Shrews on small islands: epigenetic variation elucidates population stability

Three large (4 to 8 ha) and 14 small islands (0.3 to 2 ha) in a lake in eastern Finland, all situated less than 0.5 km from the mainland, were surveyed for small mammals. Three species of shrew and two species of vole were resident in July 1982: Sorex araneus on 10, S. caecutiens on 2, S. minutus on 5, Microtus agrestis on 12 and Clethrionomys glareolus on 4 islands. Immigrants were trapped from tiny islets, and the data indicate that S. caecutiens and M. agrestis are better dispersers than S. minutus and C. glareolus , respectively. Microtus agrestis, S. araneus and C. glareolus occurred non‐randomly, on subsets of the larger islands, while the two small Sorex species occurred more erratically, possibly because of competition with S. araneus . Juvenile sex ratio was male‐biased on the mainland but female‐biased on large islands, possibly because juvenile males move more and are more likely to emigrate from an island than juvenile females. Metrical and non‐metrical (epigenetic) cranial traits gave similar patterns of population differentiation in S. araneus . Two of the three large‐island populations have differentiated from the mainland populations and from each other, suggesting that the populations are relatively stable. Small‐island populations, which are often less than 10 individuals in size, showed little differentiation but had more epigenetic traits fixed than large‐island and mainland populations (founder effect). This suggests that the small‐island populations are unstable, have a high extinction rate.