behavioral changes to lesions in the body wall, arm loss, and dissolution. To determine if such changes were unusual, we compared our results to those from surveys conducted twice annually from 2000 to 2013.
In agreement with observations elsewhere, wasting devastated Pisaster populations with population densities declining six-fold. Abundances had fluctuated from 2000-2013 but previously such rapid declines were never observed. Overall, we counted 31,955 sea stars through September. Disease frequency initially (April 2014) was low (<1%), but by June had skyrocketed to 50-80% at all sites, across ~350 km of coast. Sea stars in tidepools had higher disease rates (53%) than those out of water on rocks (46%), suggesting disease transmission was enhanced by remaining submerged at low tide. Oddly, orange individuals (22% of the population) were more susceptible to disease than were purple individuals (78%), regardless of habitat (pool or emerged). Though disease initially afflicted proportionately more adults, by summer’s end juveniles were equally affected. Juvenile relative abundance appeared to increase over the summer, but this was likely due to loss of adults, which rest atop of and obscure juveniles from view (we made counts without disturbing sea stars to reduce chances of disease transmission) in crevices and channels at low tide. Whether these juveniles or more recent recruits will survive to help recovery of the populations remains an open question. Wasting disease devastated Pisaster populations, but ecological consequences remain unclear.