Predictive accuracy regarding future wildlife responses to environmental change can be greatly enhanced by knowledge of past responses. Furthermore, organisms themselves can serve as environmental data recorders. To gain a historical perspective on climate and land-use change across a range of spatio-temporal scales over the past century, we applied stable-isotopic approaches to historical museum specimens of the most widespread generalist in North America (Peromyscus maniculatus), a highly useful environmental data recorder. Specifically, we analyzed hair samples from deer mice collected over the past 100 years for oxygen (δ18O) and hydrogen (δ2H) isotopic composition, targeting specimens from two regions of the western United States, the Pacific Northwest (PNW) and southern California (CA). These two regions capture a wide range of temperature and precipitation regimes and anthropogenic pressures (deforestation and urbanization, respectively). We focused on oxygen and hydrogen in order to investigate regional changes in water resources and aridity. We used machine learning techniques to 1) generate isotopic landscape models and 2) test which climate variables control the isotopic composition of P. maniculatus at three time intervals: before 1930, mid-century, and after 1980.
Results/Conclusions
Small mammals record aspects of their diet and thus local environment in their tissues via stable isotopes; the δ18O and δ2H values of hair, in particular, reflect aspects of the hydroclimate via drinking and consumed vegetation water. In the PNW, we found strong relationships between the δ2H of hair and growing season temperature and longitude. These relationships indicate that P. maniculatus accurately record growing season conditions and Rayleigh isotopic fractionation. Surprisingly, we found weak correlations between δ18Ohair and climatic variables in the PNW. The isotopic composition of mice from CA showed strong relationships between δ18Ohair and vapor pressure deficit, but not between δ2Hhair and growing season precipitation. Contrasting relationships uncovered for the PNW and CA suggest that the δ18O and δ2H of P. maniculatus may be controlled by different climatic variables regionally. Additionally, we found that the offset between δ18Ohair and δ18O of growing season precipitation is significantly correlated with aridity before 1930 across the entire dataset. Loss of this strong aridity signal after 1950 may be attributed to human alteration of the water cycle. Thus, stable isotopic studies of museum specimens can provide valuable insights into rapid environmental change over the 20th century.