Biodiversity Matters

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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

Cedar Creek scientists discovered that the number of plant species in an ecosystem – its biodiversity – has a profound effect on ecosystem function. Long-term experiments show ecosystems with greater plant biodiversity are more productive and stable, and better able to soak up more of our carbon dioxide emissions as well. Moreover, the value of diversity grew over time in two long-term experiments. Consequently, local loss of even a few species will likely have greater negative impacts on ecosystem functioning than has been suggested by short-term experiments.

Importantly, plant biomass production increases 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.).

Cedar Creek’s “Big Biodiversity” experiment 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.

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
Tilman et al. Science 2001
For further reading: 
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
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
Reich, P.; Knops, J.; Tilman, D.; Craine, J.; Ellsworth, D.; Tjoelker, M.; Lee, T.; Wedin, D.; Naeem, S.; Bahauddin, D.; Hendrey, G.; Jose, S.; Wrage, K.; Goth, J.; Bengston, W.; Plant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition. Nature 410:809-812. 2001
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
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
For further information: 
Dave Tilman
Peter Reich
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