Sensitivity of Primary Production to Changes in the Architecture of Belowground Food Webs

The proposed mechanisms for the species diversity-function relationship in plant communities stress the recognition of functional properties of species, and interactions between plants and soil processes. As resource availability to plants is influenced by the architecture of decomposer food webs, it has been hypothesised that the diversity of decomposers can also control ecosystem processes, including primary production. We manipulated the complexity of soil animal communities in a miniecosystem experiment in which a boreal forest floor with birch seedlings infected with mycorrhizal fungi was created. The soil animal diversity ranged from zero to typical species richness of soil fauna (approximately 50 taxa) in coniferous forests. Between these extremes was a nested factorial design with manually assembled communities consisting of two sets of one-species, and two sets of five-species animal communities within fungivorous and microbi-detritivorous trophic groups. To investigate the role of predators in system functioning, the miniecosystems with fungivorous and microbi-detritivorous fauna were established either with or without mesostigmatid mites as top predators. The miniecosystems were incubated in a climate chamber with varying illumination and temperature regimes for 40 weeks. Our experiment provides evidence that primary productivity is generally insensitive to variation at the species level or even at the level of trophic groups. Although top predators generally reduced prey population size, no effect was found on system functioning. However, the removal of microbe- or detritus-feeding fauna, especially the microbi-detritivore Cognettia sphagnetorum reduced plant N uptake and accumulation of plant biomass. The functional importance of soil fauna was inversely related to the trophic position of the group. Our results suggest that ecosystem functioning is robust against species extinctions in belowground food webs, and that primary production is predominantly controlled by organisms at low trophic positions in the decomposer food web.