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.