Quantifying bioturbation by soil macroinvertebrates in a broad leaf African savanna

Abstract

Soil macroinvertebrates, such as termites and ants, are considered ecosystem engineers that perform ecosystem processes which impact ecological function. Termites are important bioturbators that mix soil through building and foraging activities. Termite mounds influence the heterogeneity of savannas. Quantifying the volume changes of termite mounds is essential because it indicates their role as important bioturbators in savannas. However, ants are important predators of termites and may have a top-down effect on termite-mediated bioturbation. I used an evaluated photogrammetry technique to quantify the change in volume of termite mounds over time for two termite genera, Macrotermes and Nitiditermes in a broad leaf African savanna. To assess the extent of top-down control on termite-mediated bioturbation, I used large-scale suppression treatments to reduce the abundance of ants and termites and then measured the lateral and vertical movement of an optical tracer in the upper 30 cm soil layer. The photogrammetric estimated volumes and dry weight of termite mounds revealed that on average Macrotermes can add 11.17 kg. week-1 and Nitiditermes can add 72.50 g. week-1 to their mounds. The mean weight of soil added over a 40-week period to Macrotermes mounds was 446.61 kg and 2.92 kg to Nitiditermes mounds. Termite-mediated bioturbation was influenced by changes in either ant or termite abundance at certain depths and time frames. The suppression of ant abundance promoted termite-mediated bioturbation, and the suppression of termite abundance reduced bioturbation. Therefore, the amount of soil moved by termites to build their epigeal structures highlights them as important bioturbators in a savanna and ants have a potential top-down control on termite-mediated bioturbation.