Action of earthworms on stone burial – a return to Darwin’s estate

Abstract

Charles Darwin famously documented the disappearance of stones in the grounds of Down House in Kent, over a period of decades at a location originally known as the “Stony Field”. This site (Great Pucklands’ Meadow - GPM) was visited in 2007 and an experiment was set up by laying locally-derived flints on the soil surface of this ungrazed grassland. The stones (either large or small of mean mass 1,100 and 250 g respectively) were arranged at high and low density (n= 50 or 25, and 100 or 50 respectively for large and small stones) within 1 m2 plots in a Latin square design. The area was distant from public access routes and went unmown throughout the duration here reported. Fixed point photographs were taken and location recorded at the outset. After 6 years, the site was revisited and examined. The stones had generally been incorporated into the soil. Photographs were re-taken and measurements made of depth of burial from a quadrant of each plot. A controlled laboratory experiment was also conducted over a period of 6 months using Aporrectodea longa (the dominant species shown to be present in GPM) to assess the amount of possible casting by this species in the absence of other biotic factors. Here a single flint was placed on the soil surface within a 2 litre vessel and 4 earthworms added. Flints in earthworm-free vessels were also monitored. A small scale collection of A. longa casts from a mown lawn area close to GPM was also conducted on a fortnightly basis over a period of 12 months. Results from these experiments are presented to show the action of earthworms on stone burial.