INCREASED INSECT HERBIVORY DURING THE LATEST PALEOCENE AND PALEOCENE-EOCENE THERMAL MAXIMUM IN THE BIGHORN BASIN, WYOMING, USA

Climates, biodiversity, and organismal distributions changed significantly across the Paleocene-Eocene boundary. However, the responses of insect herbivores to the climatic and floral change across this boundary have not been well-studied. The research presented here is part of a larger project to examine changes in insect damage on angiosperm leaves in the Western Interior, USA, through the Paleogene. We focus here on insect damage in Wyoming's Bighorn Basin during the late Paleocene and early Eocene (59-55 Ma), with particular emphasis on the Paleocene-Eocene Thermal Maximum (PETM). Mean annual temperatures gradually warmed by ~4oC through the late Paleocene, abruptly increased by ~5oC in the PETM, and returned to pre-PETM values in the early Eocene. We test how these changes affected the diversity and frequency of insect damage.

We conducted insect damage censuses at five sites in the Bighorn Basin, including a site within the PETM. At each locality, over 800 identifiable dicot leaves were scored for insect damage using the damage morphotypes (DTs) of Labandeira et al. These DTs can be divided into those typical of generalist insects and those strongly associated with specialist herbivores. Damage metrics were compared using resampling analyses to standardize for sample size.

Both total and specialized damage diversity show a significant increase during the latest Paleocene (Clarkforkian 3), reach a maximum in the PETM, and decline slightly in the early Eocene. This pattern is visible on the bulk floral assemblages, as well as individual plant species. Less than 38% of angiosperm leaves from the three Paleocene localities contain insect damage, whereas 56% of the PETM leaves are damaged. In this study, increased insect damage diversity and frequency correlate with increases in temperature. The elevated herbivory on the thermophilic PETM flora is consistent with modern ecological studies suggesting that raising temperature increases the abundance and diversity of insect herbivores.