Some Like It Hot: The Correlation Between Temperature and Insect Herbivory during the Paleocene and Eocene in the Bighorn Basin, Wyoming, USA

Although many studies have analyzed the responses of individual taxonomic groups to climate change, very few have focused on how global warming affects interactions among organisms (e.g. Wilf and Labandeira 1999). Studies of insect herbivory on fossil leaves provide important information on the association of plants and their insect herbivores that cannot be obtained separately from plant macrofossils or insect body fossils. In this study, we track changes in insect herbivory in a single basin during the late Paleocene and early Eocene (59–52.5 Ma). Four major temperature changes occurred during this interval: a gradual warming during the last two million years of the Paleocene; the abrupt and transient Paleocene-Eocene Thermal Maximum (PETM); an early Eocene cooling; and a warming to the sustained Cenozoic maximum ~53 Ma.

Insect damage censuses were conducted at nine stratigraphic levels and 9071 fossil angiosperm leaves were examined for the presence/absence of 71 damage types (DTs). Damage frequency, number of DTs observed (standardized by the number of leaves in the analysis), and damage composition were analyzed on the bulk floras and individual host species. In general, the number of observed DTs increased as temperature increased through the late Paleocene, peaked in the PETM, decreased during the early Eocene cooling, and then increased again during the warming to the sustained Cenozoic maximum. The strong correlation between the number of DTs observed and mean annual temperature remains when the data are detrended using first differences. There is also a weak, marginally-significant correlation between temperature and damage frequency, and the abundance of highly specialized insect herbivore damage increases with warming. Temperature likely affects insect herbivory by allowing diverse insect populations from lower latitudes to migrate northwards and by influencing insect metabolism and population density. Thus, insect diversity, insect population density, and insect herbivory all increase as warming occurs.