How predictable are aphid population responses to elevated CO₂?

Experiments investigating the population responses of aphids to CO₂ enrichment have yielded results suggesting that aphid populations will be both larger under elevated CO₂ and that they will be smaller under elevated CO₂. Most studies have failed to reject the null hypothesis of no difference in population sizes due to atmospheric CO₂ concentration. This diversity of results has led some investigators to conclude that aphid responses are not general, and that every aphid-plant interaction may be unique and unpredictable a priori. We use a single, general, mathematical model to consider the population responses of cereal aphids to grass grown under different CO₂ concentrations. The model shows that it is possible to explain any of the three observed results: larger populations, smaller populations, or no difference, and that which of these three outcomes arises may depend critically on the interaction between aphid nitrogen requirements and the nitrogen fertility of the soil. The model also shows that the qualitative results will depend on how sensitive the aphid species is to increases in its own density. Past studies have shown that aphids increase their production of winged offspring in response to increasing aphid density. The model predicts that, in general, aphid species that have lower nitrogen requirements and that are less sensitive to their own density will be more likely to have larger populations in elevated CO₂ compared to ambient CO₂. Differences between aphid species (and clones) in their nitrogen requirements and the strength of their density-dependent response have not been widely reported in the literature. Also, the nitrogen fertility of the soil has rarely been manipulated in experiments on aphid responses to rising CO₂ levels. The model suggests that the diversity of population responses of aphids may be both understandable and predictable in the context of such an interaction.