Soil surface macrofaunal communities associated with earthworm casts in grasslands of the Eastern Plains of Colombia

Abstract

Earthworms are known to modify life conditions for other soil organisms through their drilospheric activities. The effects of a large anecic species, Martiodrilus carimaguensis Jiménez and Moreno, on soil macrofaunal communities were investigated in a natural and a man-made grassland of the Eastern Plains of Colombia. Invertebrates were sampled by a standard hand sorting method at different spatial scales: (i) at the scale of a few centimetres during the course of one year, we compared the dynamics of the density, biomass, diversity and individual weights of macroinvertebrates in earthworm surface casts, in the soil located below casts and in a control soil without casts; (2) at the scale of a few decimetres and at a fixed time, macroinvertebrates were sampled in soil monoliths with different densities of casts on their surfaces. Macroinvertebrates colonised casts 4 and 6 weeks after their formation in the pasture and the savanna, respectively. In the two grasslands, at the spatial scale of the cast, macroinvertebrate density was significantly higher below casts (1660–5310 ind m −2) than in the control soil (400–1700 ind m −2), while no differences were recorded in biomass, richness, diversity and evenness. The presence of casts at the soil surface increased the relative dominance of epigeic populations (+185% to +250%) at the expense of larger endogeic species (−39% to −76%). Individual weights of invertebrates were lower in casts (1.2–5.7 g ind −1) and the underlying soil (8.2–11.1 g ind −1) when compared with the control soil (25.0–39.6 g ind −1). These results were mainly confirmed at the larger spatial scale. Taxonomic richness was positively correlated with the number of casts at the soil surface at the scale of a few decimetres, while diversity was unchanged and evenness decreased. These results were mainly attributed to the creation of new habitats (macropores, galleries) by earthworm activity and to the concentration of litter and soil organic matter in casts. They support the “Nested Biodiversity Hypothesis” according to which soil ecosystem engineers may have determinant effects on the abundance and diversity of other soil organisms.