Sensitivity of ecosystem functioning to changes in trophic structure, functional group composition and species diversity in belowground food webs

The objective of the present paper, using decomposer food webs as a tool, is to explore the levels of the ecological hierarchy (trophic groups, feeding guilds, species populations) at which reduction in complexity brings about significant changes in ecosystem performance. A review is given of various mini‐ecosystem studies that have recently been conducted at the University of Jyväskylä. It is hypothesized that the typical features of soils as a habitat, and the peculiarities of belowground food webs, such as the commonness of indirect interactions (mediated through abiotic resources) among the biota, together with the high frequency of polyphagy/omnivory among soil organisms, produce a diversity‐ecosystem functioning relationship that is likely to differ from those of aquatic and aboveground food webs. Experiments showed that alterations in ‘trophic levels’ were reflected in significant changes in decomposition processes, which, in turn, had substantial impact on primary production. Similarly, heterogeneity within ‘trophic levels’ was shown to be associated with increased growth of birch and pine seedlings. In contrast, species diversity within a feeding guild had little or no influence on ecosystem‐level processes. However, the species‐specific properties of individual taxa were shown to be more influential in affecting plant growth than species number per se. For example, the presence of an omnivorous enchytraeid species in the mini‐ecosystems was observed to consistently be associated with high biomass production of tree seedlings. It is concluded that the so‐called trophic dynamic models based on direct feeding interactions are of limited value in predicting the outcomes of interactions taking place belowground.