Species included We chose this assemblage of 8 species of small forest rodents for 4 reasons. First, each species reaches a distributional limit within or close to the northern Great Lakes region. Second, each is commonly captured by the techniques most widely used by collectors. Records of other species are available but were acquired through the use of trapping or hunting techniques that have not been employed consistently across the 150 years of collecting in this region (e.g., firearms and large traps are seldom used in recent collections, and mist nets for the capture of bats did not become available until the last half of the 20th century). Third, we focused on woodland species because trapping since 1980 has concentrated heavily on forest habitats, and consequently their record is stronger than that of mammal assemblages in other habitats. Fourth, these species are relatively common and frequently captured, often in the same trap-lines. We did not consider a few species that are extremely rare in the region (e.g., woodland voles, Microtus pinetorum) or that seldom enter woodlands (southern bog lemmings, Synaptomys cooperi; meadow voles, Microtus pennsylvanicus; grassland jumping mice, Zapus hudsonius).<br /> Additionally, we report widespread changes in the distribution of common opossums. Opossums are a southern species whose range has extended gradually northwards since the early 20th century (Gardner & Sundquist, 2003). Data sources Records from 1978-2008 came primarily from extensive live-trap sampling by field crews from the University of Michigan, Michigan State University, and Miami University. The purpose of these surveys was to document the current distribution and relative abundance of species of small mammals, and all captures were recorded. In almost all cases localities are believed to be accurate to within less than 500 m (Appendix). Questionable species identifications were confirmed using molecular techniques (Appendix). When identifications could not be confirmed, animals not readily identified using field characters were eliminated from the analysis (64 out of 10,273 Peromyscus and 11 out of 293 Glaucomys were deleted).<br /> Recent opossum records were based on field observations and especially, records of road-killed animals made from 2006-present. Coordinates of road-killed animals were recorded using a GPS unit.<br /> Most records prior to 1978 came from the specimens and field notes housed in the University of Michigan Museum of Zoology and the Michigan State University Museum. Additional specimen records were obtained from the MaNIS network <a href="http://www.manisnet.org">http://www.manisnet.org</a> (Appendix). Error in estimating locality coordinates varied widely (Appendix). We examined the estimated error associated with the coordinates of each specimen with the intent of eliminating records whose error overlapped either previously reported range limits or boundaries of the geographic regions on which comparisons of community composition are based (Appendix). A few records were not mapped because their estimated errors were extremely large, but in every case specimens were unambiguously assignable to one of the geographic regions of the study. For some critical records with uncertain localities, we were able to reduce estimated error considerably by referring to field notes and/or published descriptions of collecting expeditions. We examined and verified the identifications of all museum specimens that suggested significant changes in distribution. A few records were also provided by individual collectors or taken from published papers. In most cases they involve unexpected findings, usually occurrences outside of the normal range of a species (e.g., Ozoga & Verme, 1966; Haveman, 1976; Stormer & Sloane, 1976; Wells-Gosling, 1982). These records provide documentation of range expansion and are included below in maps and calculation of range change, but as no information was usually provided on what other species were trapped, these records were excluded from analyses of faunal composition. Regions included Published range maps of species of mammals in Michigan suggest a transition between a fauna associated with the oak hickory woodlands and savannahs typical of the southern part of the state, and a northern fauna associated with northern hardwood and coniferous forests (Hall, 1981; Baker, 1983). At the time these maps were compiled, northern and southern faunas met in the middle of the Lower Peninsula, in a region (“tension zone”) that is characterized by differences in soils and a transition from a more southern to a more boreal flora (Fig. 1; Medley & Harman, 1987). Our focus is on changes concentrated to the north of this zone, and consequently we restricted our attention to records north of 44oN latitude (Fig. 1). A number of islands are found in Lakes Michigan, Superior, and Huron. Many are inhabited by small mammals, and extensive collection records are available for some. These islands have little or no opportunity to receive immigrants from the mainland, and the composition of their fauna likely reflects the species present when the islands were isolated by rising water as the lakes first formed, nearly 10,000 years ago. Records from islands separated from the mainland by at least 10 km (Beaver, High, Hog, Timm’s, Squaw, Whiskey, Trout, Gull, Garden, N and S Manitou, N and S Fox, Bois Blanc, Isle Royale) were not considered in this analysis. Further, we eliminated 4 sites in the northern Lower Peninsula, because since 1978 they were visited repeatedly, often several times a year, to obtain specimens or to follow the populations of particular species. Including them would have strongly biased the analyses in the direction of conditions at those sites, and for inferences concerning regional community composition, would represent a form of pseudoreplication (Hurlbert, 1984). These sites (and the area each encompasses) are as follows (Fig. 1): 1) 45.168 – 45.1775oN, 84.375 - 84.401oW (2.1 km2) 2) 45.088 – 45.1147 oN, 84.402 – 84.425 oW (5.34 km2) 3) 45.271 – 45.296 oN, 84.416 - 84.443 oW (5.88 km2) 4) 3 line transects, 300-500 m in length, at the University of Michigan Biological Station: 45.546°N, 84.667°W; 45.5567°N, 84.7015°W’; 45.4894°N, 84.6849°W. Huron Mountains Repeated collections made at a few sites in the Huron Mountains are especially informative. The Huron Mountains are a series of low granitic hills (maximum elevation 600 m) near the Lake Superior shoreline in the central Upper Peninsula of Michigan (Fig. 1). Approximately 7,300 ha are owned by a private association, the Huron Mountain Club, whose members support research on their property through the Huron Mountain Wildlife Foundation. This area includes a 2600 ha Nature Research Area of primary (never logged) forest. The Huron Mountain Wildlife Foundation has funded 3 surveys of the mammals of the region. The first, a comprehensive survey of vertebrates by Richard Manville, was carried out from autumn 1939 through summer 1942 (Manville, 1947, 1949). To sample small mammal populations, Manville set up 8 quadrats chosen to represent the habitats of the region. Each quadrat comprised an 11 x 11 trapping grid (30 ft between traps). Manville used live traps and trapped for 5 consecutive days 4 times over the course of the study. He deposited extensive series of voucher specimens in the collections of the University of Michigan Museum of Zoology, and we have confirmed his identifications of Peromyscus. In 1972-1973, John Laundre also conducted small mammal censuses in the Huron Mountains, trapping at or near the same locations as Manville and using similar techniques (Laundre, 1975). Unfortunately, his report does not list numbers of individuals of most species captured, and we are therefore unable to include his records in the analyses of relative abundance reported here. Nor have we been able to locate voucher specimens. His account, however, is useful in documenting the presence/absence of species in 1972-3 compared to other time periods. In 2004-2005, the survey was repeated by Allison Poor (Poor, 2005). Poor used live-trapping techniques similar to those of Manville and Laundre and located most of her quadrats at or very close to the same sites. Poor, however, trapped for 3 days/sampling period, taking 2 samples in 2004 and 1 in 2005. Like Manville, she recorded all captures, and she deposited vouchers (mainly tissue samples) in the University of Michigan Museum of Zoology. Time periods Preliminary examination of maps and capture records suggested that for small mammals, change in distributional patterns accelerated during the late 20th century. While these preliminary results also suggested some differences among species in the timing of change, to simplify comparisons of SFR assemblages we arbitrarily chose to compare collections made from 1883 (when the first records were obtained) through 1980 with those made from 1981 to the present. Data analysis A total of 14,076 records of the 8 focal species of SFRs from north of 44oN latitude were used in the analyses reported below. Of these, 4,808 came from museum catalogues and records taken from the literature, and 9,268 from our sampling. These records include 4,099 captures from 564 localities recorded during the period 1883-1980, and 9,977 captures from 591 localities from 1981-2007. The focal small forest rodents make up 96.5% of all captures of forest-dwelling rodents (including tree squirrels and rare species) and 70% of all captures of forest-dwelling small mammals (including the above species plus shrews and moles). For opossums, we included 94 capture records from MaNIS, 163 records from a survey of road-killed animals carried out in 1968 (Brocke, 1970), and 281 records from a similar survey done in 2006-8.