Faeces traits as unifying predictors of detritivore effects on organic matter turnover

In the last decade, our understanding of plant litter decomposition and soil organic matter formation substantially improved but critical blind spots remain. Particularly, the role of detritivores, i.e. soil animals that feed on litter and soil, is poorly understood and notoriously missing from biogeochemical models. This major gap results from methodological difficulties to isolate their effect and from the astonishing diversity of detritivorous organisms with few common features, thereby hampering the identification of general patterns. In this viewpoint, we propose that the characteristics of their faeces can predict detritivore effects on soil processes related to organic matter turnover across the large detritivore diversity. Indeed, faeces are common to all detritivores, and a large part of organic matter is transformed into faeces in many ecosystems. Two recent studies presented here showed that faeces characteristics are powerful predictors of the fate and turnover of this transformed organic matter. We suggest that faeces characteristics, such as water-holding capacity, size and spatial organisation of the faecal pellets and of their constituting particles, particulate organic matter connectivity, as well as the characteristics of dissolved organic matter in faecal pellets, are promising ‘effect traits’. By focusing on similar features rather than differences, this approach has the potential to break down barriers of this highly fragmented soil animal group, in particular between earthworms that are often studied as ecosystem engineers and classical litter transformers such as millipedes, woodlice, or snails. We discuss ways of tackling the complexity of using such traits, particularly regarding the composite determinism of faeces characteristics that are driven both by the detritivore identity and the ingested organic matter. Rigorous and hypothesis-based use of faeces characteristics as effect traits, including clear identification of studied processes, could allow integrating detritivores in our current understanding of organic matter turnover.