INTRODUCTION One of the aims of the contemporary ecologicalal research is to explore the mechanisms as to how bio- logical diversity within food webs is associated with ecosystem functioning (Schläpfer & Schmid 1999; Wardle 2002). Due to: (i) the many levels of the eco- logical hierarchy (species populations, functional groups, trophic levels etc.); (ii) the interconnected nature of these levels (May 1997); and (iii) the habitat-specific characteristics that are bound to affect the structure and function of the biota (Pimm 1982), the answer would remain obscure unless thorough experimentation, followed by compari- sons between various environments and biotopes, is carried out. In this Volume of Ecological Research (‘Special Issue’) investigation of food web charac- teristics from various habitats, ranging from pelagic food webs (L. Legendre & R. Rivkin, pp. 143–149) to below ground terrestrial habitats (the present article) form the backbone for such an analysis. The aim of the present paper is to shed light on biodiversity-ecosystem function using belowground decomposition food webs as a tool. I will start by introducing elements, mostly Ecological Research (2002) 17, 207–215 Sensitivity of ecosystem functioning to changes in trophic structure, functional group composition and species diversity in belowground food webs Heikki Setälä* University of Jyväskylä, Department of Ecological and Environmental Science, PO Box 35 (YAC), 40351 Jyväskylä, Finland 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 above- ground food webs. Experiments showed that alterations in ‘trophic levels’ were reflected in sig- nificant 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. Key words: decomposer food webs; ecosystem function; nutrient dynamics; omnivory. *Author to whom correspondence should be addressed. Present address: University of Helsinki, Department of Ecological and Environmental Sciences, Niemenkatu 73, FIN-15140 Lahti, Finland. Email: heikki.setala@helsinki.fi Received 21 May 2001. Accepted 20 September 2001.