Mima mounds

Abstract: A group of islands of varying size on the floodplain of the Okavango alluvial fan, were studied to establish the processes which lead to the initiation and growth of islands. It was found that islands are initiated by the mound-building activities of the termite Macrotermes michaelseni. These termites import fine grained materials to use as a mortar for the construction of epigeal mounds. Their activities create a topographic feature, raised above the level of seasonal flooding, and also change the physical properties and nutrient status of the mound soil. Shrubs and trees are able to colonize these mounds, which results in increased transpiration. As a result, precipitation of calcite and silica from the shallow ground water occurs preferentially beneath the mounds, resulting in vertical and especially lateral growth, causing island expansion. © 1998 John Wiley & Sons, Ltd.

Abstract: The Mima mounds occur on certain prairies of glacial outwash in western Washington. The mounds are closely spaced, round or oval, from 10 to 40 feet in diameter, and from 1 to 7 feet in height. The typical mound is a double-convex lens of loose, unstratified, black silt-gravel set in a shallow pit in stratified yellow outwash gravel. Mounds are found only where a thin layer of soil overlies a compact bed of gravel, not on deep prairie soils. The Mima mounds are formed by pocket gophers (Thomomys talpoides) over long periods of time. Gopher activity in any particular place destined to become a mound site starts with intensive burrowing, such as that required in the construction of a nest, which loosens the soil and stimulates the growth of vegetation. The vegetation, in turn, furnishes food for the gophers and encourages them to concentrate their activities in the vicinity. A stage is reached where the gophers find sufficient food on the mounds to maintain them the year around, making it unnecessary for th...

Abstract: 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 zon...