Ecology of Mima‐Type Mounds in Northwestern Minnesota

This paper reports on the physiognomy, soils, microclimate, vegetation and animal activity on Mima mounds in Minnesota and presents an hypothesis on origin of these mounds. The mounds vary from 10 to 130 feet in maximum dimension and from 6 to 50 inches in height and occur at a density of about one per acre on 210.5 acres of upland native prairie. Mound vegetation is always distinctly dominated by either grasses, forbs or the shrub, Symphoricarpos occidentalis. The most striking feature of most of the mounds is the disturbance on their soil by pocket gophers (Geomys bursarius), ground squirrels (Citellus spp.), badgers (Taxidae taxus), toads (Bufo hemiophrys) and other animals. This has probably caused the change in vegetation, the lower bulk density, the lack of soil structure, and the increased water permeability. Animal disturbance of the soil may also account for the lack of sod, the high organic content of the subsurface silt loam, the zone of gradation of silt into clay, the discontinuity of the zone of carbonate concentration, and most of the anomalies such as pockets of pebbles. Comparison of the mound and adjacent prairie soils suggests that the mounds are, in effect, puffs of soil formed primarily by animals disturbing and mixing the soil in a particular sport.