Aboveground macrodetritivores and belowground soil processes: Insights on species redundancy

Quantifying the level of overlap between species-specific effects in a given process - i. e. their functional redundancy-remains a central issue in predicting the shape of diversity-productivity or diversity-stability relationships. We compared the indirect effects of six aboveground macro-detritivores on two belowground processes, cellulose decay and nutrient mineralization, in single-species microcosms (n = 5) containing standardized substratum, mixed litter, and four individuals of detritivores. In all microcosms, there was an overall increase in phosphate and decrease in nitrate and ammonium over four weeks. Cellulose decay was ≈30%. Compared to fauna-free controls, there were a few noticeable fauna effects: decay was higher in microcosms with Armadillidium vulgare and Porcellio scaber, nitrate immobilization was lower in Cylindroiulus caeruleocinctus microcosms, and ammonium immobilization was lower in A. vulgare and P. scaber microcosms. The multivariate effects were idiosyncratic and related to a high intraspecific variation. Contrary to our expectations, the most noticeable difference was between P. scaber and Trachelipus rathkii. Our results point to redundancy due to high intraspecific variation. It is postulated that, if redundancy arises from high intraspecific variation, upon the loss of species, large population sizes of remaining species from the same functional group might explain positive effects of redundancy on ecosystem resilience.