QUATERNARY RANGE AND POPULATION VARIATION SHIFTS IN PACIFIC-COAST MICROTUS SPECIES IN RESPONSE TO ENVIRONMENTAL PRESSURES

Rodents of the genus Microtus have undergone extensive environmental changes throughout the Quaternary. They reacted to glacial inundations during the last glacial maximum, faced major ecosystem shifts at the Pleistocene-Holocene boundary, and experienced the near-complete replacement in their main food supply, native grasses, when European invasives swept across the Pacific coast just 300 years ago. These rodents’ biological reactions to these environmental shifts have not yet been documented, because it has been very difficult to distinguish between species in the fossil record except in rare cases of exceptional preservation.

Using geometric morphometrics and a discriminant analysis, I can distinguish between the five species of the genus Microtus living in California today (M. californicus, M. longicaudus, M. montanus, M. oregoni and M. townsendii) by examining only their first lower molar. This method allows me to identify many more fragmentary specimens than were previously recognizable to species. As such, I can begin to trace how Microtus has moved across the landscape throughout the Quaternary and how intraspecific morphological variation changed during periods of climatic or ecological stress.

Using geometric morphometrics, I examine Microtus specimens from localities in California and Oregon and trace shifts in species’ ranges and population variation across the landscape and through time. I find that M. longicaudus has undergone a range contraction of up to 160 km and ascension of approximately 1000 m within the Holocene. Additionally, I see a recent reduction in M. californicus dental variability in recent populations relative to fossil assemblages. These results indicate that human and climatic alterations to the environment may have already reduced the baseline evolvability of this small mammal species, making it more difficult for them to react to impending climate change. As I analyze more localities at higher temporal resolutions, a more detailed picture of Microtus’ history comes into focus that demonstrates that they, like large mammals, faced ecological and evolutionary challenges at the Pleistocene-Holocene boundary.