SOIL MIXING RATES OF THE WESTERN HARVESTER ANT: A NEOICHNOLOGICAL PERSPECTIVE ON THE IMPORTANCE OF THE ANT IN THE PALEOPEDOLOGICAL RECORD

Ants are one of the most important soil-dwelling organisms in both modern and ancient environments because they are common, spatially abundant, and have been they since the Jurassic. Ants have long been recognized for their importance in influencing soil organic carbon and other soil properties; however, little if any recognition has been offered to the subsurface mixing and deliberate downward movement of clastic sediments by ants. In light of this, we present new measurements on ant-clastic-sediment mixing rates from a neoichnological study. We propose these results are corollary to ant-soil-mixing rates in modern and ancient environments. For our study, ants were observed through a glass enclosure filled with several layers of colored sand, gravel, and shells. After twelve weeks, several patterns of sediment distribution were observed, including the upward relocation of sediment to the surface, the inter-horizon mixing of sediment, and the deliberate downward movement of sediment. Both the amount and distance of excavated sediment was calculated providing a conservative estimate of the total volume of excavated material. An estimated 16% (8,523 cm³) of the total volume of sediment within the enclosure (~54,099 cm³) was excavated by the ants. Our estimation was compared to the volume of the nest mound to calculate the total amount of sediment excavated to the surface. Data indicates that nearly 55% of all sediment excavated within the enclosure was not placed on the surface, but incorporated into the backfill of unused chambers and galleries within the nest. A simple comparison of the estimated and measured tunnel diameter indicates that our initial estimates are conservative and most likely represent a lower mean average than what actually occurs in nature. Our experiment shows new empirical data on the downward mixing of clastic sediments by ants, mixing that may have a greater impact on soil development than the traditionally viewed. We suggest these patterns of mixing, and, implication of such mixing, also apply to other social insect activity, which is often overlooked, but remains a significant pedological component of modern and ancient environments.