Downward Thinking: Rethinking the “Up ” in Soil Bioturbation

The nests of western harvester ants (Pogonomyrmex occidentalis) are extensive structures usually capped by a large conical mound. These ants have been documented to translocate significant amounts of soil and sediment to the surface; however, subsurface mixing patterns have not been investigated and documented. One thousand western harvester ants were added to a glass enclosure filled with discrete layers of homogeneous sediment to document this mixing. A 12-week experiment on nest development was conducted, during which (remarkable) translocation and mixing of sediment occurred. At the end of the experiment the distance and quantity of material translocated was measured. In sum, the ants excavated ~16 % of the enclosure. Large amounts of this sediment were moved upward and significant amounts of material were moved down into the nest. Notably, ~51 % of all excavated sediment was shown to be incorporated into the nest as backfill or tunnel lining. This material was neither translocated to the surface nor incorporated into the nest mound as traditionally thought. Our study demonstrates the significance of downward movement of sediments within soil. It also emphasises the underestimated role of soil-dwelling organisms, specifically the ant, in soil bioturbation and pedogenesis