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Effects of Elevated CO2 on Insect Herbivores
Stiling, P., Rossi, A.M., Hungate, B., Dijkstra, P., Hinkle, C.R., Knot III, W.M., and Drake, B.  1999.  Decreased leaf-miner abundance in elevated CO2: Reduced leaf quality and increased parasitoid attack.  Ecological Applications 9: 240-244.

What was done
In what may possibly be the first attempt to study the effects of elevated CO2 on trophic webs in an ecosystem, the authors enclosed portions of a native scrub-oak community, located in Florida, USA, in open-top chambers of 3.6-m diameter and fumigated them with atmospheric CO2 concentrations of 350 and 700 ppm for approximately one year to determine whether or not elevated CO2 impacts leaf miner (herbivore) densities, feeding rates, and mortality in this nutrient-poor woody ecosystem.

What was learned
Although there were no chamber effects on herbivore densities, total leaf miner densities were 38% less on CO2-enriched foliage than they were on foliage of ambiently-grown vegetation.  Moreover, atmospheric CO2 enrichment consistently reduced the numbers of the six individual species of leaf miners studied on CO2-enriched foliage relative to that observed on foliage grown at ambient CO2 concentration.

However, atmospheric CO2 enrichment increased the leaf area consumed by the less abundant herbivore miners by approximately 40% relative to areas mined by the more abundant herbivores present on control foliage exposed to 350 ppm CO2.  Despite this increase in feeding, leaf miners in CO2-enriched chambers experienced significantly greater mortality than those in ambient control chambers.  Although CO2-induced reductions in leaf nitrogen, and thus nutrient, content played a minor role in increasing herbivore mortality, the greatest factor contributing to this phenomenon was a four-fold increase in parasitization by various wasps that could more readily detect the more-exposed leaf miners in the CO2-enriched foliage.

What it means
As the CO2 content of the air rises, it is likely that this phenomenon will indirectly affect the abundances of herbivorous leaf miners in scrub-oak communities by increasing their mortality due to greater successful predation by their natural enemies.  If extended to agricultural ecosystems, these findings suggest that crops may experience less damage from these herbivores in the future, thus increasing potential harvest and economic gains.  In addition, with reduced leaf miner abundances in CO2-enriched atmospheres, crop producers can reduce their dependency upon chemical pesticides to control these damaging herbivores, thus reducing the impact of these agricultural chemicals on the environment.

Reviewed 1 July 2000