Cave guano deposits, which can accumulate over millennia, represent a relatively untapped paleoecological archive that can provide information about past vegetation, climate, and bat diet. Recent research suggests that carbon isotope values (δ13C) measured in guano accumulations from insectivorous bats reflect the relative abundance of C3 and C4 plants on the landscape while nitrogen isotope values (δ15N) may reflect precipitation amount. To better calibrate and ground truth these proxies, we compared isotope values measured in historical bat guano from Mary Lawson Cave, Missouri with instrumental records and aerial photographs. We additionally characterized the bats’ modern diet by combining two complementary approaches: guano isotopic analysis and DNA metabarcoding. DNA metabarcoding provides a high-resolution picture of the diversity of species consumed by bats, but not their relative abundances, whereas stable isotopes provide an overall view of diet, including inferences about relative abundance of food types.
Results/Conclusions
Mary Lawson Cave, a large limestone cavern located in central Missouri, is home to a maternity colony of insectivorous gray bats (Myotis grisescens) and as such, hosts significant guano accumulations. In the fall of 2018, we collected a 60 cm long guano core that dates to 1958 cal AD at its base, suggesting that guano has accumulated at a rate of ~1 cm/year. Guano core δ13C values decrease from the base toward the present (from -26.1 to -28.0 ‰) whereas δ15N values increase toward the present (from 9.6 to 15.5 ‰). Presently, the landscape around Mary Lawson Cave is dominated by deciduous forest and pasture. We hypothesize that the decline in δ13C values toward the present reflects an increase in forest cover, perhaps due to fire suppression, and/or a change from more natural savannah communities to pasture. Historic rainfall data from nearby Lebanon, Missouri show a small but significant increase in total yearly precipitation from 1958 to present (R2 = 0.383, p = 0.0048), indicating that higher guano δ15N values could imply greater water availability. Forthcoming results on modern bat diet will further contextualize this relationship.