David Tilman


Prix Balzan 2014 écologie des plantes: aspects fondamentaux et appliqués

Biodiversity: Causes, Consequences and Conservation

G. David Tilman is Regents Professor and McKnight Presidential Chair in Ecology at the University of Minnesota and Director of the Cedar Creek Ecosystem Science Reserve. He is also Professor at the University of California-Santa Barbara and Honorary Professor at the China Agricultural University in Beijing. Tilman has spent his career pursuing answers to three major scientific questions related to biodiversity. First, why is life so diverse? Second, how do changes in biodiversity impact the productivity, stability and other ways that ecosystems function? And finally, why and how are human actions leading to the loss of biodiversity, and how might such losses be minimized or prevented?

In trying to answer these questions he has formulated a “universal tradeoff” hypothesis, which suggests that a deep underlying unity of causation explains why the world became so diverse and why biodiversity has such large impacts on how ecosystems function.

The second half of Tilman’s Balzan Prize is being used for a project in which he is working with younger scholars to address three issues related to this “universal tradeoff” hypothesis and its implications:
1) how do evolutionary and ecological processes interact to cause coexistence or competitive displacement, and to determine which species can invade into new regions;
2) why are the effects of biodiversity on ecosystem functioning so unexpectedly large;
3) what are the mechanisms whereby human actions cause species extinctions, the number of species so threatened, and ways to prevent such extinctions.

1. Causes of Biodiversity
Theory predicts that high biodiversity – particularly the long-term coexistence of many competing species – requires that all coexisting species have tradeoffs in their traits. Tilman’s research has pursued the goal of testing the applicability of this universal tradeoff hypothesis using both experimental and observational approaches. For both cases, data gathered on the traits of plant species were used to predict species abundances or species diversity when many different plant species compete with each other. Work currently in progress is expanding such studies to include herbivory and predation along with competition.

2. Why are Biodiversity Effects so Large?
Current mathematical theory predicts that the productivity of a plant community is an increasing function of its plant diversity. However, the observed effects of diversity on productivity in long-term experiments are much greater than theory predicts. Analyses of experimental data suggest that current theory omits a major factor. The initial diversity- dependent increase in productivity has a positive feedback effect on soil fertility, and this increased soil fertility causes productivity to increase through time. Tilman and his researchers are working to modify theory to include this feedback effect.

3. Causes of Extinctions, and Ways to Prevent Extinction
Because of the urgency of global, human-caused extinction risks, much of our efforts have focused on various aspects of this issue. Existing evidence and related mathematical theory on human-caused extinction threats are being synthesized, with the aim of testing existing theories and seeking new theories that can integrate the simultaneous effects of multiple stressors to predict their interactive effects on extinctions. Tilman’s group has found that the greatest single current and future threat to biodiversity is caused by the ways that humans meet their food needs. They have also identified how both expansion of classical conservation methods and how adoption of pro-active conservation policies aimed at preventing further land clearing and halting “bushmeat” hunting could help prevent extinction of the Earth’s remaining large animals.


Burgess, M. G., C. Costello, A. Fredston-Hermann, M. Pinsky, S. Gaines, D. Tilman and S. Polasky. 2017. Range contraction enables harvesting to extinction. Proceeding of the National Academy of Sciences (USA) 114: 3945-3950.
Catford, J.A., M. Bode and D. Tilman. 2018. Introduced species that overcome life history tradeoffs can cause native extinctions. Nature Communications, in press.
Clark, M. and D. Tilman. 2017. Comparative analysis of environmental impacts of agricultural production systems, agricultural input efficiency, and food choice. Environmental Research Ltrs, 12(6).
Clark, M. and D. Tilman. The Diet, Health, and Environment Trilemma. Annual Reviews of Environment and Resources, in press.
Clark, A. T., C. Lehman and D. Tilman. 2018. Tradeoff-Based Coexistence Mechanisms Predict Productivity and Species Abundances in Grassland Plant Communities. Ecology Letters, in press.
Clark, A.T., H. Ye, F. Isbell, E.R. Deyle, J.M. Cowles, D. Tilman and G. Sugihara. 2015. Spatial convergent cross mapping to detect causal relationships from short time-series. Ecology 96: 1174-1181.
Cowles, J. M., P. D. Wragg, A. J. Wright, J. S. Powers and D. Tilman. 2016. Shifting grassland plant community structure drives positive interactive effects of warming and diversity on aboveground net primary productivity. Global Change Biology 22: 741-749.
Hautier, Y., D. Tilman, F. Isbell, E.W. Seabloom, E.T. Borer and P.B. Reich. 2015. Anthropogenic environmental changes impact ecosystem stability via biodiversity. Science 348: 336-340.
Isbell, F., D. Tilman, S. Polasky and M. Loreau. The biodiversity-dependent ecosystem service debt. Ecology Letters. 2015; 18: 119-134.
Marks, C. O., H. C. Muller-Landau and D. Tilman. 2016. Tree diversity, tree height and environmental harshness in eastern and western North America. Ecology Letters 19: 743-751.
Tilman, D. and F. Isbell. 2015. Biodiversity: Recovery as nitrogen declines. Nature 528: 336-337.
Tilman, D. and E. T. Borer. 2015. African mammals, foodwebs, and coexistence.
Proceedings of the National Academy of Sciences 112: 7890-7891.
Tilman, D. and M. Clark. 2015. Food, agriculture, and the environment: can we feed the world and save the Earth? Daedelus 144: 8-23.
Tilman, D., M. Clark, D. Williams, K. Kimmel, S. Polasky and C. Packer. 2017. Future Threats to Biodiversity and Pathways to their Prevention. Nature 546: 73-81.
Tilman, D., N. Hartline and M. Clark. 2017. Saving Biodiversity in the Era of Human- Dominated Ecosystems. In T Lovejoy and L Hannah, Eds, Biodiversity and Climate Change, Yale Univ. Press.

Excerpt from the: The Balzan Prizewinners’ Research Projects: An Overview 2018