Plants respond to changes in atmospheric carbon dioxide. To herbivores, the decreased leaf protein contents and increased C/N ratios common to all leaves under elevated atmospheric carbon dioxide imply a reduction in food quality. In addition to these fine-scale adjustments, the abundance of C₃ and C₄ plants (particularly grasses) are affected by atmospheric carbon dioxide. C₄ grasses currently predominate over C₃ grasses in warmer climates and their distributions expand as atmospheric carbon dioxide levels decreased during glacial periods. C₄ grasses are a less nutritious food resource than C₃ grasses both in terms of reduced protein content and increased C/N ratios. There is an indication that as C₄-dominated ecosystems expanded 6–8 Ma b.p., there were significant species-level changes in mammalian grazers. Today there is evidence that mammalian herbivores differ in their preference for C₃ versus C₄ food resources, although the factors contributing to these patterns are not clear. Elevated carbon dioxide levels will likely alter food quality to grazers both in terms of fine-scale (protein content, C/N ratio) and coarse-scale (C₃ versus C₄) changes.