Introduction
This experiment (often called the "Big" Biodiversity Experiment; the "small" experiment is no longer maintained) determines effects of plant species numbers and functional traits on community and ecosystem dynamics and functioning. It manipulates the number of plant species in 168 plots, each 9 m x 9 m, by imposing plant species numbers of 1, 2, 4, 8, or 16 perennial grassland species. The species planted in a plot were randomly chosen from a pool of 18 species (4 species, each, of C4 grasses, C3 grasses, legumes, non-legume forbs; 2 species of woody plants). Its high replication (about 35 plots at each level of diversity) and large plots allow observation of responses of herbivorous, parasitoid and predator insects and allow additional treatments to be nested within plots. Planted in 1994, it has been annually sampled since 1996 for plant aboveground biomass and plant species abundances and for insect diversity and species abundances. Root mass, soil nitrate, light interception, biomass of invading plant species, and C and N levels in soils, roots, and aboveground biomass have been determined periodically. In addition, soil microbial processes and abundances of mycorrhizal fungi, soil bacteria and other fungi, N mineralization rates, patterns of N uptake by various species, and invading plant species, have been periodically measured in subprojects in the Biodiversity Experiment.
Key Results
Plant biomass production increased with diversity (Fig 1) because of complementary interactions among species and not because of selection (sampling) effects (Figs 2 Tilman et al. 2001b, Pacala and Tilman 2002, Hille Ris Lambers et al. 2004; Fargione et al. in prep.).
Foliar fungal disease incidence decreased at higher diversity because of greater distance between individuals of a species, and resultant lower rates of disease spread (Mitchell et al. 2002).
Greater plant diversity led to greater diversity of herbivorous insects, and this effect continued up the food web to predator and parasitoid insects (Haddad et al. 2001).
Fewer novel plant species invaded higher diversity treatments because of their lower soil NO3 levels, greater neighborhood crowding and competition, and greater chance that functionally similar species would occur in a given neighborhood (Figs 3; Naeem et al. 2000, Kennedy et al. 2002, Fargione et al. 2003, Fargione and Tilman 2005a, 2005b).
Greater plant species numbers led to greater ecosystem stability (lower year-to-year variation in total plant biomass) but to lower species stability (greater year-to-year variation in abundances of individual species), with the stabilizing effect of diversity mainly attributable to statistical averaging effects and overyielding effects (Fig 4; Tilman et al, submitted).
Data gathered this past field season shows that soil total C has now become an increasing function of plant species numbers (Fig 5).
Our results have helped resolve a debate about why plant diversity affects ecosystem functioning. Such resolution was accomplished by a Paris symposium in which we made CDR biodiversity data available so others could test their alternative hypotheses; by a paper by 12 ecologists with divergent views that explored areas of agreement and articulated areas in need of 10 further testing (Loreau et al. 2001); and by our analyses of alternative hypotheses using results of the CDR biodiversity experiment (Tilman et al. 2001b).
Future Research
Major questions about why biodiversity affects population, community and ecosystem properties and dynamics remain unanswered:
Why Does Higher Diversity Lead to Stability? We have found strong support for the diversity-stability hypothesis. During the coming six years, we will gather annual data on plant species abundances, total plant biomass, and their relationships to climatic and arthropod variation to test among alternative hypotheses of why ecosystem stability depends on diversity.
How do arthropod communities depend on plant diversity and composition, and what feedback effects do arthropods have on plant communities? The arthropod communities that had assembled in the first years of the biodiversity experiment had diversities and compositions that depended on the diversity and composition of the plant community (Haddad et al. 2001). Preliminary analyses of subsequent arthropod dynamics (based on annual samples, sorted to species and counted) show increases in arthropod diversity and shifts in the ratios of herbivores to parasitoids and herbivores to predators, relationships that we will explore in detail. We are especially interested in food chain structure and stability, and possible feedback effects of arthropods on plant dynamics, such as via seed predation and resultant recruitment limitation of species abundances. These questions will also be explored by new experimental treatments nested within a subset of the plots (see Set 3: Enemies and Biodiversity).
How do Biodiversity and Composition Influence Carbon Sequestration and Other Ecosystem Services and How Sustainable Will These Services Be? We will focus on four ecosystem services (Daily 1997) potentially produced at different rates in response to plant biodiversity - C sequestration, quality of ground water, restoration of soil fertility, and provisioning of pollinators and predator/parasitoids of value to agriculture - by gathering data on these and relevant underlying processes. For instance, preliminary analysis of results from the 10th year of this experiment, 2005, suggested that annual belowground C sequestration was equal to about 1/3 of C in annual aboveground biomass. Soil N levels, and resultant soil fertility, also increased with diversity (Fig 31). Soil NO3 is about 60% lower in high diversity plots than in monocultures, which suggests the possibility that higher plant diversity might lead to lower leaching losses and higher ground water quality. We propose exploring these possibilities, and potential agricultural impacts of arthropods, in our work on ecosystem services (see: D. Synthetic Research)
Biofuels and Ecosystem Services: Another grant (Polasky et al.) will use CDR data on biomass energy (Ragauskas et al. 2006) produced with prairie plantings of various compositions and diversities to explore use of prairie biomass for heating and electric generation (pelletized biomass) and for cellulose-based ethanol as a transportation biofuel. We will build on recent analyses (Farrell et al. 2006) to perform full-cost accounting of net energy balance and of full life cycle environmental benefits/costs in an approach that explicitly considers and economically values ecosystem services.
How Generalizable are the Results of the CDR Biodiversity Experiment? We are collaborating with a "sister" biodiversity experiment, the Jena Biodiversity Experiment, a largescale well-replicated grassland biodiversity experiment established a few years ago by the Max Plank Institute for Biogeochemistry in Jena, Germany. We have agreed to share data, exchange scholars, and, through comparative analyses, determine which of the results of biodiversity experiments are site-specific and which are general.
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| Fig. 1. Change in total (above-ground plus 0±20 cm belowground) biomass (compared with ambient levels of both CO2 and N) in response to elevated CO2 alone (at ambient soil N), to enriched N alone (at ambient CO2), and to the combination of elevated CO and enriched soil N, for plots containing 1, 4, 9 or 16 species. Data were averaged for 4 harvests over 2 yr. Per cent change is shown above each histogram for each diversity treatment. (From Reich et al. 2001.) |
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| Fig. 3. Neighborhood characteristics and plot diversity treatment. Diverse plots tend to have neighborhoods with more neighboring plants (a, solid symbols and line), neighborhoods that are more species rich (a, open symbols and dashed line), and neighborhoods that are more crowded (b). Data points are mean values of 100 null points randomly placed into each of the 147 maps from 1998 that were then averaged within diversity treatments, �one standard error. (From Kennedy et al. 2002.) |
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| Fig. 2. The dependence of aboveground (A and B) and of total (C and D) biomass of each plot on planted species number for 1999 and 2000. The broken line shows the biomass of the top monoculture for a given year. The solid line is a regression of biomass on the logarithm of species number. Logarithm of species number was used in the figure because it gave slightly better fits, but was not used in Table 1 because it often gave slightly lower R2 values than species number. (From Tilman et al. Science 2001) |
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| Fig. 5. Annual rate of accrual of carbon in soil in the Biodiversity Experiment plots. Soil cores for depths of 0- 20 cm, 20-40 cm and 40-60 cm were collected in 1994 and in 2005 in the Biodiversity Experiment, sieved to remove roots, analyzed for total C, and used to calculate the annual rate of storage of carbon in the soil for the 0- 60 depths. Note that there was no net storage of C, on average, in monoculture plots but that plots planted with 16 prairie perennials accrued about 750 kg of C per hectare each year (Tilman et al., in preparation). |
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| Fig. 4. Dependence of temporal stability of each plot on experimentally-controlled plant species numbers. (a) Ecosystem temporal stability for the decade from 1996-2005 was an increasing function of the number of planted species. Ecosystem stability is the ratio of mean plot total biomass to its temporal standard deviation determined after detrending. Regression line and its 95% confidence interval are shown, with F1, 159 = 43.7, P<0.0001. To reduce y-axis scale difference between the two parts of this figure, a single data point (species number of 16, ecosystem stability of 15.76) is not shown but was included in analysis. (b) Plot-average species temporal stability, determined using species biomass data for 2001-2005, was a declining function of the number of planted species. Regression curve and 95% confidence intervals based on fit of Log[Species Stability] on Log[Species Number], with F1, 159 = 72.3, P<0.0001. (From Tilman et al. Nature, submitted.) |
Associated Publications
Biondini, Mario; Plant Diversity, Production, Stability, and Susceptibility to Invasion in Restored Northern Tall Grass Prairies (United States). Restoration Ecology. 15:77-87 2007 [Full Text]
Burt-Smith, G. S.; Grime, J. P.; Tilman, D.; Seedling resistance to herbivory as a predictor of relative abundance in a synthesized prairie community. Oikos 101:345-353. 2003 [Full Text]
Cadotte MW, Cavender-Bares J, Tilman D, Oakley TH (2009) Using Phylogenetic, Functional and Trait Diversity to Understand Patterns of Plant Community Productivity. PLoS ONE 4(5): e5695. doi:10.1371/journal.pone.0005695 2009 [Full Text]
Cadotte, Marc W.; Carscadden, Kelly; Mirotchnick, Nicholas; Beyond species: functional diversity and the maintenance of ecological processes and services. Journal of Applied Ecology. 48:1079-1087 2011[Full Text]
Cadottea, Marc W.; Cardinalec, Bradely J.; Oakleyc, Todd H.; Evolutionary history and the effect of biodiversity on plant productivity. PNAS. 105:17012-17017 2008
Cardinale, Bradley J.; Srivastava, Diane S.; Duffy, Emmett; Wright, Justin P.; Downing, Amy L.; Sankaran, Mahesh; Jouseau, Claire; Effects of biodiversity on the functioning of trophic groups and ecosystems. Nature. 443:989-992 2006
Chapin~III, F. S.; Sala, O. E.; Burke, I. C.; Grime, J. P.; Hooper, D. U.; Lauenroth, W. K.; Lombard, A.; Mooney, H. A.; Mosier, A. R.; Naeem, S.; Pacala, S. W.; Roy, J.; Steffen, W. L.; Tilman, D.; Ecosystem consequences of changing biodiversity. BioScience 48:45-52. 1998 [Full Text]
Chapin~III, F. S; Walker, B. H.; Hobbs, R. J.; Hooper, D. U.; Lawton, J. H.; Sala, O. E.; Tilman, D.; Biotic control over the functioning of ecosystems. Science 277:500-504. 1997 [Full Text]
Craine, Joseph M.; Competition for Nutrients and Optimal Root Allocation. Plant and Soil. 285:171-185 2006 [Full Text]
Diaz, S.; Tilman, D.; Fargoine, J.; Stuart Chapin III, F.; Daily, G.C.; Dirzo, R.; Galetti, M.; Gemmill, B.; Harvell, D.; Kitzberger, T.; Laurance, W.F; "Biodiversity regulation of ecosystem services. In Millenium Ecosystem Assessmen Eds., Ecosystems and Human Well-Being Current State and Trends, Voulme 1. Island Press." 2005 [Full Text]
Dybzinsk, Ray; Fargione, Joseph E.; Zak, Donald R.; Fornara, Dario; Tilman, David; 1998. Soil fertility increases with plant species diversity in a long-term biodiversity experiment. Oecologia. 158:85-93 2008 [Full Text]
Elser, James J.; Bracken, Matthew E.S.; Cleland, Elsa E.; Gruner, Daniel S.; Harpole, W. Stanley; Hillebrand, Helmut; Ngai, Jacqueline T.; Seabloom, Eric W.; Shurin, Jonathan B.; Smith, Jennifer E.; Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters. 10:1135�1142 2007 [Full Text]
Fargione, J. E.; Tilman, D.; Diversity decreases invasion via both sampling and complementarity effects. Ecology Letters 8:604-611 2005 [Full Text]
Fargione, J.; Tilman, D.; Predicting relative yield and abundance in competition with plant species traits. Functional Ecology 20:533-450. 2006 [Full Text]
Fargione, J.; Tilman, D; Dybzinski, R; Lambers, J. H. R.; Clark, C; Harpole, W. S.; Knops, J. M. H.; Reich, P. B.; Loreau, M; From selection to complementarity: shifts in the causes of biodiversity-productivity relationships in a long-term biodiversity experiment. Proceedings of The Royal Society B 274:871-876. 2007 [Full Text]
Fargione, J; Hill, J; Tilman, D; Polasky, S; Hawthorne, P, "Biofuels: Effects on land and fire - Response", SCIENCE, p. 199, vol. 321. Published 2008
Fargione, J; Hill, J; Tilman, D; Polasky, S; Hawthorne, P, "Biofuels: Putting current practices in perspective - Response", SCIENCE, p. 1420, vol. 320, Published 2008
Fargione, J; Hill, J; Tilman, D; Polasky, S; Hawthorne, P, "Land clearing and the biofuel carbon debt", SCIENCE, p. 1235, vol. 319, Published, 10.1126/science.115274 2008 [Full Text]
Fornara, D. A., D. Tilman and S. E. Hobbie, "Linkages between plant functional composition, fine root processes and potential soil N mineralization rates", Journal of Ecology 97:48-56 2009 [Full Text]
Fornara, D. A.; Tilman, D.; Ecological mechanisms associated with the positive diversity-productivity relationship in an N-limited grassland. Ecology, 90: 408�418 2009
Fornara, DA; Tilman, D, "Plant functional composition influences rates of soil carbon and nitrogen accumulation", JOURNAL OF ECOLOGY, p. 314, vol. 96 Published, 10.1111/j.1365-2745.2007.01345. 2008 [Full Text]
Fox, Jeremy W.; Harpole, W. Stanley; REVEALING HOW SPECIES LOSS AFFECTS ECOSYSTEM FUNCTION: THE TRAIT-BASED PRICE EQUATION PARTITION. Ecology. 89:269�279 2008 [Full Text]
Haddad, N. M.; Tilman, D.; Haarstad, J.; Ritchie, M.; Knops, J. M. H.; Contrasting effects of plant richness and composition on insect communities: a field experiment. The American Naturalist 158:17-35. 2001 [Full Text]
Hille Ris Lambers, J.; Harpole, W. S.; Tilman, D.; Knops, J.; Reich, P.; Mechanisms responsible for the positive diversity-productivity relationship in Minnesota grasslands. Ecology Letters 7:661-668 2004[Full Text]
Kennedy, T. A.; Naeem, S.; Howe, K. M.; Knops, J. M. H.; Tilman, D.; Reich, P.; Biodiversity as a barrier to ecological invasion. Nature 417:636-638. 2002 [Full Text]
Kennedy, T.; "The causes and consequences of plant invasions. Ph.D. Thesis, University of Minnesota." 2002 [Full Text]
Knops, J. M. H.; Tilman, D.; Haddad, N. M.; Naeem, S.; Mitchell, C. E.; Haarstad, J.; Ritchie, M. E.; Howe, K. M.; Reich, P. B.; Siemann, E.; Groth, J.; "Effects of plant species richness on invasion dynamics, disease outbreaks, insect abundances and diversity. Ecological Letters 2:286-293." 1999[Full Text]
Knops, J. M. H.; Wedin, D.; Tilman, D.; Biodiversity and decomposition in experimental grassland ecosystems. Oecologia 126:429-433. 2001 [Full Text]
Lehman, C. L.; Tilman, D.; "Biodiversity, stability, and productivity in competitive communities. The American Naturalist 156:534-552." 2000 [Full Text]
Lin, Brenda B.; Flynn, Dan F.B.; Bunker, Daniel E.; Uriarte, Mar�┤a; Naeem, Shahid; The effect of agricultural diversity and crop choice on functional capacity change in grassland conversions. Journal of Applied Ecology. 48:609�618 2011 [Full Text]
Loreau, M.; Naeem, S.; Inchausti, P.; Bengtsson, J.; Grime, J. P.; Hector, A.; Hooper, D. U.; Huston, M. A.; Raffaelli, D.; Schmid, B.; Tilman, D.; Wardle, D. A.; Biodiversity and ecosystem functioning: current knowledge and future challenges. Science 294:804-808. 2001 [Full Text]
L�pez-Villalta, JS.; A metabolic view of the diversity-stability relationship. J Theor Biol. 252:39-42 2008 [Full Text]
Mitchell, C.; Tilman, D.; Groth, J. V.; "Effects of grassland plant species diversity, abundance, and composition on foliar fungal disease. Ecology 83:1713-1726." 2002 [Full Text]
Pacala, S.; Tilman, D.; "The transition from sampling to complementarity. Pages 151-166, in, A. P. Kinzig, S. W. Pacala and D. Tilman, Eds., The Functional Consequences of Biodiversity: Empirical Progress and Theoretical Extensions. Princeton University Press, Princeton and Oxford." 2002
Siemann, E.; Tilman, D.; Haarstad, J.; Ritchie, M.; Experimental tests of the dependence of arthropod diversity on plant diversity. American Naturalist 152:738-750. 1998 [Full Text]
Strengbom, J.; Reich, P.B.; Elevated [CO2] and increased N supply reduce leaf disease and related photosynthetic impacts on Solidago rigida. OECOLOGIA 149:519-525. 2006 [Full Text]
Tilman, D.; "Causes, consequences and ethics of biodiversity. Nature 405:208-211." 2000 [Full Text]
Tilman, D.; "Effects of diversity and composition on grassland stability and productivity. Pages 183-207 in, M. C. Press, N. J. Huntly and S. Levin, Eds., Ecology: Achievement and Challenge. Blackwell Science, Oxford." 2001 [Full Text]
Tilman, D.; "Biodiversity and Ecosystem Services: Does Biodiversity Loss Matter?. M. Loreau Eds., Biodiversity: Science and Governance. Proceedings of Biodiversity Conference in Paris 2005." 2005
Tilman, D.; "Functional diversity. Pages 109-120, in, S. A. Levin, Editor-in-Chief, Encyclopedia of Biodiversity, Vol. 3. Academic Press, San Diego, CA." 2001 [Full Text]
Tilman, D.; Commentary - An evolutionary approach to ecosystem functioning. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 98:10979-10980. 2001 [Full Text]
Tilman, D.; Distinguishing between the effects of species diversity and species composition. Oikos 80(1):185. 1997 [Full Text]
Tilman, D.; Diversity and production in European grasslands. Science 286:1099-1100. 1999 [Full Text]
Tilman, D.; Fargione, J.; Wolff, B.; Antonio, C. D.; Dobson, A.; Howarth, R.; Schindler, D.; Schlesinger, W. H.; Simberloff, D.; Swackhamer, D.; Forecasting agriculturally driven global environmental change. Science 292:281-284. 2001 [Full Text]
Tilman, D.; Hill, J.; Lehman, C.; Carbon-negative biofuels from low-input high-diversity grassland biomass. SCIENCE 314:1598-1600. 2006 [Full Text]
Tilman, D.; Hill, J.; Lehman, C.; Response to comment on "Carbon-negative biofuels from low-input high-diversity grassland biomass". SCIENCE 316:1598-1600. 2007 [Full Text]
Tilman, D.; Knops, J.; Wedin, D.; Reich, P.; "Experimental and observational studies of diversity, productivity and stability. Pages 42-70, in, A. P. Kinzig, S. W. Pacala and D. Tilman, Eds., The Functional Consequences of Biodiversity: Empirical Progress and Theoretical Extensions. Princeton University Press, Princeton and Oxford." 2002
Tilman, D.; Knops, J.; Wedin, D.; Reich, P.; "Plant diversity and composition: effects on productivity and nutrient dynamics of experimental grasslands. Pages 21-35 in, Loreau, M., S. Naeem and P. Inchausti, eds., Biodiversity and Ecosystem Functioning: Synthesis and Perspectives. Oxford University Press, Oxford." 2002
Tilman, D.; Knops, J.; Wedin, D.; Reich, P.; Ritchie, M.; Siemann, E.; The influence of functional diversity and composition on ecosystem processes. Science 277:1300-1302. (highlighted in Perspectives by Grime) 1997 [Full Text]
Tilman, D.; Lehman, C.; "Biodiversity, composition, and ecosystem processes: theory and concepts. Pages 9-41, in, A. Kinzig, S. Pacala and D. Tilman, Eds., Functional Consequences of Biodiversity: Empirical Progress and Theoretical Extensions. Princeton University Press, New Jersey." 2002
Tilman, D.; Lehman, C.; Human-caused environmental change: Impacts on plant diversity and evolution. Proceedings of the National Academy of Science 98:5433-5440. 2001 [Full Text]
Tilman, D.; Naeem, S.; Knops, J.; Reich, P.; Siemann, E.; Wedin, D.; Ritchie, M.; Lawton, J.; Biodiversity and ecosystem properties. Science 278:1866-1867. 1997 [Full Text]
Tilman, D.; Reich, P. B.; Knops, J.M.H.; Wedin, D.; Mielke, T.; Lehman, C.; Diversity and productivity in a long-term grassland experiment. Science 294:843-845. 2001 [Full Text]
Tilman, D.; Reich, P.B.; Knops, J.; Diversity and stability in plant communities Reply. NATURE 446:E6-E8. 2007 [Full Text]
Tilman, D.; Reich, P.B.; Knops, J.M.H.; Biodiversity and ecosystem stability in a decade-long grassland experiment. NATURE 441:629-632. 2006 [Full Text]
Tilman, D.; The ecological consequences of changes in biodiversity: a search for general principles. The Robert H. MacArthur Award Lecture. Ecology 80:1455-1474. 1999 [Full Text]
Tilman, G.; Duvick, D. N.; Brush, S. B.; Cook, R. J.; Daily, G. C.; Heal, G. M.; Naeem, S.; Notter, D.; "Benefits of Biodiversity. Task Force Report No. 133, Council for Agricultural Science and Technology." 1999
Waldrop, M.; Zak, D.R.; Blackwood, C.B.; Curtis, C.D.; Tilman, D.; Resource availability controls fungal diversity across a plant diversity gradient. ECOLOGY LETTERS 9:1127-1135. 2006 [Full Text]
Wright, J.; Naeem, S.; Hector, A.; Lehman, C.; Reich, P.B.; Schmid, B.; Tilman, D.; Conventional functional classification schemes underestimate the relationship with ecosystem functioning. ECOLOGY LETTERS 9:111-120. 2006 [Full Text]
Zak, D.; Holmes, W. E.; White, D. C.; Peacock , A. D.; Tilman, D.; "Plant diversity, soil microbial communities, and ecosystem function: Are there any links? Ecology 84:2042-2050." 2003 [Full Text]
