The coming century could be the age of the big crash, he says. One-third of the world's estimated 300,000 species of plants could vanish by 2050, and two-thirds by 2100 â€“ the worst mass extinction in the last 65 million years, and the first caused by people.
"We are driving an extinction event that is like the extinction event that occurred at the end of the Cretaceous Period," Dr. Raven said during a stop in Fort Worth last week.
Dr. Raven, director of the Missouri Botanical Garden in St. Louis, is globe-trotting as an adviser to conservation groups, national governments and other scientists. He was the kickoff speaker in the Botanical Research Institute of Texas' fall lecture series.
The series, which is free and open to the public, focuses on what scientists are calling the "sixth extinction" â€“ the sudden and massive loss of species that some researchers have predicted will become apparent during the 21st century.
Scientists still debate the causes of the five mass extinctions that have occurred during Earth's geological past. But it's certain that they happened because of natural phenomena, such as the meteor impact that many paleontologists think led to the end of the dinosaurs.
This time, researchers foresee a massive die-off because of habitat loss and other imbalances caused by humans.
"The numbers of people, the expectations of consumption by those people, the amounts of natural material that we consume ... are nothing like anything that has existed in the past," Dr. Raven said.
The idea that a mass extinction is under way has won acceptance among many scientists who monitor the planet's plant life.
Last year in St. Louis, thousands of botanists from 85 countries gathered for the International Botanical Congress, which is conducted every six years. They voiced concerns about the future of the world's plant life and called for urgent international and local action.
Thousands of plants are becoming extinct faster than scientists can catalog them, they said.
Dr. Raven, who chaired the congress, said the coming age of huge biological advances to benefit people is being undercut by the loss of the Earth's living raw materials.
"We know that hundreds of [species of] plants and animals are disappearing for every new one that's being produced," Dr. Raven said.
Conservation strategies that might work elsewhere â€“ relying largely on big tracts of public land, for example â€“ won't work in Texas, Dr. Raven said. Texas has only about 3 percent of its land in public ownership.
"You've got to find ways that will address the remaining 97 percent of the land," he said.
North Texas is the site of a sprawling metropolis with about 5 million people, but 200 years ago it was prairie. The blackland prairie, named for its rich black soil, ran through Central Texas, straight through the present-day site of Dallas.
The original 12 million acres of blackland prairie is down to about 5,000 â€“ making it one of the nation's most endangered ecosystems. It's a small reminder, Dr. Raven said, of how quickly natural values can vanish.
"Ten thousand years ago, the population of the entire Earth was less than the estimated population of this metropolitan area at the present time," he said.
That local growth echoes a worldwide population that has leaped from 2.5 billion in 1950 to 6 billion today.
Those 50 years, Dr. Raven said, have seen the loss of a quarter of the world's topsoil, a fifth of its cropland, one-third of its forests â€“ and a one-sixth increase in the concentration of carbon dioxide, blamed for global warming.
"We are not living off the interest; we're living off the principal," he said. "We're using what the Earth has â€“ on which we base our life-support systems â€“ faster than it can be replenished."
But the situation isn't hopeless, he added. The same brains that led to modern technology, Dr. Raven said, can lead to a sustainable way of life.
"We are supposed to be intelligent," he said. "We can do something about it."