UC Santa Barbara Ecologist Awarded National Medal of Sciences for Showing Diversity Matters

Dr. David Tilman Receives U.S.’s Highest Scientific Honor for His Discoveries About Importance of Earth’s Biodiversity

Arati Prabhakar, PhD, Director of the White House Office of Science and Technology Policy (OSTP), awards G. David Tilman the National Medal of Science during an awards ceremony at the Eisenhower Executive Office Building in Washington, D.C., January 3, 2025. | Credit: Ryan K. Morris and the National Science & Technology Medals Foundation.

UC Santa Barbara professor Dr. David Tilman has spent a lifetime learning that, above all, Earth’s biodiversity is vital. But when he first presented the idea, his fellow ecologists were not convinced.

Fast-forward 30 years, and Tilman was one of the Biden administration’s recipients of the 2025 National Medal of Science — America’s highest scientific honor — for his work. “It’s special,” said the professor of 49 years. “It’s my government telling me they appreciate how I spent my life.”

Through his research, Tilman discovered that the number of species in an ecosystem is “the single most important factor” for the health of that ecosystem. This idea intrigued him, he said, more so than the study of a singular plant or animal species, like others in his field.

Tilman posed an important question: Why has the Earth become so diverse?

To find an answer, he studied grasses in America’s prairies to learn more about the effects of competition. When the plants competed with each other, they became more specialized as a kind of “trade off.” Specializations allow plants to coexist, even as they compete, he explained.

He compared an ecosystem to an economic system in this way, with many professions filled by individuals who have honed their crafts. This diversity, he said, makes a system more productive and stable.

He first presented these important findings back in 1994. However, most ecologists at the time didn’t believe diversity mattered at all. If anything, they believed it made an ecosystem more unstable. If plumbers suddenly disappeared from Santa Barbara, for example, the consequences would be devastating. Lawyers couldn’t suddenly step in to fill the void. No amount of law school could teach you how to fix a leak. In this way, specializations wouldn’t allow for essential overlap to bridge any potential gaps in the environment, ecologists thought.

But ecological diversity isn’t so simple. Diversity in nature leads to “functional overlap,” allowing organisms to take over certain parts of other species’ roles if that species disappears, Tilman said. (No doubt some lawyers know how to fix a pipe.)

In his grassland studies, for instance, Tilman found that plots with more species were more resilient to drought than those that had fewer.

A loss of diversity, on the other hand, leads to ecosystems less able to provide society with goods and services — like clean water, air, and habitat. Right now, we are in the middle of a historic loss of this precious diversity, with humanity as the driver. Enterprises like agriculture and the burning of fossil fuels have led to habitat degradation and Earth’s sixth mass extinction, known as the anthropocene.

Tilman, in response, has turned his attention toward how human impacts on nature come back to affect us, and how biodiversity can be a part of the solution. His work includes research on crop diversification, and both healthy and sustainable changes to the modern diet, according to UCSB’s The Current.

After coming to Santa Barbara as a visiting professor, Tilman was drawn to the UCSB Bren School of Environmental Science and Management. “Unlike a traditional ecology program,” he said, “the Bren School is dedicated to solving major environmental problems, focusing on long-term problems for the benefit of humanity.”

Tilman now teaches at UCSB every winter quarter. Otherwise, he works in his role as Regents’ Professor at the University of Minnesota’s College of Biological Sciences.

“When I look at my students, tackling these big issues, I’m thinking, ‘Wow, what a great group of scholars,’” he smiled.

While reflecting on teaching the next generation of ecologists, he said he never imagined that there would be a “rapid” societal response to environmental problems.

Our total impact is now 100 times worse than it was 100 years ago, he said. Rules and regulations have not kept pace with that impact.

“Every nation has to be involved in the climate solution, like every nation has to be involved in preventing extinction,” he said. “If we keep doing what we’re doing, what kind of world are we creating? We are the dominant force on Earth — we need to ask ourselves these questions.”

However, Tilman said he is optimistic about the future. He’s “discouraged we’re causing problems that don’t need to be here,” but we will “have to turn the corner” and address our actions eventually. “I’m hoping sooner rather than later, because it’ll save a lot of hardship and pain,” he continued, giving the example of the recent L.A. fires. “And personally, I don’t think these issues have anything to do with a political party. It has to do with logic.”

Tilman, 76, said he is going to continue to look for workable solutions to these problems. He’s still doing research and writing papers. He has the “health and the enthusiasm” to keep going.

“Right now, I can’t imagine why I would retire. I haven’t saved the world yet,” he chuckled.