Spatial and temporal expression of vegetation and atmospheric variability from stable carbon and nitrogen isotope analysis of bat guano in the southern United States

Abstract

Stable isotopes of faeces contain information related to the animals feeding ecology. The use of stable isotope values from subfossil faeces as a palaeoenvironmental indicator depends on how faithfully the animal records their local environment. Here we present insectivorous bat guano δ13C and δ15N values from a precipitation gradient across the southern United States and northern Mexico to compare with local vegetation and climate. We find δ13C values to be an excellent predictor of expected C4/CAM vegetation, indicating that the bats are non-selective in their diet. Moreover, we find bat guano δ13C values to be strongly correlated with summer precipitation amount and winter precipitation ratio. We also find evidence for a significant relationship with mean annual temperature. In general, we do not find δ15N values to be related to any parameters along the climatic gradient we examined. Additionally, we measured δ13C and δ15N values of bulk guano deposited annually from 1968 to 1987 in a varved guano deposit at Eagle Creek Cave, Arizona. Neither δ13C nor δ15N values were significantly related to various local meteorological variables; however, we found δ13C values of guano to be significantly related to drought and to the North American Monsoon indicating bat guano δ13C values preserve an interpretable record of large-scale atmospheric variability.