Conclusions
Based on the Anna Karenina Principle, we used the variability of
microbial communities (i.e., beta diversity) among replicated holobionts
as a proxy for the ratio between stochastic and deterministic processes
acting on the associated microbial community – and therewith as a proxy
of stability within the holobiont. Using the invasive seaweed G.
vermiculophylla , we simulated an invasion process in a common garden,
and demonstrated that epibiota beta diversity increases in response to
thermal stress, exactly as the Anna Karenina Principle predicts. Under
stressed conditions non-native populations – descending from algae that
may have undergone selection as a result of the invasion – increased
less in terms of beta diversity compared to native populations. We argue
that these results imply that traits with which the host manipulates
epibiota are less susceptible to thermal stress in non-native
populations and that the invasion process may have selected for this,
resulting in non-native populations that are more tolerant toward
microbial pressures under a wider range of conditions.
Acknowledgements The authors are especially grateful to Nadja Stärck from the GEOMAR
Helmholtz Centre in Kiel who supported with the fieldwork and the
experiment.