Warmer climate spurred ancient plant pests.

In one of the longest-running wars on Earth, plants and insects have been battling each other for more than 300 million years. The fiercest skirmishes play out in the tropics, where hordes of hungry pests attack vegetation protected by multiple defensive weapons. A pair of paleontologists has now used ancient leaf fossils to decipher what it is about the tropics that brings out the worst in insects and plants.

Ecologists have long recognized that life is most exuberant near the equator, whereas other regions have fewer species. With the increase in tropical species come enhanced opportunities for insects to munch various plants. Researchers have wondered whether temperature, light, topography, or other factors can explain the latitudinal differences in insect herbivory.

The rocks of western Wyoming record a natural experiment capable of answering that question, report Peter Wilf and Conrad C. Labandeira of the Smithsonian Institution's National Museum of Natural History in Washington, D.C. From these rock formations, the two researchers excavated two sets of leaf fossils. The older group dates from the Paleocene epoch, 56 million years ago, when global temperatures were rising. The younger group hails from the Eocene epoch, 53 million years ago, during which temperatures reached a peak.

Comparing the damage patterns found on the fossil specimens, the researchers determined that insects took a larger bite out of plants in the Eocene. In that epoch, "plants were being more intensely attacked, and there were more types of things doing the attacking," says Labandeira. He and Wilf describe their results in the June 25 SCIENCE.

Most studies of modern plant herbivory make comparisons among sites at various latitudes, but the fossil study examines two different populations from the same latitude and the same type of floodplain environment. The prime difference between the samples is temperature, say the researchers. The later time was 7 [degrees] C warmer than the earlier.

In their recent work, the two scientists have extended the study by looking at fossils postdating the peak Eocene warmth. As temperatures dropped, so did the frequency of insect damage to leaves, says Labandeira.

The researchers caution against applying the results of their study to current climatic concerns. "This is not a prediction about what will happen in the next 100 years as temperatures go up," says Wilf. The Paleocene and Eocene warming evolved over millions of years, much slower than the one occurring today.

"Although Wilf and Labandeira's study may not be able to directly predict future changes in plant-herbivore interactions, it goes a long way toward explaining present and past communities," says Phyllis D. Coley of the University of Utah in Salt Lake City, who wrote a commentary in the same issue of SCIENCE. "Wilf and Labandeira," she says, "push the fossil evidence farther than ever before by quantitatively testing ecological hypotheses."