Sediment, Gophers, and Time: A Model for the Origin and Persistence of Mima Mound—Vernal Pool Topography in the Great Central Valley

Vernal pools are seasonal wetlands that harbor rare and endemic plants and animals. In California, vernal pools are often associated with Mima mounds, roughly circular soil mounds, found in grassland landscapes. The processes that form and maintain the mound-pool complexes have not yet been conclusively identified, even though such information is necessary to understand the effects that land use and climate change may have on the resilience and longevity of these important landscapes. One hypothesis for the origin and persistence of Mima mound—vernal pool systems proposes that pocket gophers (Rodentia: Geomyidae) maintain and possibly create the mounds by preferentially translocating soils towards mound centers as an adaptive response to perched water tables. The topographic characteristics and above-ground gopher activity of one of the largest remaining Mima mound—vernal pool systems in California were studied in order to investigate the gopher hypothesis. A detailed topographic survey of a 3,507 m region was conducted near the city of Merced, CA. Downslope erosion rates off Mima mounds were estimated using a mass balance model that incorporates a diffusive sediment transport law. The median calculated net erosion rate was 15 cm of soil per 1,000 years, while the measured rate of above-ground gopher sediment movement was approximately 57 cm of soil per 1,000 years. Assuming that some portion of the gopher sediment movement is in an upslope direction, these results suggest that gopher soil transport may be large enough to compensate for erosion, and this activity may play a dominant role in maintaining Mima mound—vernal pool systems. The results of this study were used to guide the development of a quantitative model of gopherdriven sediment transport on Mima mound—vernal pool landscapes. This model will be used to infer the origin of the landscapes and the rates of processes critical to their continued functioning and will also help to determine, quantitatively, the role of burrowing animals as “keystone” species on these landscapes. CITATION. REED, S. AND R. AMUNDSON. 2007. Sediment, gophers and time: a model for the origin and persistence of mima mound—vernal pool topography in the Great Central Valley. Pages 15-27 in R. A. Schlising and D. G. Alexander (Editors), Vernal Pool Landscapes. Studies from the Herbarium, Number 14. California State University, Chico, CA.