Individual host taxa may resist the climate-mediated trend in herbivory: Cenozoic herbivory patterns in western North American oaks

Abstract

The interactions between plant-eating insects and their hosts are hypothesized to shift dramatically as global temperatures increase. Examination of modern and fossil leaves has led to predictions of increased herbivory in a warmer world. However, climate induced changes in insect feeding may differ between individual host plants and their broader plant community. To examine how climate affects herbivory in a single, ecologically widespread host taxon, we studied insect feeding damage on the fossilized leaves of oaks (Quercus), which are host to a diverse group of herbivores and have been broadly distributed in temperate Northern Hemisphere floras throughout the Cenozoic. We examined oak leaves with ages that range from Eocene to Pliocene (38 to 5Ma) with a range of mean annual temperatures (6.5–17.6°C) and 3-month average precipitation levels (9.8–78.2cm) to test whether insect damage on oaks is more intense in floras that grew under warmer and wetter conditions.Examination of leaves from thirteen floras from the western United States shows that neither mean annual temperature nor average precipitation was a significant determinant of overall insect damage. However, herbivory made by endophagous insects (gallers and leaf miners) was more prevalent in dry conditions. Temperature also was a significant determinant of gall and leaf mine damage; warmer floras tended to have more galls and fewer leaf mines. In addition, gallers and leaf miners show a high degree of host-fidelity through time. This pattern of host-fidelity suggests that feeding of endophagous insect herbivores is more climatically sensitive than that of more generalized, exophagous herbivores, and that future climatic change may not affect all host-herbivore systems, nor all herbivores within a single host system, in the same ways. Therefore, an increase in outbreaks of specialized insects on their hosts may be more likely than broadly increased herbivory as global temperatures increase.