Within-population beta diversity as a proxy for stability
Changes in beta diversity within a common garden may be informative in different ways. Whereas beta diversity among individuals from different populations (between-population beta diversity) relates to changes within the holobiont with respect to the environment and other populations (microbiome flexibility; Voolstra & Ziegler, 2020 or host promiscuity; Bonthond et al., 2021), the within-population beta diversity reflects the ratio between deterministic drivers (e.g., abiotic variables, mechanisms from the host) and stochastic drivers (e.g., historical contingency, mass effects, microbe-microbe interactions) that act on the holobiont. Metabolites involved in attracting or repelling microbes (Saha & Weinberger, 2019), chemicals related to defense against fouling (Saha et al., 2016), e.g., interfering with quorum sensing (Harder et al., 2012), traits related to morphology affecting the associated epibiota (Lemay et al., 2021), or promiscuity of the host towards potential symbionts (Bonthond et al., 2021; Klock et al., 2015) could represent important deterministic drivers that stabilize microbial communities. The Anna Karenina Principle posits that as such host mechanisms are compromised or affected in response to stress, the associated microbial community is less shaped by these deterministic drivers and becomes more unstable (Zaneveld et al., 2017). Instead, stochastic processes acting on the microbial community become relatively more important, and consequently, beta diversity among replicated holobionts will increase.
Unlike in their natural habitat where each population is exposed to a unique environment, in a common garden different populations are exposed to the same environment, and therefore to the same environmental deterministic and stochastic processes. Turning the Anna Karenina Principle around in the common garden, differences in within-group beta diversity among groups of study, could thus be used as a relative measure for the contribution of host mechanisms acting on the associated microbiota. Based on this idea, we used within population beta diversity as a surrogate for stability. The present study provides an example of how different groups (here native and non-native populations) can be compared.