DRIVERS OF HOLOCENE ABUNDANCE DYNAMICS IN DESERT MICE

Global temperature is increasing at an alarming rate, placing many species at increased risk of extinction. Paleontologists and neontologists alike have long recognized abundance as a key variable for identifying vulnerable species. Abundance, however, reflects complex interactions of both abiotic and biotic factors. For example, niche breadth flexibility could be an important biotic buffer to environmental change. Little is known about how climate, niche breadth, and species’ abundance dynamics interact, however, making it difficult to identify the species most vulnerable to future environmental changes.

We used the mice of Two Ledges Chamber -- a Holocene fossil record in the Great Basin, USA -- to disentangle how the temporal dynamics of climate and a species’ niche breadth influence its abundance. Using stable isotopes (d¹³C and d¹⁵N), we estimated the niche breadth of three species (Perognathus longimembris, Peromyscus cf. maniculatus, and Reithrodontomys megalotis) that encompass a range of physiological, morphological, and behavioral adaptations of mice to desert environments. Sample-standardized abundance dynamics for these species were estimated from fossil specimens spanning the last 8,000 years. Local high-resolution climate dynamics for the same time-interval were obtained from a published lake core sediment d¹⁸O record. We then used structural equation modeling to assess and contrast alternative hypotheses regarding the relationship(s) among climate, niche-breadth and abundance.

Species’ abundances exhibited marked taxonomic and temporal variation. Isotopic niche-breadth also varied significantly among species and over time, and did not always reflect a species’ modern d¹³C and d¹⁵N values. Preliminary AIC model-comparisons support a direct-effects-only causal hypothesis in which climate warming, increases in d¹³C, and decreases in d¹⁵N contribute independently to increases in abundance. This suggests that both biotic and abiotic factors must be considered when assessing species vulnerability to environmental change. Further work is needed to determine whether temporal changes in isotopic niche-breadth reflect changes in vegetation, diet preferences, or the isotopic composition of mouse food.