Isotopic values of hair tissues in small mammal populations reflect differences in ecological behavior and long-term responses to environmental change

Background/Question/Methods

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.

Results/Conclusions

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.