Blol., Eastern Michigan Univ., Ypsilanti, Ml 48197. USA) OECOLOGIA, vol. 87, no. 1. Jun. 1991. pp. 102-105. En;en. This study ... Microtus pennsylvanicus is prone to large supra-annual fluctuations in population size, while M. brew- en is essentialry acyclical. ... A radiocarbon-dated pollen diagram from a typical lowland raised bog in County Antrim is presented, together with a limited stratigraphic study.