DO WORMHOLES PREVENT LANDSLIDES? THE LINK BETWEEN BIOLOGICAL RESTRUCTURING OF SOIL AND HILLSLOPE PROCESSES

Critical zone science explores the processes that link natural systems – and this leads to some surprising questions, such as: can an earthworm prevent a landslide? Earthworm tunneling generates macropore networks that flush rainfall into the subsurface effectively. This potential impact on hydraulic conductivity and soil permeability has large-scale geomorphic implications. The tunnel networks reduce surface runoff and topsoil erosion, increase soil pore pressure by compaction, and increase the depth of weathering. Hillslope curvature and erosive mechanisms depend on these environmental factors. The Luquillo Critical Zone Observatory in northeastern Puerto Rico is a tropical mountain site where the occurrence of high magnitude, low frequency erosion events – landsliding – may depend on changes to the soil structure caused by earthworm tunneling. The Luquillo Mountains have a very deep soil profile, steep slopes controlled predominantly by landslide erosion, high annual rainfall, and high density of earthworms. The mid-elevation Tabonuco forest has a density of ~220 worms per m² [1]. From this forest type, we collected individual earthworms and housed them in their native soil in a unique nesting environment constructed to be equal in width to the individual worm, thus allowing their tunnel excavations to appear in 2D. Repeat photography and image analysis was used to measure the total volume of soil moved by tunnel excavating through time. The change in infiltration rate of the soil was physically measured after two weeks. The data from this experiment is used to inform a thought exercise – how differently would erosive processes behave in the Luquillo Mountains in the absence of worms?

[1] González et al. 2007. Earthworm communities along an elevation gradient in Northeastern Puerto Rico. European Journal of Soil Biology.