USING BUG BITES ON FOSSIL LEAVES TO RECONSTRUCT FOREST HEALTH AND ECOSYSTEM STABILITY (Invited Presentation)

Insect herbivore damage on fossil leaves provides an independent line of paleoecological evidence that complements plant and insect body fossils, morphological data, and molecular phylogenies. The late Oligocene Chilga floras from northwestern Ethiopia provide a straightforward case study of the utility of fossil folivory in reconstructing ecological disturbance and ecosystem health. The Guang River and Bull’s Bellow fossil leaf localities are stratigraphically equivalent and only 1.5 km apart. Sedimentary structures, plant diversity, leaf physiognomy, and botanical affinities indicate that the Bull’s Bellow flora represents a recently disturbed early successional forest, whereas the Guang River flora is a more mature forest, with a mix of early and late successional species. A quantitative analysis of folivory on these two floras indicates important differences. First, damage diversity and frequency are significantly higher on both the bulk flora and individual plant hosts at Guang than at Bull’s Bellow. Second, both the percent of leaves with specialized feeding and the percent of feeding occurrences that are specialized are significantly higher at Guang.

These conclusions were applied to the early Paleogene of the Bighorn Basin, Wyoming, to test whether Paleocene-Eocene Thermal Maximum forests were stressed, unbalanced ecosystems. Because insect damage diversity and frequency are correlated with temperature, the PETM flora (20.1 ± 2.8^oC mean annual temperature, or MAT) was compared with a flora from the Early Eocene Climatic Optimum (22.2 ± 2^oC MAT). Leaf area damaged during the PETM is significantly higher than during the EECO, suggesting decreased leaf nitrogen concentrations and a corresponding increase in insect feeding rates. However, both floras have the combination of high plant diversity, insect damage diversity, and insect damage specialization that suggest balanced and fully functional ecosystems. In particular. the diversity and prevalence of host-specialized damage types on many different PETM plant species is striking, indicating well-developed specialized feeding associations on a broad spectrum of host lineages.