ANCIENT BUG-BITTEN LEAVES REVEAL THE IMPACTS OF CLIMATE AND PLANT NUTRIENT CONCENTRATIONS ON INSECT HERBIVORE COMMUNITY ASSEMBLY (Invited Presentation)

Plants and insects account for up to 75% of non-microbial terrestrial biodiversity, form the base of terrestrial ecosystems, and play a critical role in nutrient cycling. Unlike most biotic interactions, whose evolutionary history must be pieced together using indirect evidence from species co-occurrences, consumer morphology, or observations of modern ecosystems, plant-insect herbivore associations can be studied directly using leaf compression fossils with insect feeding traces. Paleoecological studies have documented the initial colonization of land plants by insect herbivores over 400 million years ago, subsequent increases in trophic complexity and feeding diversity, and the ecological and evolutionary impacts of climate change and mass extinction events on plants and insect herbivores.

High resolution paleoecological studies of intervals of environmental quiescence complement research focusing on abiotic perturbations and provide insight into biological factors affecting plant and insect herbivore community assembly. A several thousand year record from the early Miocene Mush Valley in north-central Ethiopia provides one such example. Twenty-two hundred fossil leaves were censused from six stratigraphic levels: two carbonaceous shales below a ~10 cm thick ash bed, the ash bed itself, and three carbonaceous shales above the ash bed. Both plant and insect damage diversity are lowest within the ash bed, likely the result of environmental perturbation caused by the eruption, but insect damage diversity does not track plant diversity throughout the rest of the section. Rather, the strongest predictor of insect herbivore damage diversity and frequency is the prevalence of legumes, whose symbiotic relationships with nitrogen-fixing bacteria make them more nutrient-rich. This pattern was also observed across the climatically variable Paleocene-Eocene boundary in the western US, suggesting that plant nutrient composition is at least as important as climate in controlling insect herbivore distributions.