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Insect Herbivores, Insectivores and Detritivores in a Scrub-Oak Ecoystem
Stiling, P., Forkner, R. and Drake, B. 2010. Long-term exposure to elevated CO2 in a Florida scrub-oak forest increases herbivore densities but has no effect on other arthropod guilds. Insect Conservation and Diversity 3: 152-156.

The authors write that in a CO2-enriched atmosphere, "plant quality for herbivores is reduced because of decreases in plant nitrogen concentrations and increases in secondary metabolites (Poorter et al., 1997; Curtis and Wang, 1998; Penuelas and Estiarte, 1998)," but that "Stiling et al. (2009) recently showed that, over long time periods under continuously elevated CO2, the increases in plant biomass under elevated CO2 over compensate for reductions in foliage quality by providing a greater quantity of biomass for herbivores."

What was done
Seeking to determine if such might also be the case for insectivores and detritivores, Stiling et al. employed pitfall trapping over the last three of eleven years of continuous CO2 enrichment (to 350 ppm above ambient) in eight of sixteen open-top chambers that enclosed portions of a scrub-oak ecosystem in Florida (USA) that was comprised of a mix of three oak tree species (Quercus myrtifolia, Q. geminata and Q. chapmanii) that accounted for approximately 90% of the total aboveground ecosystem biomass, while they employed sticky traps over another six months of the study.

What was learned
The three researchers report that 110,618 insects and other arthropods from 25 orders were recovered from the pitfall traps, and that 39,305 insects and other arthropods from 14 orders were found on the sticky traps. And these data, in their words, "revealed increases of insect herbivore species such as Thysanoptera (thrips), Hemiptera, and Lepidoptera, but no effects on insectivores such as spiders, parasitic wasps, and ants, or on detritivores such as Diptera (flies), Psocoptera (book lice), Blattodea (cockroaches), Collembola (spring tails), Orthoptera (crickets) and Coleoptera (beetles)."

What it means
Stiling et al. conclude that the "increase in the number of herbivores, including raspers, suckers, and chewers, under elevated CO2" was "likely caused by an increase in plant biomass under elevated CO2," which for the scrub-oak ecosystem they studied amounted to 67% (Seiler et al., 2009). But they additionally conclude that "increases in plant biomass and herbivore abundance brought about by elevated CO2 do not influence insect abundance at other trophic levels such as insectivores, parasitoids and predators, or decomposers," noting that their results are similar to those of two other studies (Sanders et al., 2004; Hillstrom and Lindroth, 2008) that also "failed to detect effects of elevated CO2 on most guilds." Hence, the "bottom-up effect brought about by higher plant biomass as a result of many years of elevated CO2" would at this point in time appear to only enhance herbivore abundance.

Curtis, P.S. and Wang, X. 1998. A meta-analysis of elevated CO2 effects on woody plant mass, form, and physiology. Oecologia 113: 299-313.

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.

Penuelas, J. and Estiarte, M. 1998. Can elevated CO2 affect secondary metabolism and ecosystem function? Trends in Ecology and Evolution 13: 20-24.

Poorter, H., van Berkel, Y., Baxter, B., Den Hertog, J., Dijkstra, P., Gifford, R.M., Griffin, K.L., Roumet, C., Roy, J. and Wong, S.C. 1997. The effect of elevated CO2 on the chemical composition and construction costs of leaves. Plant, Cell and Environment 10: 472-482.

Sanders, N.J., Belote, R.T. and Weltzen, J.F. 2004. Multitrophic effects of elevated atmospheric CO2 on understory plant and arthropod communities. Environmental Entomology 33: 1609-1616.

Seiler, T.J., Rasse, D.P., Li, J., Dijkstra, P., Anderson, H.P., Johnson, D.P., Powell, T.L., Hungate, B.A., Hinkle, C.R. and Drake, B.G. 2009. Disturbance, rainfall and contrasting species responses mediated aboveground biomass response to 11 years of CO2 enrichment in a Florida scrub-oak ecosystem. Global Change Biology 15: 356-367.

Stiling, P., Moon, D.C., Rossi, A.M., Hungate, B. and Drake, B. 2009. Seeing the forest for the trees: long term exposure to elevated CO2 increases some herbivore densities. Global Change Biology 15: 1895-1902.

Reviewed 16 June 2010