Data from: Reciprocal abundance shifts of the intertidal sea stars, Evasterias troschelii and Pisaster ochraceus, following sea star wasting disease

Disease emergence occurs within the context of ecological communities, and
disease driven declines in host populations can lead to complex direct and
indirect ecological effects. Varying effects of a single disease among
multiple susceptible hosts could benefit relatively resistant species.
Beginning in 2013, an outbreak of sea star wasting disease (SSWD) led to
population declines of many sea star species along the west coast of North
America. Through field surveys and laboratory experiments, we investigated
how and why the relative abundances of two co-occurring sea star species,
Evasterias troschelii and Pisaster ochraceus, shifted during the ongoing
wasting epidemic in Burrard Inlet, British Columbia, Canada. We
hypothesized that Evasterias is competitively inferior to Pisaster but
more resistant to SSWD. Thus, we predicted that SSWD-induced declines of
Pisaster could mitigate the negative effects of SSWD on Evasterias, as the
latter would experience competitive release. We document shifts in sea
star abundance from 2007-2017: Pisaster abundance and mean size declined
during the outbreak, while Evasterias abundance increased from relatively
rare to numerically dominant within the intertidal. When exposed to
symptomatic sea stars, Pisaster and Evasterias both showed signs of SSWD,
but transmission and susceptibility was lower in Evasterias. Despite diet
overlap documented in our field surveys, Evasterias was not outcompeted by
Pisaster in laboratory trails conducted with the relatively small Pisaster
available after the outbreak. Interference competition with larger
Pisaster, or prey exploitation by Pisaster during summer when Evasterias
is primarily subtidal, may explain the rarity of Evasterias prior to
Pisaster declines. Our results suggest that indirect effects mediated by
competition can mask some of the direct effects of disease outbreaks, and
the combination of direct and indirect effects will determine the
restructuring of a community after disturbance.