I have had two major intellectual loves in life, mathematics and biology, and I’ve always had a personal love of the out-of-doors. I found in ecology a career in which I could combine all three of those. If you look over the list of papers that have been frequently cited, several are theoretical and mathematical treatments of ecological issues, others are experimental treatments. I have been very fortunate to stumble into a career where someone with my way of thinking, likes, and loves has been able to have an impact.

It was an experiment that we began in 1982, which is still continuing. Several serendipitous events really led to that paper coming into being. One was a major drought in 1988, a disturbance that had a big impact on the ecosystem we were studying—the grasslands. The second was that I was invited to a conference in Germany in 1992, where the issue was raised as to whether or not diversity could affect how ecosystems function. That led me to wonder what the impact of the drought had been, and we had a rich data set that allowed us to very thoroughly explore how the drought had affected the stability of these grassland systems. The final serendipitous factor was that John Downing was in Minnesota with me for a year—he is the second author on that paper—and he had skills in analyzing these large data sets that I didn’t have. Together, he and I were able to analyze this data set and explore the hypothesis that there might be an effect of diversity on stability. In doing so, we found strong support for the hypothesis and were able to reject many alternative hypotheses.

Why do you think the paper had such an impact?

It and another paper, written by Shahid Naeem and his colleagues (S. Naeem, L.J. Thompson, S.P. Lawler, J.H. Lawton, R.M. Woodfin, "Declining biodiversity can alter the performance of ecosystems," Nature 368[6473]: 734-7, 21 April 1994) raised an issue that had been considered resolved in ecology and that challenged a major paradigm of the discipline. It raised the possibility that diversity—the number of species in an ecosystem—might greatly influence how that ecosystem functions. This was an old idea in ecology that had been rejected in the 1970s. Our paper and Naeem’s, which both came out in 1994, revisited that issue with large data sets and really forced our discipline to start re-exploring that issue. It has since been examined very thoroughly, both experimentally and theoretically. So those papers led to what is probably one of the busier areas of research in the discipline and one of the biggest areas of controversy. When a 20-year paradigm is challenged, it leads to a host of new questions.

What was the basis of misunderstanding in the old paradigm?

It turns out that, as in most of life, there wasn’t really a mistake, just an incomplete understanding. There was some wonderful math done by Sir Robert May, a fabulous theoretical ecologist, which correctly showed that higher diversity tended to destabilize individual species. Thus, a species living in an ecosystem with high diversity would be less stable than if it lived in a lower diversity ecosystem. That is exactly what our recent field work showed. The problem was that many ecologists assumed in the 1970s that, if each individual species was less stable at higher diversity, then the sum of these species, the abundance of the whole ecosystem, would also be less stable. As Robert May had mentioned, that need not be the case. Our field work showed that the stabilizing effect of diversity on the whole ecosystem was much greater than the destabilizing effect of diversity on individual species, which was a surprise.

Your 1994 Ecology paper, "Competition and Biodiversity in Spatially Structured Habitats"(Ecology 75[1]: 2-16, January 1994), has been cited almost as many times as the Nature paper. Tell us about that research and why you think it’s had such an impact.

I was surprised by how frequently that article was cited, if for no other reason than Ecology is not as visible a journal as Nature or Science. That paper deals with the other side of diversity. It’s an attempt to understand why so many species can coexist in a given ecosystem. This has been one of the mysteries of the discipline since the time of Darwin. Darwin and his contemporaries were amazed at how many species there were, not just in the world, but in a single forest or a single prairie. A century later, our models still didn’t predict so much diversity. This was one of the mysteries that attracted me to ecology in the first place. I spent much of my career dealing with the question of why there are so many species. This paper probably constitutes one of my best insights into what might explain the high diversity of nature. It mathematically showed how a large number of species could coexist in very simple systems if there was a tradeoff between how they could compete and how they could disperse: if the best competitors were the poorest dispersers and vice versa. The theory was inspired by our experimental work that had increasingly shown this tradeoff was a major way in which coexisting species differed. It led me to explore that possibility mathematically. I was amazed at the prediction of this relatively simple model, which effectively showed no limit to local diversity. That tradeoff could, in theory, allow an unlimited number of species to coexist even if they were competing for one single factor, the same limiting nutrient.

How would you describe the evolution of your discipline? Where is it leading us and the researchers in it?

One major theme is an increased emphasis on global patterns. With human domination of global ecosystems now evident to all, it’s equally evident in our discipline that we need to better understand the underlying causes of global patterns if we’re going to better understand what impact humans will have in the long term, and how those changes feed back on human society. This is happening at the same time as another major trend in the discipline, the increasing integration of mathematical theory with experiments and long-term observations. In the last 25 years, ecology has changed from being mainly descriptive natural history into a discipline that, to me, looks a lot like chemistry and physics. We now have a rich integration of observation, theory, and experiment. Frankly, that integration didn’t come at all too soon. These are exactly the kinds of approaches that are needed to address major questions about the impact that humans are very rapidly imposing on the globe.

Are much of these advances technologically inspired?

Certainly when it comes to gathering and handling large data sets, technology has been absolutely essential. Many analyses that are routine now could never have been done 30 years ago. There weren’t the software packages or computers to do so. Just handling the amounts of data we routinely gather would have been impossible. At our research site, about 80 people gather data each summer as part of various teams of undergrads, grad students, post-docs, and faculty. All this would be logistically impossible without the computer technology. In addition, computers now allow us to do simulation studies on models that you could never solve analytically.
     Moreover, our discipline has gone from a tradition of individual scientists doing their own personal research by themselves to one of large teams, intellectually diverse, attacking much larger questions. This award really honors of all the people with whom I’ve worked. Team science is what is now most productive in our discipline.

What are you working on now?

We are still deeply involved in large-scale experiments looking at the effects of biological diversity. We are continually being surprised by the large number of ways diversity affects the function of ecosystems. Another major project is one in which we are trying to predict where the ecosystems of the world are heading because of human impacts. We’re looking at what the world might be like in 20 or 50 years if the current trends continue. And we’re asking, what are the implications of these changes on human society? At same time, we’re trying to better understand where the holes are in our underlying scientific knowledge.

What was the most difficult professional moment in the course of your career?

I would say the challenges and the difficult moments have all come from disagreements over concepts and interpretation of results.

It sounds like you’re talking about the 1994 Nature paper again. Did that ignite such disagreements?

Yes, as has virtually every other paper since. There has been a long string of disagreements. Some of those, in retrospect, were mainly semantic. Some were philosophical, about how one might validly interpret results. Others were deeply philosophical, only to be addressed by more research into what the underlying relationships between diversity and stability might be. These disagreements are stimulating and fun but disconcerting, as well.

What achievement has brought you the most professional satisfaction?

I would say it’s our work on diversity and stability, the work that challenged the existing paradigm. It was satisfying because it was a surprising result to us, and because after the result appeared, it led to an incredible richness of new ideas that we’ve been able to explore.

If you could solve one big question in the next ten years, what would it be?

I would love to finally understand why the world is so richly endowed with biological diversity. That’s the question that motivated my career. I’d love to know the answer.

Dr. G. David Tilman
University of Minnesota
Department of Ecology, Evolution, and Behavior
St. Paul, MN, USA