Climate change and cereal aphids: the relative effects of increasing CO₂ and temperature on aphid population dynamics

Experimental evidence regarding the responses of cereal aphids to rising atmospheric CO 2 has been ambiguous. Some studies suggest increased population sizes under future CO 2 levels, others suggest decreased population sizes, and still others suggest little or no difference. Recently, Newman et al . (2003) constructed a general mathematical model of the aphid–grass interaction to investigate whether or not we should, in fact, expect a general aphid response to rising CO 2 . They concluded that aphid populations are likely to be larger under future CO 2 concentrations if soil N levels are high, the aphid species' nitrogen requirement is low and the aphid species' density‐dependent response in winged morph production is weak. In that model, and in field experiments, CO 2 concentration influences aphid population dynamics through the effect it has on plant quality. However, future CO 2 concentrations are also likely to be accompanied by higher ambient temperatures, a combination that has received little focus to date. In the present paper, the Newman et al . model is used to consider the combined effects of increased CO 2 concentrations and temperature on aphid population sizes. It is concluded that, when both factors are elevated, aphid population dynamics will be more similar to current ambient conditions than expected from the results of experiments studying either factor alone. This result has important implications for future experimentation.