Inventory of the Small Mammal Community of the Apostle Islands National Lakeshore 2017-2020

Mallinger, Elyse; Khadka, Bijit; Morales, Morgan; Morrison, Megan; Van Stappen, Julie; Van Deelen, Timothy; Olson, Erik

Martes americana (American marten) are the only state endangered mammal in Wisconsin. American martens are known to inhabit eight of the islands of the Apostle Islands archipelago, as well as, the mainland within the Apostle Islands National Lakeshore. Like other mesopredator species, martens rely heavily on small mammals as prey resources, especially Myodes gapperi (Southern red-backed vole). To better understand how small mammal prey communities may influence marten abundance and distribution within the archipelago we inventoried small mammal communities in the summers of 2017-2020. Small mammal trapping was performed on20 islands and the mainland. We also compared the results of our surveys with the results of historical small mammal surveys going back to 1961 to better understand the dynamics of small mammal community in the archipelago. Trap grids were deployed for four days and three nights at each study area. Throughout 2017-2020, a total of 7,672 individual trap nights produced 1,366captures of ten small mammal species. In 2017, we had a trapping effort of 2,720 trap nights resulting in 746 captures of ten species from eight islands and the mainland. In 2018, we had a trapping effort of 2,225 trap nights resulting in 243 captures of eight species from seven islands and the mainland. In 2019, we had a trapping effort of 2,304 trap nights resulting in 280 captures of six species from seven islands. In 2020, we had a trapping effort of 423 trap nights resulting in 97 captures of six species from Stockton and Oak Islands. Compared to historical data, the small mammal community of the archipelago has changed over time. Myodes gapperi abundance and distribution appeared relatively stable historically. The abundance of Sorex cinereus (Masked shrew) has increased from the 1960's to the 2000's (p=0.0268). Peromyscus spp. (Mice spp.) abundance has also increased but not significantly (p= 0.2482). Contrarily, Microtus pennsylvanicus (Meadow vole) abundance has declined dramatically since the 1960's (p=0.013).These changes are likely a result of vegetative succession, interspecific interactions, and biogeographical forces influencing island-level extinction and colonization events. For example, larger islands tended to have more species, especially when examining patterns of species distribution going back to the 1960’s (p =8.54e-09). This suggests that island biogeographylikely plays an important role in influencing the small mammal diversity of islands within thearchipelago, and that the small mammal communities are dynamic over time. The presence of parasitic dermatophytes was confirmed on M. gapperi, Peromyscus spp., and T. hudsonicus species, but were mainly observed on M. gapperi. Density of small mammals at both the island and local scales appears to be a significant factor in the spread of these parasites. The influences of habitat on the small mammal community was less clear. Although, Myodes gapperi abundance was significantly higher (p= 0.0001) at sites where the understory was dominated by Taxus canadensis (Canada yew), suggesting that Ta. canadensis may be an important structural factor influencing M. gapperi abundance. While M. gapperi abundance was higher on average on islands where M. americana were present the relationship was not statistically significant (p=0.6035), suggesting that more fine scale analyses exploring the relationships between Ta. canadensis, M. gapperi, and M. americana may be warranted.