Background
The authors write that "the response of complex plant and animal communities to global change is highly variable (Tylianakis et al., 2008)," but they note that "recent studies have documented that loss of foliage to arthropod herbivores decreases under elevated CO2 in woody communities (Hamilton et al., 2004; Knepp et al., 2005; Stiling and Cornelissen, 2007)," and they say that the fitness and in some cases the population size of herbivorous insects may decline in communities exposed to elevated CO2 (Hillstrom and Lindroth, 2008; Hillstrom et al., 2010)," although they indicate that the "effects of elevated CO2 on naturally occurring arthropod assemblages have not yet been widely characterized."

What was done
Working at the Duke Forest FACE facility in the Piedmont region of North Carolina (USA) - where three 30-meter-diameter plots of an expansive stand of loblolly pine had their atmospheric CO2 concentrations boosted by approximately 200 ppm, and where three other such plots were maintained at the normal ambient CO2 concentration - Hamilton et al. counted the numbers of different types of arthropods found in each of the six plots every two weeks throughout June and July of 2005, in order to be able to assign them to different feeding guilds. In addition, they say that "stable isotope data for spiders collected in ambient and elevated CO2 plots were analyzed to determine the extent to which herbivorous prey species moved into and out of the elevated CO2 plots."

What was learned
The seven U.S. scientists say their isotopic data "gave no indication that the treatment plots represented a 'boundary' to the movement of insects." In addition, they determined there was no detectable effect of elevated CO2 on the total number of individual arthropods in the two sets of treatment plots. However, they say "there was an increase in the numbers of individuals collected in primarily predaceous orders (Araneae and Hymenoptera; from 60% to more than 150%) under elevated CO2 and a decrease in the numbers in primarily herbivorous orders (Lepidoptera and Coleoptera; from -30 to -45%)."

What it means
In the closing sentence of their paper, Hamilton et al. conclude what is pretty obvious, i.e. that "decreases in herbivorous arthropods and increases in predaceous arthropods may contribute to reduced herbivory under elevated CO2 in forest systems," which is basically what has also been observed in the other studies that have experimentally explored the subject, all of which bodes well for earth's forests in a CO2-enriched world of the future.

References
Hamilton, J.G., Zangerl, A.R., Berenbaum, M.R., Pippen, J.S., Aldea, M. and DeLucia, E.H. 2004. Insect herbivory in an intact forest understory under experimental CO2 enrichment. Oecologia 138: 566-573.

Hillstrom, M.L. and Lindroth, R.L. 2008. Elevated atmospheric carbon dioxide and ozone alter forest insect abundance and community composition. Insect Conservation and Diversity 1: 233-241.

Hillstrom, M.L., Vigue, L.M., Coyle, D.R., Raffa, K.F. and Lindroth, R.L. 2010. Performance of the invasive weevil Polydrusus sericeus is influenced by atmospheric CO2 and host species. Agricultural and Forest Entomology 12: 285-292.

Knepp, R.G., Hamilton, J.G., Mohan, J.E., Zangerl, A.R., Berenbaum, M.R. and DeLucia, E.H. 2005. Elevated CO2 reduces leaf damage by insect herbivores in a forest community. New Phytologist 167: 207-218.

Stiling, P. and Cornelissen, T. 2007. How does elevated carbon dioxide (CO2) affect plant-herbivore interactions? A field experiment and meta-analysis of CO2-mediated changes on plant chemistry and herbivore performance. Global Change Biology 13: 1-20.

Tylianakis, J.M., Didham, R.K., Bascompte, J. and Wardle, D.A. 2008. Global change and species interactions in terrestrial ecosystems. Ecology Letters 11: 1351-1363.