FIG. 47.18

Does species diversity promote primary productivity?

BACKGROUND The different plant species found in communities have combinations of leaves, stems, and roots that tap different sources of water and nutrients in the soil and capture light at different levels of sun and shade. Do the differences among plant species result in higher levels of primary production than would be possible with fewer species?

HYPOTHESIS Plant diversity promotes primary production within a community.

EXPERIMENT Ecologist David Tilman conducted a long-term experiment in Minnesota, seeding each of 11 plots with 1, 2, 4, or 16 grassland species, and controlling each plot’s amount of nutrients, water, and carbon dioxide, as well as manipulating the plots’ exposure to herbivory and disturbance by fire. The primary production of each plot was measured annually for up to 23 years.

RESULTS The graph shows that the addition of nitrogen fertilizer [quantified as kg of N per hectare (ha)] increased primary production significantly, and additional water, higher CO₂, herbivory, and disturbance also resulted in at least a modest increase in productivity. The difference in primary production between high-diversity (16 species) and low-diversity (1, 2, or 4 species) plots was at least as great as the largest additions of fertilizer—and much greater than any of the other treatments.

CONCLUSION The hypothesis was supported: In the grassland community under study, species diversity promoted primary production.

FOLLOW-UP WORK Researchers continue to study the ecosystem effects of species diversity, steadily accumulating data that show the importance of conserving the species diversity found in nature.

SOURCE Data from Tilman, D., P. B. Reich, and F. Isbell. 2012. “Biodiversity Impacts Ecosystem Productivity as Much as Resources, Disturbance, or Herbivory.” Proceedings of the National Academy of Sciences USA, 109:10394–10397.