Disease outbreaks occur within the context of communities, and the interplay between community and disease dynamics can lead to complex effects on the host populations. This is particularly true when multiple interacting hosts are exposed and susceptible to a single disease. Sea star wasting disease (SSWD) has killed millions of sea stars from Mexico to Alaska, and can infect up to 20 species of sea star, many of which co-occur and compete for resources. We explore the population response following disease outbreak of two sets of competing sea stars with differing susceptibility to disease; the intertidal Pisaster ochraceus and Evasterias troschellii and the subtidal Pycnopodia helianthoides and Dermasterias imbricata. Our work was conducted in British Colubmia, Canada. Pisaster and Pycnopodia are highly susceptible to disease, whereas Evasterias and Dermasterias less susceptible, respectively. Pisaster and Evasterias were quantified annually pre and post SSWD (2008-2020) at set sites around Burrard Inlet. Pycnopodia and Dermasterias were quantified annually pre and post SSWD (2013-2020) at set sites around Calvert Island.
Results/Conclusions
In the intertidal, we found that while Pisaster populations decreased following disease outbreak, Evasterias populations increased, becoming the dominant species in the intertidal. Further, as Pisaster populations began to rebound in Burrard Inlet in 2018, there was a corresponding decrease in Evasterias populations. In the subtidal there was a marked decline in mean and max size of Pycnopodia following SSWD outbreak, and a corresponding increase in Dermasterias mean and max size. As with the intertidal stars, as Pycnopodia size began to increase several years after SSWD outbreak, Dermasterias size correspondingly dropped to pre-wasting size levels. In both case studies presented, we found that the comparatively less susceptible host population appeared to temporarily benefit following disease outbreak. Our work highlights the importance of considering community interactions when predicting host response to disease outbreak.