Acknowledgements
We would like to thank B. Kryštufek, J. Purger, N. Bulatova and J. Uhlíková for their help in collecting samples. We thank W.E. Bradshaw, C.M. Holzapfel, D.G. Hazlerigg and two anonymous reviewers for their critical and valuable comments on the manuscript. This work was funded by the Ecology fund of the Royal Netherlands Academy of Arts and Sciences (KNAW Fonds Ecologie). Sample collection was financed by the National Science Centre in Poland (UMO-2013/09/N/NZ8/03205 to J.S. and N N304 058340 to J.M.W.).