California Institute for Energy and Environment (CIEE)

Small mammals have shifted their elevation ranges in the Sierra Nevada. We questioned whether this shift can be linked to changes in habitat distribution, whether changes in population abundance match range dynamics, and how the shift affects predictions of future small mammal distribution. We merged data from mammal records of the Grinnell Resurvey Project, vegetation from the Wieslander Vegetation Type Maps and CALVEG and National Park Service, and downscaled PRISM climate data to meet these objectives.   We found that species that expanded their elevational distribution range tracked suitable habitats, and their ecological niche broadened over time. Species whose elevation range has contracted did not track suitable habitats, and their ecological niche remained constant.   Species that tracked their habitat dynamics showed an average decrease in abundance at the leading edge of their distribution range, whereas species that did not track their habitat dynamics showed either no change or increase in abundance at the lagging edge of their distribution range over time.   Life zone (vegetation types across elevation bands as a response to gradients in temperature and precipitation) and climate models performed better than vegetation models when changes over 80 years were analyzed, suggesting that species are responding more rapidly to climate than to vegetation change. Nonetheless, in all of these models, expanding species were harder to model as their ecological niche shifted, whereas contracting species produced more reliable models.   These results imply that modeling future distributions of sensitive species will vary according to the direction and magnitude of their sensitivity to both climate and vegetation changes.   The results of this analysis highlight the need to determine these species life history traits, habitat preferences and temporal dynamics, in order to identify which species are positively and negatively sensitive, and which are relatively insensitive to future climate and land cover change.