Småpattedyrfaunaen på Anholt og Sprogø

The small mammals of the Islands of Anholt and Sprogø<br/>After the latest Ice Age’s glacial maximum Denmark was a contin-<br/>uous landmass, landlocked with both England, Sweden, Germa-<br/>ny, and Poland until approximately 8.500 years ago. The island of<br/>Sprogø was formed as part of a marginal moraine from the Young<br/>Baltic glacier advance. The isolation occurred when land sub-<br/>sidence and water level rises caused rivers from the Baltic Sea to<br/>break through the land area between Lolland-Falster and Rügen,<br/>and the Great Belt was formed. During the subsequent extensive<br/>sea level rise, many other Danish islands were formed, including<br/>Anholt. When these events occurred, Denmark was covered by a<br/>mixed deciduous forest with e.g., hazel, birch, and pine, and many<br/>of the animal species we know today, were present including<br/>small mammal species such as rodents, shrews, and small preda-<br/>tors. The now isolated islands therefore probably predominantly<br/>supported populations of small mammals which typified forest. It<br/>should, however, be mentioned, that Sprogø since 1998 has been<br/>connected to Funen and Zealand by the Great Belt Bridges and<br/>the Eastern railway tunnel.<br/>Anholt is one of the most isolated among smaller Danish is-<br/>lands and until recently only one small mammal species, the<br/>house mouse (Mus musculus), was present probably represent-<br/>ing an introduction in historical times. In 2022, several dead, yellow-necked mice (Apodemus flavicollis) were found in houses,<br/>and subsequent livetraping in the fall confirmed its presence.<br/>In October 2022, small mammals were livetraped on the island<br/>of Sprogø. Ninety-six trap nights resulted in 85 captured small<br/>mammals, comprising 70 bank voles (Clethrionomys glareolus),<br/>12 wood mice (Apodemus sylvaticus), and 3 yellow-necked mice.<br/>Bank voles and wood mice were present in all habitats on the<br/>island, but bank voles were more numerous.<br/>Presumably all three species were present in Denmark before<br/>Sprogø became an island. Based on this history, we consider it<br/>most likely that our catches of species typifying forest show that<br/>the small mammal fauna has been preserved intact from the time<br/>when Sprogø was part of a forest-covered mainland. However, we<br/>cannot rule out that later introductions have occurred. On the<br/>other hand, the small mammals on Anholt must have been intro-<br/>duced by humans.<br/>The house mouse on Anholt and the bank vole on Sprogø are<br/>examples of small mammal species that, in the absence of com-<br/>petition, can occupy habitats in which they would not occur in<br/>areas with richer small mammal faunas and mammal predators.