Both at UIA and CEFE-CNRS Montpellier, ongoing research investigates the role of biological diversity in ecosystems. To this end the influence is examined of diversity on processes like productivity, nutrient acquisition, water household, etc., but also on resistance and resilience to disturbance. The approach is both experimental, by construction of synthesised model ecosystems (mediterrenean and temperate grasslands) and theoretical, by development of conceptual models. Recently, explanatory mechanisms were proposed for the first time in the literature regarding the influence of biological diversity on productivity (Tilman et al. 1997, Loreau 1998, Nijs & Roy 2000). However, the picture is gradually emerging that probably several mechanisms are operating simultaneously, and interact in a complex way. The aim of the current project is to epxlore an underlying mechansim of diversity which has not yet been examined in great detail: the complementary use of natural resources. According to Hooper (1998) addition of species to plant communities leads to niche-differentiation and additional nutrients becoming available, for example owing to differences between species in rooting depth or in timing of the growth cycle. Effects of increasing diversity on biomass production, as well as on other processes, may therefore ensue from a more efficient exploitation of the environment