Elevated atmospheric carbon dioxide and ozone alter forest insect abundance and community composition

1. Human-induced climate changes threaten the health of forest ecosystems. In particular, carbon dioxide (CO2) and tropospheric ozone (O3) will likely have significant but opposing impacts on forests and their associated insect communities. Compared with other animal groups, insect communities are expected to be especially sensitive to changes in global climate. 2. This study examined the effects of elevated CO2 and O3 (eCO2 and eO2) individually and in combination on the abundance, diversity and composition of forest insect communities. Insects were sampled using yellow pan traps in an aggrading aspen-birch forest at the Aspen Free Air CO2 Enrichment (FACE) site in northern Wisconsin, USA. We trapped for 24 h every 10–15 days throughout the summers (June to September) of 2000–2003. 3. We examined 47 415 insects from 4 orders and 83 families. Elevated CO2 reduced abundance of phloem-feeding herbivores and increased abundance of chewing herbivores, although results were not statistically significant. Enriched CO2 increased numbers of some parasitoids. The effects of eO3 on insect abundance were generally opposite those of eCO2. No significant differences in arthropod family richness were found among treatments. However, eCO2, eO3, or both significantly affected insect community composition in all years. 4. Carbon dioxide and tropospheric ozone have the potential to alter significantly forest insect communities. Feeding guild may strongly influence insect response to environmental change and may provide the best opportunity to generalise for conservation efforts. Because insect communities influence forest health and ecosystem services, continued research on their response to global change is critically important to forest management and conservation.