Epibiota dispersed more with increased temperature (ii)
As host performance declined with increased temperature, within-population beta diversity increased as well. This observation is in line with the Anna Karenina principle which predicts that conditions stressful to the host promote a decline in stability in the associated microbial community. It also supports our second hypothesis that thermal stress increases beta diversity in epibiota among individuals from the same population. Along with this increase in beta diversity, alpha diversity (both richness and evenness) increased as well. This could reflect higher metabolic rates at elevated temperature (Clarke & Fraser, 2004; Gillooly, Brown, West, Savage, & Charnov, 2001) promoting proliferation, dispersal and succession rates. However, the increase in alpha-diversity could at the same time be another indicator of a relaxation of influence from the host (and thus a relative increase in stochasticity) at elevated temperature. The production of defense related metabolites (Saha et al., 2016; Wang, Wang, et al., 2017) could, for instance, be reduced under stress, resulting in a higher probability for random microbes to temporarily settle in the community. On the other hand, increased alpha-diversity may benefit the host under stressful conditions by promoting functional redundancy (Girvan, Campbell, Killham, Prosser, & Glover, 2005) and support the holobiont to retain essential bacterial functions.