The selection Committee, established by the Committee on the International Prize for Biology and chaired Masamitsu Wada, Professor President , Botanical Society of Japan, distributed a total of 1,885 recommendation forms to Japanese and foreign universities, research centers, academic associations, individual researchers, and international academic organizations involved in this field of biology, and received a total of 38 recommendations in response. As some of these recommendations named the same individuals, the actual number of individuals recommended was 34, from 17 countries throughout the world. The Selection Committee met a total of four times and very carefully reviewed all the candidates. Ultimately, the Committee decided to recommend Dr. George David Tilman of the United States of America to the Prize Committee as the recipient of the 24th International Prize for Biology.

Dr. Tilman has been a major influence in ecology and related fields thanks to the singularly outstanding work he has done, both in terms of theory and in long-term field experiments, on the formation and conservation mechanisms of earth’s biodiversity and the relationship between biodiversity and ecosystem functioning and stability.

In ecology, resource competition was long believed to diminish diversity by not allowing species to coexist. Dr. Tilman has furthered the theoretical understanding of competition by determining that species coexistence can be explained if a tradeoff in the use of resources is involved, and that such tradeoffs play an important role in the organization of ecosystems, which are based upon relationships among multiple species.

Further, by amassing data from long-term grassland experiments and adopting a synthetic theoretical approach, he has effectively settled the controversy among ecologists over the relationship between diversity and stability. He demonstrated, in fact, that while diversity causes the populations of individual species to fluctuate over time, it also acts to stabilize the functioning of the ecosystem as a whole. One has only to look at the frequency with which Dr. Tilman’s publications are cited to appreciate the enormous influence that this work has had in ecology and related fields.

In a recent series of papers founded on ecological theory and his own research, Dr. Tilman has offered many thought-provoking insights into sustainable agriculture and biofuel production, accompanied by clearly presented information. With humanity facing an ever-deepening environmental crisis, Dr. Tilman’s research achievements have contributed greatly to the renewal and development of ecology and related disciplines.

The award ceremony was convened on Monday, December 8, 2008, at the Japan Academy (7-32 Ueno Koen, Taito-ku, Tokyo). Their Majesties the Emperor and Empress attended the ceremony and a party in honor of the award recipient. To commemorate the award to Dr. George David Tilman, the 24rd International Prize for Biology Commemorative Symposium on Ecology took place on December 10 & 11, 2008, at the Sendai International Center. At the symposium, Dr. Tilman gave a lecture to attendances on his latest research accomplishment.

1) Theoretical and Experimental Work on Biodiversity and Ecosystem Stability

Competition is an ecological process that greatly influences the structure and functioning of biotic communities. Ecologists long considered competition to be an inhibitor of species coexistence, as more competitive species eliminated weaker competitors. In reality, however, communities harbor many different species in mutual competition. Thus, the search for the principle that enables competing species to coexist poses one of the key challenges in ecology.

The resource-based competition theory developed by Dr. Tilman early in his career (Tilman 1982) focuses on the minimum resource requirements of each species as the factor that determines competitive success or failure, and the validity of this approach has been demonstrated by experimental work on algae and grasslands. A likely mechanism by which competition could lead to diversity is that of the tradeoff, in which a species that is superior in one trait is inferior in another; Dr. Tilman has shown theoretically that tradeoffs between resource utilization and dispersal ability, for example, lead to species coexistence (Tilman 1994). His theory predicts that at higher plant diversity, more efficient utilization of limiting resources will lead to higher biomass production, due in part to the higher probability that a more productive species will be present in a more diverse plot (the sampling effect), and in part to complementary exploitation of different growing conditions by different species (the niche differentiation effect) (Tilman 1999). Further, he has predicted theoretically that high diversity, coupled with a low supply of a limiting resource, should reduce the success of invasions by new species.

These predictions have been validated by decade-long experiments. In well over a hundred grassland plots where he manipulated species diversity, Dr. Tilman found that productivity increased with plant species diversity, and that soil inorganic nitrogen, the main limiting nutrient, was utilized more completely in the more species-diverse plots (Tilman et al. 1996). Further, even in adjacent natural grassland, plant productivity and the availability of soil inorganic nitrogen were found to increase with increasing species diversity.

The relationship between diversity and stability had become the most intensely debated issue in ecology: on the one hand, the widely supported “diversity-stability theory” held that diversity contributes to stability; the counterargument that diversity begets instability, championed by Dr. Robert May, was also very influential. Through long-term research using 207 grasslands plots, Dr. Tilman showed that while diversity stabilizes the functioning of communities and ecosystems, it destabilizes the population dynamics of individual species (Tilman 1996). At higher diversity, annual fluctuations of above-ground biomass in the plant community as a whole decreased (i.e., temporal stability increased), even in the presence of drought conditions, yet the more diverse the community, the more variable the abundance of each constituent species became. This difference between the stability of the ecosystem as a whole and that of individual populations can be explained theoretically by interspecies competition. If a dominant competitor becomes less abundant due to drought, more drought-resistant latent competitors will gain a competitive advantage and their abundance will increase, with the net result that community biomass remains the same. This complementary oscillation of species leads to stability of the community as a whole. These results agree with Dr. May’s predictions, in that diversity does reduce population stability; at the same time, they support the diversity-stability hypothesis on a community- and ecosystem-wide scale. In this way, Dr. Tilman settled the hottest debate in ecology by adopting a more synthetic theoretical approach and supplying experimental proof (Tilman 1999). Further, the stability that species diversity brings to an ecosystem is observable not only in terms of temporal variability but also in the drought resistance and resilience of biomass production; the dependence of these properties on species diversity has been approximated to a saturation curve (Tilman et al. 1994). In other words, when there are fewer species present, the loss of a single species has a greater impact. With biodiversity currently in rapid decline on both the global and the local scale, these findings sound an alarm for our times.

As a more general theory of diversity in biotic communities, Dr. Tilman developed the stochastic niche model and tested its validity in long-term grassland experiments (Tilman 2004). The stochastic niche theory unites the neutrality theory (which is concerned with the meaning of community diversity) with the classic niche theory based on tradeoffs, and builds on them both. It does this by addressing the fact that the survival of newly invading disseminules is affected by resource availability as well as stochastic processes. Long-term experiments have shown that the establishment of new species is most strongly inhibited by the presence of established species belonging to the same one of four functional groups, and these results are in good agreement with predictions based on the model that incorporates niche effects.


  Under the combined impact of fossil fuel combustion and the use of agricultural fertilizers, the rate of addition of biologically-active nitrogen to the atmosphere has already reached two to seven times the preindustrial level. Very little research had been done on the effects of this chronic nitrogen surplus on plants, but in a joint study with Dr. Christopher Clark (Clark and Tilman 2008), Dr. Tilman used grassland plots to test the effects of an input of 10 kilograms of nitrogen per hectare per year, which is somewhat higher than the ambient background level of atmospheric nitrogen deposition (about 6 kg/ha/year) and about the same as the amounts of nitrogen deposited in many industrialized countries today. The study showed that at chronically elevated nitrogen levels, plant species diversity decreases sharply due to the loss of rare species. Also, an analysis of the relationship between plant species diversity and disease in grassland plots (Mitchell, Tilman and Groth 2002) showed that the pathogen load of fungal disease was three times higher in the average monoculture than in the average plot planted with 24 grassland species, and that nearly all the diseases increased in severity in the less species-rich plots. Disease severity was positively correlated with host plant abundance, and disease was especially severe in plots that contained no resistant species. This analysis supports the general theory that low species diversity leads to more disease as the abundance of susceptible plants increases.

In an overview of grassland experiments in which he grew perennial grasses for a decade, Dr. Tilman reported large annual oscillations in species abundance and biomass production due to fluctuations in climatic conditions during the growing period, but found that, over time, the more species-rich plots showed greater stability in their annual above-ground productivity, and that this stability increased as the vegetation in the plots matured (Tilman 2006). These experiments revealed that ecosystem stability depends on the quantity of roots maintained from year to year, thus bearing out the theoretically predicted portfolio effect (also known as the statistical averaging effect) and the yield effect.

This series of theoretical and experimental studies suggests that biodiversity may enhance the production and utilization of food (including livestock feed), biofuels, and ecosystem services.

2) Research on Sustainable Agriculture and Biofuel Production

Seeking ways to reduce the heavy environmental burden imposed by modern agriculture, Dr. Tilman has been exploring the implications of converting to sustainable farming practices that respect ecological principles, such as the diversity effects he himself has identified (Tilman 1998). With a large group of coauthors, he forecast the environmental impact of the expansion of cultivated area 50 years into the future, assuming that existing agricultural practices remain unchanged (Tilman et al. 2001). The predicted effects include the conversion of 109 hectares of natural ecosystems to agriculture, accompanied by a 2.4- to 2.7-fold increase in nitrogen- and phosphorus-driven eutrophication of terrestrial, freshwater, and near-shore ecosystems, and comparable increases in the use of water and pesticides. This eutrophication and habitat destruction would cause unprecedented ecosystem simplification, loss of ecosystem services, and species extinctions. The study concluded that significant scientific investigation and regulatory, technological, and policy changes are needed in order to avoid the negative impacts of agricultural expansion by converting to sustainable practices. Dr. Tilman has further highlighted the urgent need for new incentives and policies to ensure the sustainability of agriculture and ecosystem services (Tilman 2002).

Recently, Dr. Tilman has been vigorously pursuing joint research on the environmental assessment of biofuels, whose relative merits are a controversial theme. Whether biofuels offer carbon savings depends to a large extent on how they are produced (Hill et al. 2006), and Dr. Tilman and his coauthors have quantified the “carbon debt” created in the process of converting rainforests, peatlands, savannas or grasslands to produce food crop-based biofuels. This process releases 17 to 420 times more carbon dioxide than the annual greenhouse gas reductions that the biofuels would permit by displacing fossil fuels (Fargione et al. 2008). In contrast, biofuels made from waste biomass or from biomass grown on degraded and abandoned agricultural lands planted with perennials incur little or no carbon debt and can offer immediate and sustained mitigation effects. In particular, biofuels derived from low-input high-diversity (LIHD) mixtures of native grassland perennials (which may be grown on agriculturally degraded lands) can provide more usable energy, greater greenhouse gas reductions, and less agrichemical pollution than food crop-based biofuels (Tilman et al. 2006). LIHD biofuels are considered “carbon negative” because the amount of carbon accumulated in the roots and soil (net carbon sequestration) greatly exceeds fossil carbon dioxide release during the biofuel production. Moreover, they can be produced on agriculturally degraded lands, and thus have the advantage that they need neither displace food production nor cause loss of biodiversity via habitat destruction. Through these biofuel-related proposals, Dr. Tilman has maximized the effectiveness of his long-term experimental work on grasslands and his theoretical work on diversity.

Thus, through his theoretical work and its testing in long-term experiments, Dr. Tilman has not only made major contributions to the basic research on biodiversity and on ecosystem functioning and stability; he has also endeavored to provide the foundation of scientific knowledge that society needs if we are to make appropriate choices in the areas of sustainable agricultural production and carbon mitigation, which are pressing issues in the current environmental crisis. By availing himself of the full range of current ecological knowledge and analytical techniques, and by undertaking an array of multidisciplinary joint research projects, Dr. Tilman has actively put forward policy proposals whose broad perspective rises above the confines of the individual disciplines. Dr. Tilman’s energetic career as a pioneer of holistic research in an area of crucial importance to society makes him, in the truest sense, a leading player in contemporary ecology.

1. Clark, C.M. and D. Tilman. 2008. Loss of plant species after chronic low-level nitrogen deposition to prairie grasslands. Nature 451:712-715. Fargione, J., J. Hill, D. Tilman, S. Polasky and P. Hawthorne. 2008. Land clearing and the biofuel carbon debt. Science 319: 1235-1238.
2. Hill, J., E. Nelson, D. Tilman, S. Polasky and D. Tiffany. 2006. Environmental, economic, and energetic costs and benefits of biodiesel and ethanol biofuels. Proceedings of the National Academy of Sciences of the United Sates of America 103:11206-11210.
3. Reich P.B., S.E. Hobbie, T. Lee, D.S. Ellsworth, J.B. West, D. Tilman, J.M.H. Knops, S. Naeem and J. Trost. 2006. Nitrogen limitation constrains sustainability of ecosystem response to CO2. Nature 440:922-925.
4. Tilman, D., J. Hill and C. Lehman. 2006. Carbon-negative biofuels from low-input high-diversity grassland biomass. Science 314:1598-1600.
5. Tilman, D., P.B. Reich and J.M.H. Knops. 2006. Biodiversity and ecosystem stability in a decade-long grassland experiment. Nature 441:629-632.
6. Tilman, D. 2004. Niche tradeoffs, neutrality, and community structure: A stochastic theory of resource competition, invasion, and community assembly. Proceedings of the National Academy of Sciences of the United Sates of America 101:10854-10861.
7. Kennedy, T.A., S. Naeem, K.M. Howe, J.M.H. Knops, D. Tilman and P. Reich. 2002. Biodiversity as a barrier to ecological invasion. Nature 417:636-638.
8. Mitchell, C., D. Tilman and J.V. Groth. 2002. Effects of grassland plant species diversity, abundance, and composition on foliar fungal disease. Ecology 83:1713-1726.
9. Tilman, D., K.G. Cassman, P.A. Matson, R. Naylor and S. Polasky. 2002. Agricultural sustainability and intensive production practices. Nature 418:671-677.
10. Tilman, D., J. Fargione, B. Wolff, C. D'Antonio, A. Dobson, R. Howarth, D. Schindler, W. Schlesinger, D. Simberloff, and D. Swackhamer. 2001. Forecasting agriculturally driven global environmental change. Science 292:281-284.
11. Tilman, D., P.B. Reich, J. Knops, D. Wedin, T. Mielke and C. Lehman. 2001. Diversity and productivity in a long-term grassland experiment. Science 294:843-845.
12. Tilman, D. 2000. Causes, consequences and ethics of biodiversity. Nature 405:208-211.
13. Tilman, D. 1999. The ecological consequences of changes in biodiversity: a search for general principles. The Robert H. MacArthur Award Lecture. Ecology 80:1455-1474.
14. Tilman, D. 1998. The greening of the green revolution. Nature 396:211-212.
15. Cohen, J.E. and D. Tilman. 1996. Biosphere 2 and biodiversity: the lessons so far. Science 274:1150-1151.
16. Siemann, E., D. Tilman and J. Haarstad. 1996. Insect species diversity, abundance and body size relationships. Nature 380:704-706.
17. Tilman, D. 1996. Biodiversity: Population versus ecosystem stability. Ecology 77:350-363.
18. Tilman, D., D. Wedin and J. Knops. 1996. Productivity and sustainability influenced by biodiversity in grassland ecosystems. Nature 379:718-720.
19. Wedin, D.A. and D. Tilman. 1996. Influence of nitrogen loading and species composition on the carbon balance of grasslands. Science 274:1720-1723.
20. Tilman, D. and J.A. Downing. 1994. Biodiversity and stability in grasslands. Nature 367:363-365.
21. Tilman, D., R.M. May, C.L. Lehman and M.A. Nowak. 1994. Habitat destruction and the extinction debt. Nature 371:65-66.