During the last decades, the importance of biodiversity for ecosystems and their functionality has received increasing attention in ecological research (Hooper et al. 2005 , Tilman et al. 2014) . According to the convention on biological diversity (1992) biodiversity is defined as "the variability among living organisms from all sources including inter alia, terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part: this includes diversity within

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... es, between species, and of their habitats". More comprehensively, it concerns all variety in life with respect to genes, species, communities, and processes (Cardinale et al. 2012). Most of the biomes and ecosystems worldwide are actual facing substantial losses in species richness and diversity (at least for the past 60 yrs.), as a consequence of human activities (Millennium Ecosystem Assessment 2005). At the global scale, climate and land use change, nitrogen deposition, increasing atmospheric CO 2 , and biotic exchange and species invasion are presumably the most relevant drivers diminishing biodiversity (Sala et al. 2000) . Due to the ecological, genetic, economic and recreational values of biodiversity, its loss is known to affect key processes for ecosystem functioning and services like productivity, element and energy fluxes, soil formation and retention (Loreau et al. 2002 , Hooper et al. 2005 , Naeem et al. 2009). In fact, there is mounting evidence that the impact of species loss on ecosystem functioning is even as severe as of other major determinants of global change, i.e. global warming (Hooper et al. 2012. A positive relation between biodiversity and ecosystem functioning (BEF) is usually known to be based on the interplay of three mechanisms: the selection effect, the complementarity effect, and facilitation (Vandermeer 1992, Loreau and. Through the selection effect (or sampling effect), a diverse community can be dominated by the most productive and/or strongest consumer species, which might enhance overall performance in comparison to the average of monocultures (overyielding). In case of species complementarity, niche differentiation (i.e. partitioning in root or crown space) and a more complete resource acquisition may provide reduced inter-specific competition in comparison to intra-specific competition, causing enhanced turnover rates and overyielding in growth. Furthermore, species in mixtures might be facilitated by others, like for instance by nitrogen fixation or hydraulic redistribution; such processes might be difficult to separate from complementary behavior (Forrester 2014).