Longitudinal trends of the small mammal community of the Apostle Islands archipelago

Documenting longitudinal changes in small mammal communities provides insights into ecosystem dynamics. We examined changes in small mammal communities of the Apostle Islands archipelago (Wisconsin, USA) since the establishment of the Apostle Islands National Lakeshore in 1970. We trapped small mammals from 20 of the 22 islands of the archipelago (2017-2020) and compared those results to historical (1961-2004) records. Microtus pennsylvanicus (p = 0.0076) and Sorex cinereus (p = 0.0268) exhibited significant changes in distribution. Microtus pennsylvanicus was likely extirpated from 10 of the 11 islands where it was previously detected, while S. cinereus increased in distribution. Peromyscus spp. colonized at least three islands since the establishment of the National Lakeshore, potentially through human-facilitated dispersal (i.e., boating, kayaking). Myodes gapperi remained widespread and abundant. Recent trends (2003-2004 to 2017-2020) in abundance indicated that S. cinereus may be declining locally on the islands, whereas Tamiasciurus hudsonicus may be increasing. Community diversity was driven by island size, regardless of variation over time. Long-term changes in small mammal populations across the archipelago likely reflect reduction of human extractive activities following the establishment of the National Lakeshore and the corresponding succession of vegetative communities. Our work suggests that even though small mammal communities of the archipelago have changed over time, larger islands may be able to better retain species that have been lost on others; therefore, island size remains an important predictor of community diversity.