Atmospheric CO2 as a Global Change Driver Influencing Plant-Animal Interactions

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 C(3) and C(4) plants (particularly grasses) are affected by atmospheric carbon dioxide. C(4) grasses currently predominate over C(3) grasses in warmer climates and their distributions expand as atmospheric carbon dioxide levels decreased during glacial periods. C(4) grasses are a less nutritious food resource than C(3) grasses both in terms of reduced protein content and increased C/N ratios. There is an indication that as C(4)-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 C(3) versus C(4) 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 (C(3) versus C(4)) changes.