Isotopic values of hair tissues in small mammal populations reflect differences in ecological behavior and long-term responses to environmental change
The northeastern U.S. has undergone extensive land use changes over the past 150 years including agricultural expansion, increased nitrogen deposition, and increased coal production. These changes can have long-term impacts on resource availability and use by organisms by altering the magnitude and chemistry of nutrient inputs into ecosystems. This study applies stable isotope analysis to museum specimen collections spanning a 120-year period in order to elucidate long-term shifts in resource use by ground-dwelling small mammal populations within this region. The goal of this study was to test for effects of time and sex on the isotopic composition of hair tissues in two common small mammal species, Blarina brevicauda and Microtus pennsylvanicus, having known differences in ecological behavior. We measured the isotopic composition (δ13C, δ15N, δ18O, δ2H, and δ34S) of hair tissues from museum specimens collected from 1889 – 2000 across six counties located in the Great Lakes Basin of New York state. We evaluated differences in population mean values of δ13C vs. δ15N and δ18O vs. δ2H, and related the latter to the isotopic composition of regional precipitation. We then examined the effects of year, season, sex, and the interaction of these factors on isotopic values using a three-factor linear model.
B. brevicauda (Northern short-tailed shrew) populations showed a significant 3‰ enrichment in δ13C and 2‰ enrichment in δ15N relative to M. pennsylvanicus (meadow vole) populations; suggesting that variability within food sources of insectivorous B. brevicauda diets is large relative to differences between food sources of herbivorous M. pennsylvanicus diets. Seasonal variation in B. brevicauda diet was consistent across sexes, with depleted δ15N values in the fall; δ15N values were consistent across seasons in M. pennsylvanicus populations, which suggests no dietary-switching. Hair δ13C values in both populations were consistent across years, suggesting limited use of C4 agricultural lands. Hair δ15N values in both populations showed a significant 4‰ depletion over the 120-year period, suggesting landscape-level depletion in baseline N inputs with increased atmospheric deposition of isotopically depleted NH4, or increased N fixation via leguminous cover crops. For both species, regional precipitation only explained a small amount of variation in hair δ2H and δ18O values, suggesting greater proportions of O and H derived from plant or sub-surface waters. Our results provide insights into resource use patterns among small mammal populations, indicate potential drivers of environmental change, and highlight the utility of museum specimen collections in understanding ecological responses long-term environmental change.