The geomorphology of burrowing bioturbation in the Central French Pyrenees

Abstract. Burrowing mammals can alter mountain hillslopes by excavating subsurface voids, producing mounds of loose sediment, and creating local weaknesses in banks and slopes. Their geomorphic role remains poorly documented in alpine and subalpine catchments, where steep slopes, shallow soils, seasonal snowmelt, and grazing can rapidly rework small disturbances. We quantified the geomorphic imprint of marmots, voles, and moles in the eastern Bastan catchment, Central French Pyrenees, using field surveys, burrow and mound measurements, and soil analyses. We recorded 439 vole burrow entrances, 131 marmot burrow entrances, and 357 molehills. Vole activity was especially associated with gully banks, where 274 entrances were counted along 50 surveyed gully segments and local densities reached 3 entrances m⁻¹. Marmot burrows were less frequent but larger: 42 measured internal voids had a mean volume of 4.68 × 10⁻² m³ and a total measured volume of 1.97 m³. Marmot mounds averaged 1.95 × 10⁻² m³, whereas molehills were smaller (5.26 × 10⁻³ m³) but more numerous. Soil pH and median grain size did not differ significantly between mounds and adjacent soil. The clearest soil-property effect was the higher bulk density of marmot mounds relative to adjacent soil, suggesting post-excavation compaction. Overall, burrowing mammals in the Bastan create a visible microtopographic and subsurface signature: molehills roughen fine-grained grassland surfaces, marmot burrows generate large local voids and mounds, and vole galleries are concentrated in gully banks where they may favour bank collapse and gully widening.