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science, health & technology

Leaves leave more questions than answers about post-destruction biodiversity

by Eric Jackson

The popular culture has yet to dispense with the public demand for omniscient gods and wizards, even as it would embrace the advance of science and technology. Thus we get the personality cults of a few media-hyped scientific superstars, and the common notion of scientists as better-educated and less comical versions of Bullwinkle the Moose’s “Mr. Know-it-all” persona. But modesty about what one actually knows and skepticism about what is suspected are the true hallmarks of a scientific attitude. Science is about asking good questions, sometimes about things to which it’s believed that there are already correct answers, devising ways to put these things to the test, then honestly recording and reporting the results whether or not they match the scientist’s starting hunches.

Peter Wilf is a paleobotanist at Penn State, a man with eclectic scientific interests. In his first visit to Panama and the Smithsonian Tropical Research Institute he spoke to a crowd mostly composed of fellow scientists from diverse fields about his research into the fossil record about plant and insect diversity before and after the mass extinction that marked the boundary between the Cretaceous and Paleocene epochs.

The end of the Cretaceous coincided with the end of the dinosaurs and most of their contemporary plants and animals. It is widely believed that a comet or meteorite smashed into the Earth near what is today the Yucatan Peninsula in Mexico, causing massive earthquakes, floods, firestorms and environmental changes to which only a few species were able to successfully adapt.

It turns out that for those who want to look at the geological boundary marked by this cataclysm, North Dakota is the place to go. Especially in the Hell Creek Badlands, this is the place where what is called the K-P Boundary that includes the detritus of the collision and the “before” and “after” sedimentary layers are found in exposed rocks. There you can dig up the fossil records of what lived at the end of the Cretaceous and of the terrestrial fauna and flora of the Paleocene that followed. (The world’s other exposed K-P Boundary sites were all underwater at the time of the event, so are mainly useful for the study of changes in the marine biology.)

The fossil evidence he examined, Wilf said, is “maybe alarming.” “It raises a lot more questions that it answers.”

Wilf, a prolific collector of fossil leaves himself, went to a colleague with a truly vast collection with a proposal. Why not analyze, tabulate and create a database out of this collection of more than 20,000 specimens taken from before and after the event that ended the Cretaceous? The collaboration was agreed to, a methodology to exclude doubtful identifications was devised and cross-references were made in the database.

“There was a major plant extinction in North Dakota,” Wilf concluded. Of the plant species identified there at the end of the Cretaceous, 29 survived and 130 did not. The massive plant extinction trend is corroborated by extinctions noted worldwide in fossil pollens, he noted.

So what did that do to the interactions between insects and plants? “We really don’t have anywhere to go” for the most direct evidence of insect populations, Wilf said, pointing out that there are no known places where insects are found preserved in amber that straddles the K-P Boundary.

However, there are fossil leaves with evidence of insect damage on them --- they are chewed, there are galls in them, they have been mined, or there are the scars that come from piercing to suck out plant juices. That lets Wilf and his colleagues include insect damage to fossil leaves in their database and ask the question: “Is there any change in the insect damage across the K-P Boundary?”

In a few cases, both the plants and the insects that ate them survived --- a species of ginger, and a species of beetle with a taste for that spice fare, for example.

However, the specialized plant - insect relationships that existed below the K-P Boundary tended to disappear. The record of fossil leaf insect damage, then, becomes “the only evidence of a major insect extinction at the end of the Cretaceous,” a record that indicates “a skewed drop in insect feeding right after the dinosaur extinction.”

So if the fossil record of the immediate aftermath of a planetary catastrophe looks fairly clear, what do the leaves preserved in stone say about the recovery process? If one were to project modern trends back to the Paleocene, it would be expected that insect herbivore diversity goes up or down with plant diversity.

But alas, said Wilf, “that’s not what we found.”

Wilf’s search for the recovery of specialized relationships among plants and the insects that eat them as the Paleocene progressed has shifted from the North Dakota Badlands to Mexican Hat, Montana and Castle Rock, Colorado.

At the Montana site, there wasn’t much plant diversity in the Paleocene layers --- 91 percent of plant leaf samples were from just four species and only 16 species of dicots were identified. Castle Rock, on the Rocky Mountain side of the Denver Basin, however, was the site of something like a rainforest back in the Paleocene --- many of the leaves reflected the high rainfall with their large leaves and drip tips, some of the tree trunks were rainforest-huge, and there was plant diversity typical of what one might find in a rainforest. According to what had been previously known, Wilf said, there was “much too much diversity for the Paleocene.”

Based on the way things are in today’s rainforests, then, certain expectations arose about the insect predation record that would be found in the fossil leaves from Castle Rock. “We thought it would be absolutely packed,” Wilf noted.

Not so. There was hardly any insect feeding in the fossil record at Castle Rock. Meanwhile among the much less diverse flora of Mexican Hat, there was a much higher incidence of leaves with insect damage and a much larger number of insect species left their special marks in the fossil record. It was “an inversion of expectations,” according to Wilf.

So why did insect feeding come back before tree diversity at Mexican Hat, while the situation was reversed at Castle Rock? It raises a host of questions and suggests many hypotheses, and calls for more research at other Paleocene sites to gather more evidence against which these local discrepancies might be put into context.

What we learn about the recovery of biodiversity after the dinosaur extinction may give us further insights into the ways that evolution works, and may even result in some better reforestation techniques. Or maybe not. A lot of times it’s very hard to predict where scientific research might lead.