The impact of ant bioturbation and foraging activities on surrounding soil properties

Abstract

Nest building and foraging activities are two of the many ways that ants impact on the surrounding soil environment within and beyond the mound. These activities have both long- and short-term effects on the soil part of the ecosystem through structural alterations, nutrient accumulation and release, with possible enhancement of soil quality. This study illustrates the impact of ant foraging activities on the soil and the pattern of arrangement existing in soil properties in the mound soil environment. Eight active mounds of Iridomyrmex greensladei, greater than 50 cm in diameter, were randomly selected from four blocks in a vegetation remnant adjacent to the main Wee Waa highway, Narrabri, New South Wales. The soil volume, soil mass and slope for each mound was characterised. Soil samples were collected in the plots at 0-10 cm depth from the top of the mound, the mound perimeter, 5 m radius from the mound perimeter, the foraging tracks, and from other locations unaffected by ant activity. The soil samples were analysed for physical and chemical properties. The extent of pore distribution as cavities and galleries, was evaluated by taking photographs of a cross section of an ant mound after pouring in water miscible paint (1:8 paint to water suspension) into an open cut on the top of the mound. The paint moved through the pores, to a depth of about 150-200 cm from the surface through vertical and lateral galleries. In comparison to the surrounding soil, ant-impacted soils were lower in clay, higher in sand and silt, and lower in exchangeable Ca, Mg, K and Na. The top of the mound was higher in NO₃, P and more compacted than soils not modified by ant mounds. Ant-impacted soils had low dispersion indices compared with unmodified soil. Ant bioturbation activities increased soil porosity in the mounds extending to about 200 cm down the soil profile. Ant bioturbation and foraging activities were found to affect soil properties beyond the perimeter of the mound and into the surrounding ecosystem.