Positive interactions and complementary use of resources are pointed out as potential mechanisms responsible for  increases in ecosystem functioning with increases in biodiversity. However, few efforts have been made to understand the joint action of both of them. Our objective was to analyze the simultaneous effects of niche complementarity and positive interactions on ecosystem functioning. We developed a model that mimicked the functioning of a Patagonian steppe ecosystem, and simulated shrub and grass transpiration and water movement through three soil layers. We calculated aboveground net primary production (ANPP) based on plant transpiration and an estimate of water use efficiency (WUE). We simulated a gradient of niche complementarity by modifying root distribution and a gradient of facilitation by modifying grass-WUE. We found that ANPP increased with niche complementarity, that ANPP increased with WUE, and that niche complementarity modulated the interaction between the two life forms. We estimated the biodiversity effect as the difference between the simulated and expected ANPP; expected values were the average ANPP obtained from life form monocultures. Along niche complementarity and positive interactions space, we estimated isopleths of the proportion of ANPP explained by the biodiversity effect. We found an uneven contribution of both mechanisms to the biodiversity effect that resulted from non-linear water absorption by plants.