MIMA MOUNDS MOVING OVER TIME

The evidence that Mima mounds are maintained by rodent bioturbation is strong, but the origins of the mound landscapes are still debated and may be multicausal. Proposed origin mechanisms for some mound landscapes rely on the spacing or locations of the individual Mima mounds within the landscape, assuming that current locations are the locations of the original mounds or their causes. Analysis of the chronosequence of extensive mound landscapes near Merced, California has found two relationships between mound organization and underlying landform characteristics: 1) the density and regularity of mound packing increases with terrace age; and 2) on erosional slopes they are aligned and fused on fall lines. We present an explicit cellular automaton model of soil movement up- and down-slope due to decimeter-scale hydrology, erosion, and rodent activity. This model reproduces the increase in mound density and regularity over time only if high rainfall variability, e.g. ENSO variations and larger, are included. Modeled responses to drought years are derived from observations of current mounds at Jepson Prairie, California, 2011-2013. Analyzing LiDAR maps of mounds on Merced erosional slopes finds that mound locations are predicted by current headward erosion into 2 million year old plains. Taken together, these suggest a response of mound locations to age and geologic changes. This weakens our ability to deduce original mound locations from current mound locations, and improves our ability to estimate the effect of climate changes on mound perseverance and on the species that live in Mima mound-vernal pool landscapes.