Species-specific responses to atmospheric CO2 enrichment may affect biodiversity, which in turn may alter ecosystem functioning. Here we have explored biodiversity effects in model assemblages of semi-arid grassland of the northern Negev, Israel, at 280 ppm (pre-industrial era), 440 ppm (early 21st century) and 600 ppm CO2 (mid to late 21st century). Thirty-two mostly annual species were grown together in large containers (ca 400 kg each) on native soil and under a dynamic simulation of the Negev winter climate. CO2 enrichment increased concentrations of total non-structural carbohydrates and C/N ratios, and reduced specific leaf area and nitrogen concentrations in leaves of all species. In contrast to these uniform CO2 effects on leaf quality, biomass and reproductive output remained unchanged in most species, and varied greatly among the few responsive ones (−80 to +145%). Biomass was significantly increased at elevated CO2 in Onobrychis crista-galli (one of the six legume species) and was reduced in Biscutella didyma (Brassicaceae). Seed yield increased in three out of six legumes and in the root hemiparasite Parentucellia flaviflora, and decreased in the grass Aegilops peregrina. Fruit dry matter tended to be reduced in two Brassicaceae. Onobrychis, the largest and most responsive species present, was the most ‘mesic’ legume, and might have profited most from the higher soil moisture induced by CO2 enrichment. The significant CO2 response of only 5–6 out of 32 species, in particular their altered seed yield, suggests a potential shift in biodiversity. In a future CO2-enriched atmosphere, ‘mesic’ legumes and root hemiparasites might be favored, while some Brassicaceae and grasses might decline. As indicated by significant 280- vs 440-ppm differences, reductions in leaf nitrogen concentration of grasses and legumes are likely to be under way right now, and may negatively affect grazers. Altered seed yields were more pronounced between 440 and 600 ppm, suggesting that these changes could intensify as the atmospheric CO2 concentration continues to rise.