As Arctic climate warms, how will terrestrial ecosystems and interconnected communities respond in the coming decades? Small mammals, such as shrews, and their parasites can serve as key indicators for anticipating the consequences of accelerating environmental perturbation and changing interactions among species. Some Arctic shrews have recently acquired new parasites indicating poleward shifts of sub-Arctic faunas, and demonstrating increases in both Arctic biodiversity and overall complexity within these novel species networks. Spatially broad and temporally deep specimen collections of small mammals and parasites are revealing the extent and nature of community reorganization as northern faunas adjust to accelerating climate change.