Ecosystem engineering by hummock‐building earthworms in seasonal wetlands of eastern South Africa: Insights into the mechanics of biomorphodynamic feedbacks in wetland ecosystems

Abstract

This paper resolves the origin of clay hummock micro‐topography in seasonal wetlands of the Drakensberg Foothills, providing a review and appraisal of previously‐suggested mechanisms of hummock formation in the context of new field and laboratory data. Field surveys revealed neo‐formation of clay hummocks in a river channel that had been abandoned in c.1984. Fresh earthworm castings were located atop hummocks protruding from inundated abandoned channel margins. Earthworm castings, and sediment cores taken in hummocks and adjacent hollows, were analysed for soil‐adsorbed carbon and nitrogen using an HCN analyser, and for 210Pb activity using alpha‐geochronology. 210Pb activity profiles suggest relative enrichment of the isotope in hummocks, and relative depletion in adjacent hollows. Earthworm castings are characterised by very high 210Pb activity, as well as high C and N contents. Hummocks have significantly higher C and N contents than adjacent hollows. Results suggest that it is the foraging activity of earthworms in litter‐rich seasonal wetland hollows, and repeated excretion of castings atop adjacent hummocks, that is responsible for the elemental enrichment observed. The paper presents a conceptual model of hummock formation in wetlands through interactions between hydrogeomorphology and earthworm activity, and illustrates a mechanism of biogeomorphic inheritance through which ordered patterns of preferential flow can emerge in ecosystems. Further implications of hummock formation and nodal accumulation of nutrients are considered in relation to wetland resilience and regulatory ecosystem service provision.© 2018 John Wiley & Sons, Ltd.

Biogeomorphology, quo vadis? On processes, time, and space in biogeomorphology

“…(2019) explore the potential of case‐building caddisfly larvae and fish feeding behaviour in changing in‐channel sediment transport, while Grenfell et al . (2019) show for the first time how the ecosystem engineering of earthworms changes surface processes in riparian wetlands in South Africa.…”

“…Grenfell et al . (2019) find that earthworms create a hummock–hollow microtopography in seasonal wetlands in South Africa. The authors were able to quantify the slow ecosystem engineering of earthworms by using a combination of field observations of earthworm activity and Pb210 sediment dating technique.…”

“…Yet, many contributions within this issue highlight that our understanding of biogeomorphic feedback systems is incomplete and, importantly, differs depending on the scale considered. For the smallest scales, a mechanistic understanding is established for some feedback systems (Grenfell et al ., 2019; Mason et al ., 2019) but many other feedback systems at the smallest scales remain unresolved to date (e.g. microbial effects on sediment dynamics, microbial effects on extraterrestrial bodies).…”

“…In agreement with the coastal sedimentary environment, a distinction between different scales and associated fluvial landforms can be made, although cross-scalar linkages are increasingly a focus area (Gurnell et al, 2019;Kleinhans et al, 2019;Rice et al, 2019). At the small scale, work is done in understanding the role of vegetation, and particularly animals, on hydrodynamics and sediment dynamics (DeAnna and Wohl, 2019;Dong et al, 2019;Grenfell et al, 2019;Gurnell et al, 2019;Mason et al, 2019;Rice et al, 2019;Jerin and Phillips, 2020). At larger spatial and temporal scales, GIS and remote sensing are increasingly used to monitor and evaluate the evolution of fluvial riparian environments (Gurnell et al, 2019;Kleinhans et al, 2019;Corenblit et al, 2020;Mossa et al, 2020).…”

“…Discoveries can still be made on how animals change surface processes through environmental feedback systems, like beavers causing long-lasting vegetation change in riparian corridors (DeAnna and Wohl, 2019). Mason et al (2019) and Rice et al (2019) explore the potential of case-building caddisfly larvae and fish feeding behaviour in changing in-channel sediment transport, while Grenfell et al (2019) show for the first time how the ecosystem engineering of earthworms changes surface processes in riparian wetlands in South Africa.…”