Researchers looking at plant communities and insect predation on leaves at both the Cretaceous-Tertiary boundary 65.51 million years ago and 10 million years later at the Paleocene-Eocene boundary, can track the changes in plants and insects through time. The K-T event, which marked the extinction of the dinosaurs and more than 50 percent of all plant species, was caused by the impact of an extraterrestrial object, while the P-E interval was a more gradual change from one climate regime to another caused by a long-term global warming trend.
"The early Eocene 52 million years ago was the warmest the Earth has been in the last 100 million years, and that warming lasted for 2 million years," says Dr. Peter Wilf, assistant professor of geosciences at Penn State. "There is strong evidence for high diversity when temperatures were warm," Wilf told attendees at the annual meeting of the American Association for the Advancement of Science today (Feb. 16) in Denver.
Plants respond to climate change by migrating, evolving and going extinct. However, Wilf notes that due to human activity, global change is occurring at breakneck speed today. Because of the geologically rapid pace of human-induced extinctions, habitat loss and climate changes, land plants currently face a situation more closely resembling the K-T than the P-E boundary.
Both the plant and insect studies used three fossil areas for samples; the K-T was represented by fossil beds in North Dakota, while the P-E was represented by two areas in Wyoming. Reporting on the fossil plant communities were Wilf; Kirk R. Johnson, curator of paleontology, Denver Museum of Nature & Science; and Scott L. Wing, National Museum of Natural History, the Smithsonian Institution. In a subsequent paper, Wilf, Johnson and Conrad C. Labandeira, National Museum of Natural History, the Smithsonian Institution, discussed the role of insects in teasing out climate change influences on ecological communities.
Fossilized leaves show a record of insect predation not unlike what is seen on leaves today. On some leaves, the imprint of piercing and sucking insects is visible. Others show the ragged margins or holey centers of leaves chewed by hole feeding and margin feeding insects. Evidence of mining insects is also preserved, as are galls. The fossil record preserves even the totally skeletonized leaves that show only veins.
By looking at the damage, the researchers can categorize the types of insects that infested these forests 65 and 55 million years ago, and trace the extinction and evolution of species. Labandeira looked at 13.5 thousand leaves across the K-T and identified 51 types of insect feeding damage. The researchers found that the K-T impact was associated with a significant and enduring loss of plant and insect species. Among insects, the most affected were the specialized feeders, those insects that fed on leaves of only one type of plant.
Insects died both from the impact and because the trees that they fed on died. Specialized feeders were less able to adapt and eat off any tree available and so were more greatly effected.
"At the K-T boundary, we see the largest spike of the last appearance of species, both for plants and insects that ate them," says Wilf. "Although climate change was occurring for a long time before the K-T, its effects could not hold a candle to the extinctions brought on by the impact."
Only 21 percent of species made it across the K-T boundary and only 11 species originate in the Paleocene indicating the recovery was not immediate. In fact, diverse vegetation does not return in the area studied until the warm early Eocene 12 million years after the impact.
"The P-E transition was a relatively long, slow change that allowed the plants and insects to adapt to the shifting environment," says Wilf. "At the K-T, species could not adapt in time because the change was so rapid. These rapid changes were much more like what we have today than the gradual ones that occurred at the P-E. Organisms cannot migrate in response to climate changes as they did during the Eocene because of because of freeways and parking lots, and the ongoing loss of habitat imposes severe and geologically sudden stress on ecosystems."