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Ant (Formica exsectoides) Pedoturbation in a Forest Soil¹

ABSTRACT

Twenty-five active mounds of the common eastern moundbuilding ant (Formica exsectoides) had an average height of 0.37 m and basal diameter of 1.1 m in a long, narrow neverplowed clearing of 1,040 m² on the edge of a woodland in southern Wisconsin. The mound volume ranged from 0.01 to 0.6 m³ and averaged 0.13 m³. The ant nests, extending at least 160 cm below the soil surface occupied 1.9% of the area of the surface of a Typic Hapludalf (Gray-Brown Podzolic), Dubuque silt loam, formed in thin loess over reddish cherty clayey residuum on dolomitic limestone. Mounds were virtually bare and were largely composed of silty clay. Detailed studies were made of a soil profile through a large mound and of an intermound soil profile nearby. Calculations indicated that the ants have moved to the surface the equivalent of 1.25% of a plow layer or 2,500 kg of soil (dry wt) per 1,040 m² and in the process have reduced the bulk density of the material from 1.5 to 0.8 g/cm³ and increased the contents of available nutrients in the soil. Voids 0.2 to 2.3 cm in diameter, on the average occupied 20% by volume of the mound and about 10% of the buried A horizons. The mound contained about 50% silt, 46% clay, 4% sand; 4% free iron oxides, and 3% organic matter. Corresponding figures for the A2 and IIB23 horizons of the Dubuque are: 78, 18, 4; 2 and 3; 15, 83, 2; 6 and 0.3. Disturbance of soil profiles by this species of ant is restricted to small areas, apparently because colonies are established and mounds developed only in temporary openings in forests.

¹ Contribution from the Department of Soil Science, University of Wisconsin, Madison, supported by USAID, Wyoming University-Kabul University Contract. Published with the permission of the Wisconsin Agr. Exp. Sta., Madison.

² Research Assistant, USAID, and Professor of Soil Science, in charge, Soil Survey Division, Wisconsin Geological and Natural History Survey, respectively.

Received for publication November 15, 1967. Accepted for publication January 22, 1968.