Influences of climate, cattle density, and lake morphology on Sporormiella abundances in modern lake sediments in the US Great Plains

Abstract

Sporormiella, a genus of coprophilous fungus, is increasingly used as a qualitative proxy for megafaunal density. Decreased spore abundances in Late-Pleistocene and Holocene sediments are thought to signal prehistoric megafaunal population declines and extinctions. However, much remains unknown regarding the dispersal, deposition, and preservation of Sporormiella, hindering interpretation. Here we collected lake-center and lake-margin surface-sediment samples from 24 lakes in a west–east transect across South Dakota, Minnesota, and Wisconsin to test whether Sporormiella abundances varied along a precipitation gradient and areas of varying grazing intensity. Unlike other pollen types, relative abundances and concentrations of Sporormiella spores were uncorrelated between lake-center and lake-margin samples. For lake-margin samples, distance-to-shoreline and county-scale cattle density were significant predictors of Sporormiella relative abundances. Only distance to shoreline was a significant predictor of Sporormiella concentrations. The positive relationship between shoreline proximity and Sporormiella abundances differs from prior reports that Sporormiella concentrations decrease away from shoreline. For lake-center samples, Sporormiella relative abundances were negatively correlated with precipitation and Sporormiella concentrations had no significant predictors. These results suggest that Sporormiella representation in lake sediments is controlled by multiple factors, including but not limited to local megaherbivore density. Sporormiella concentrations appear to be ‘noisier’ than Sporormiella relative abundances, due to within-basin sedimentary effects on pollen and spore accumulation rates. More work on Sporormiella taphonomy is critically needed to better understand the controls on its representation in late-Quaternary sediments.