Bug-bitten leaves from the early Miocene of Ethiopia elucidate the impacts of plant nutrient concentrations and climate on insect herbivore communities

High-resolution paleoecological studies elucidate the effects of biotic and abiotic factors on the diversity and distribution of insect herbivory in deep time. Previous analyses of fossil leaf assemblages from intervals of significant environmental change have shown that insect damage frequency and diversity are strongly influenced by temperature and carbon dioxide levels. Similar studies that span intervals of environmental quiescence are a necessary complement and can help disentangle the relative influences of biological versus abiotic factors. The 21.73 ± 0.03 million year old (early Miocene) Mush Valley lagerstätten, Ethiopia, provides one such example. Insect damage censuses were conducted at six stratigraphic levels that span ~50,000 years and whose plant ecology has been studied in detail. A total of 2200 leaves (47 morphotypes) were examined and 35 damage types (DTs) observed. We documented variations in insect damage frequency, diversity, and composition among plant host taxa and across stratigraphic levels and tested for correlations with abiotic and biotic factors. Interspecific variations in damage frequency and diversity among host plants at Mush with >20 leaves in the damage census could not be explained solely by leaf mass per area, presence/absence of trichomes, symbiosis with nitrogen-fixing bacteria, or host plant abundance. The strongest predictor of insect herbivore damage on the bulk floras (all leaves) was the prevalence of legumes, whose symbiotic relationships with nitrogen-fixing bacteria likely made them more nutrient-rich. This pattern was also observed across the climatically-variable Paleocene-Eocene boundary in the Bighorn Basin, USA, suggesting that the soil microbiome, plant nutrient composition, and climate work synergistically in regulating insect herbivore distribution, population sizes, and feeding.