Diet analysis of Neotoma middens from the last glacial period to present in the Snake Range, NV

Abstract

Atmospheric [CO2] is increasing at an unprecedented rate from fossil fuel emissions, resulting in rapid climate shifts and subsequent changes in plant community composition. Ancient climactic excursions and [CO2] change in the geological record may have also driven community shifts in vegetation from non-human effects, posing challenges for mammalian herbivores. During the Last Glacial Maximum (LGM), Younger Dryas, and mid-Holocene, temperatures were ~8 °C colder than modern values, ~9°C colder than modern values, and ~2°C warmer than modern values, respectively. Ancient DNA (aDNA) analysis of ancient samples allows for the characterization of ancient packrat (Neotoma sp.) diet from these time periods that was previously only possible through visual observations of macrofossils and pollen. My research examines how Neotoma diet shifted over time between 27,000 and 3300 yrs BP (from 14C dating) with respect to shifting vegetation due to changes in climate and atmospheric [CO2] over this time at the Snake Range in Nevada. I do this through the analysis of packrat middens, ancient Neotoma deposits that contain fossilized vegetation and fecal matter that represent past local vegetation. N. cinerea is a midden building species inhabiting the Snake Range and is a dietary generalist, meaning its diet likely reflects local vegetation. Evidence shows local vegetation shifts with environmental conditions across time, thus the advantage of different photosynthetic pathways shifts. Warm conditions and growing season rain favor C4 plants, which are also highly competitive under low CO2 conditions. On the other hand, C3 plants are generally favored under cooler conditions and show improved growth rates under elevated [CO2] conditions. CAM plants are generally desert-adapted and can tolerate very dry conditions and a wide range of temperatures in many cases. To interpret isotopic signatures of fecal pellets and to distinguish whether they reflected a mixed C3-C4 diet or a full CAM diet of Neotoma, I utilized a DNA metabarcoding approach to identify plant species directly from the DNA of ancient fecal pellets. This combination of stable isotope diet analysis and species identification through aDNA metabarcoding allows for a novel two-tier approach that details the presence / absence of plant species as well as gives insight into diet shifts with vegetation changes. I identified no CAM taxa within the samples but found strong evidence of a C3 diet throughout time as supported by identified taxa and δ13C isotope analyses. In addition, I observed shifts in present vegetation of the pack rats’ diet over time, notably that Juniperus sp. became more utilized in the diet in the mid-Holocene, suggesting these packrats may have evolved adaptations to subsist on this toxic juniper over time. My research identifies past dietary shifts due to changing [CO2] and climate which informs future work in understanding how modern and future plant /animal biodiversity will respond to new shifts in climate and rising [CO2] levels.