THE QUANTITATIVE INTERACTION BETWEEN BIOLOGY AND HILLSLOPE SOIL PROCESSES: A MATHEMATICAL MODEL FOCUSING ON POCKET GOPHERS (GEOMYIDAE) AS SOIL EROSION AGENTS

There is an increasing awareness of the interplay between biologic and geologic processes in shaping the earth's surface. On convex hillslopes, where overland flow is absent, burrowing organisms are often the dominant energy source for sediment transport and soil erosion. In diffusive sediment transport models, the role of biology is captured within diffusivity, but there has not been an explicit link between ecological measures of animal activity (energy consumption) and the diffusivity. We have built a model to link energy expenditure by the pocket gopher (Geomyidae) to geomorphic soil production and transport models. In the model, soil thickness is a dynamic function of gopher population density, bedrock weathering rate, and slope curvature. Values for model parameters were obtained from the literature (gopher energetics) and recent hillslope studies in Marin County, California, where bedrock weathering rate and diffusive soil transport (gopher driven) have been empirically and independently determined. Model calculations based on the empirically derived diffusivities indicate that less than 0.001% of the net primary productivity in this annual grassland is consumed in sediment transport. We evaluated the effect of gopher's preferences for thicker soils with higher biomass, and model simulations suggested that soil thicknesses was less responsive to topographic curvature as previous models suggest, and that steady state soil thickness became feasible at convergent slopes. Both these results better explained the observed soil thickness vs. curvature observations from our reference hillslope. Time-dependent model simulations suggested that there must be a dynamic feedback between gopher population density and bedrock weathering rates through soil thickness on century to millennial time scales. Gopher-mediated soil loss may ultimately decrease soil thickness and deteriorate the habitat, and thus decrease gopher populations. However bedrock weathering compensates for soil loss and may affect the population density. Thus, an interesting feedback may exist between burrowing organisms, geology, and hillslope soils. To our knowledge, this is the first attempt to mathematically solve the interactions between geological processes and biological agents in co-shaping complex landforms.