Elevated CO2 Lowers Relative And Absolute Herbivore Density Across All Species Of A Scrub-Oak Forest

The unabated increase in global atmospheric CO₂ is expected to induce physiological changes in plants, including reduced foliar nitrogen, which are likely to affect herbivore densities. This study employs a field-based CO₂ enrichment experiment at Kennedy Space Center, Florida, to examine plant-herbivore (insect) interactions inside eight open-topped chambers with elevated CO₂ (710 ppm) and eight control chambers with ambient CO₂. In elevated CO₂ we found decreased herbivore densities per 100 leaves, especially of leaf miners, across all five plant species we examined: the oak trees Quercusmyrtifolia, Q. geminata, and Q. chapmanii, the nitrogen-fixing vine Galactiaelliottii and the shrub Vacciniummyrsinites. Both direct and indirect effects of lowered plant nitrogen may influence this decrease in herbivore densities. Direct effects of lowered nitrogen resulted in increased host-plant related death and an increase in compensatory feeding: per capita herbivore leaf consumption in elevated CO₂ was higher than in ambient CO₂. Indirectly, compensatory feeding may have prolonged herbivore development and increased exposure to natural enemies. For all leaf miners we examined, mortality from natural enemies increased in elevated CO₂. These increases in host-plant induced mortality and in attack rates by natural enemies decreased leaf miner survivorship, causing a reduction in leaf miner density per 100 leaves. Despite increased leaf production in elevated CO₂ from the carbon fertilization effect, absolute herbivore abundance per chamber was also reduced in elevated CO₂. Because insects cause premature leaf abscission, we also thought that leaf abscission would be decreased in elevated CO₂. However, for all plant species, leaf abscission was increased in elevated CO₂, suggesting a direct effect of CO₂ on leaf abscission that outweighs the indirect effects of reduced insect densities on leaf abscission.