This paper deals with the contribution value of biodiversity to ecosystem performances under a viability approach. Two contrasted cases are considered. First, a no-exploitation situation where the ecosystem performances are measured through the capacity of species richness to maintain the ecosystem above a minimum ecological viability threshold measured through a Shannon index. Second, an exploitation situation where the performances of the ecosystem are measured through its economic sustainability, that is, its capacity to generate direct-use values greater than a minimum guaranteed utility level. The analysis, based on numerical simulations, shows that biodiversity has a positive effect on both ecological and economic performances and that in both cases the marginal contribution of biodiversity is positive. Furthermore, this marginal contribution exhibits maximum values. These maximum values seem however to decrease with the level of species richness. These results show interesting links with two of the main current debates on biodiversity: the Noah's Ark problem and the assumption of decreasing marginal value supported by some recent works on bio-prospecting.