Elevated Atmospheric CO2 Lowers Herbivore Abundance, But Increases Leaf Abscission Rates

Increased levels of atmospheric carbon dioxide (CO₂) are likely to affect the trophic relationships that exist between plants, their herbivores and the herbivores’ natural enemies. This study takes advantage of an open-top CO₂ fertilization experiment in a Florida scrub oak community at Kennedy Space Center, Florida, consisting of eight chambers supplied with ambient CO₂ (360 ppm) and eight chambers supplied with elevated CO₂ (710 ppm). We examined the effects of elevated CO₂ on herbivore densities and levels of leaf consumption, rates of herbivore attack by natural enemies and effects on leaf abscission. Cumulative levels of herbivores and herbivore damage were significantly lower in elevated CO₂ than in ambient CO₂. This may be because leaf nitrogen levels are lower in elevated CO₂. More herbivores die of host plant-induced death in elevated CO₂ than in ambient CO₂. Attack rates of herbivores by parasitoids are also higher in elevated CO₂, possibly because herbivores need to feed for a longer time in order to accrue sufficient nitrogen (N), thus exposing themselves longer to natural enemies. Insect herbivores cause an increase in abscission rates of leaves throughout the year. Because of the lower insect density in elevated CO₂, we thought, abscission rates would be lower in these chambers. However, abscission rates were significantly higher in elevated CO₂. Thus, the direct effects of elevated CO₂ on abscission are greater than the indirect effects on abscission mediated via lower insect densities. A consequence of increased leaf abscission in elevated CO₂ is that nutrient deposition rates to the soil surface are accelerated.